Archive for the ‘AVR’ Category

Moving code from Arduino to Raspberry Pi

Thursday, May 31st, 2012
Photo: Hello world, using the WiringPi library to control an LED display.

Photo 1: Hello world, using the WiringPi library to control an LED display.

To celebrate getting the GPIO working using WiringPi from Gordon Henderson I thought I’d have a quick look at the difference between running some code on an Arduino compared to running almost the same code on a Raspberry Pi (RasPi).

It is worth noting now that the setups I’ve been using to test with are all powered from a separate 3.3V supply with the grounds linked. Nothing is being powered via the RasPi itself to avoid drawing too much current should I make a mistake. When working with the RasPi you should be careful as the RasPi will not take kindly to static discharge, connecting things backwards or the wrong voltage levels.

The really great thing about Gordon’s library is that by using it I am able to transfer Arduino code to the RasPi with only a few simple changes. Obviously not everything in the Arduino libraries are present, but the basic I/O manipulation is plenty for some simple applications.

MAX6675 thermocouple interface example

The MAX6675 from Maxim is an SPI Bus based thermocouple convertor that I built a breakout board for a while back. You can find out more information about that project in this earlier post.

As a simple test I took the basic example from the MAX6675 thermocouple interface library and adapted it to run with the WiringPi library on the RasPi.

I initially ported the example and library to C as the WiringPi was in C and I had a couple of issues compiling mixed C and C++ properly. After a bit of investigation I found that when explicitly including the libraries as C the compiling problems go away. Below is an example of how to modify the wiringPi.h file to ensure it is included as C and not C++. Gordon has said he will make this change to the library on the next release. Once I had sorted out the makefile to use g++ rather than gcc I was able to compile and link successfully.

Listing 1: Explicitly declaring a header file as C


 #ifdef __cplusplus
 extern "C" {
 #endif
  
 // wiringPi.h Code goes Here 
 //  or 
 // #include <wiringPi.h>

 #ifdef __cplusplus
 }
 #endif

While compiling and linking of the example was now okay, the next problem was that when run, the program just exited with a “Segmentation Fault” (segfault). I didn’t even see the error message when running as root with sudo, it was only when running as the standard user (pi) that it became apparent that something was wrong with the software rather than the hardware.

When I had run previous examples as the standard user I would see a permissions related error complaining that the program could not access the “/dev/mem/” device. This error comes from the part of the WiringPi library that sets up memory access to the hardware registers in the Broadcom BCM2835. This clue told me that the wiringPiSetup routine was not being run. This in turn showed me that the global object representing the temperature sensor was being constructed before the WiringPi library was being setup.

The solution to this particular problem was simple enough. I edited the MAX6675 library so that it calls wiringPiSetup directly when initialising. I also set an internal flag if this succeeds. In addition to this I modified the methods to exit with an error if the flag has not been set. This means that none of the MAX6675 library methods should ever call WiringPi functions without the appropriate initialisation having taken place and succeeded beforehand. My code for this example is here. A more general solution might be to add this flag to the WiringPi library itself.

LED Display (MLMC) and Temperature Combined Example

Following my success with the temperature sensor I decided to push the boat out a little further and try to interface to my MLMC LED display system.

The MLMC LED display is a set of LED dot matrix control PCBs I designed to use up some unusual LED modules I had lying about. You can get the full story in this blog post or on the website.

Photo:  Using the WiringPi library to control an LED display and Thermocouple interface

Photo 2: Using the WiringPi library to control an LED display and thermocouple interface

This was a simple demonstration with the RasPi reading from the thermocouple interface and then displaying the temperature on the display. For this example I wrote everything in C. The MLMC interface is very simple and involves clocking 16 bit words of data using two pins, one for data and one for clock. Each word represents a single column on the LED display. This low speed (~5Kbps) synchronous protocol was originally implemented by directly manipulating the digital I/O pins on the Arduino and so was very simple to port to the RasPi with the WiringPi library.

While the ported program functions very well, the waveforms obtained from the RasPi are quite different to those achieved with the Arduino. This is a good example of how the same simple code can be quite different when ported between platforms.

MLMC Clock Waveforms

The function used to send serial data to the MLMC is shown below. This code has a 75µs delay between switching the level of the clock and this gave uniform pulses on both systems. The pulses also appeared consistent from one run to another.

Listing 2: MLMC serial data sending routine


const int spidelay_us =75;

void Send_byte( char data ///< 8 bit data to be sent
			   ) {
    uint8_t temp;	// local copy of the data to be shifted out
    uint8_t i;					
	
    temp = data;	
    for (i=0; i<8; i++) {
		// start the cycle with the clock pin low
        digitalWrite(clockPin, LOW);

		// clock out a single byte
        if (temp & (1<<7)) {
            digitalWrite(dataPin, HIGH);
        } else {
            digitalWrite(dataPin, LOW);
        }
        
		// wait for data bit to be set up
		delayMicroseconds(spidelay_us);
        
		// clock the data out by producing a rising edge
        digitalWrite(clockPin,HIGH); 
		
		// wait for mlmc to read the data bit in 
        delayMicroseconds(spidelay_us);
		
        // shift data along by one
        temp <<= 1;
    }
	
	// leave both pins low when idle 
    digitalWrite(clockPin, LOW);
    digitalWrite(dataPin, LOW);
}

The following scope traces are the result of executing Listing 2 on the Arduino:

Clock pulses from the Arduino to the MLMC modules

Figure 1: Clock pulses from the Arduino to the MLMC modules

Single clock pulse from the Arduino to the MLMC modules

Figure 2: Single clock pulse from the Arduino to the MLMC modules

Arduino has a total pulse time of approximately 80µs for the first clock pulse to MLMC. This extra 5µs I assign to the time taken to call the digitalWrite function, a single shift operation and iterating the for-loop.

The next two traces in figure 3 and figure 4 are produced when running the same code on the RasPi:

Clock pulses from the Raspberry Pi to the MLMC modules

Figure 3: Clock pulses from the Raspberry Pi to the MLMC modules

Single clock pulse from the Raspberry Pi to the MLMC modules

Figure 4: Single clock pulse from the Raspberry Pi to the MLMC modules

You can notice that while the pulses look identical at a quick glance the RasPi has 150µs pulse width; almost double that of the Arduino. I assume that the delayMicroseconds function, which only guarantees that a minimum of the given microseconds have elapsed before returning, is releasing control to the operating system and coming back a lot later than expected. As the delay implementation in the WiringPi library simply calls nanosleep I am surprised this operation takes twice as long. There may be a number of causes for this behaviour:

  • There might be a bug in the way the delay function is implemented.
  • The shift and for-loop iteration might have compiled in a very inefficient way.
  • The operating system simply cannot schedule other tasks quickly enough to return from nanosleep and meet our expectations.

MAX6675 thermocouple interface

This loop was clocking data in from the MAX6675 chip via a “bit-banged” interface. The main loop did not have a delay and was toggling bits as quickly as possible while shifting in the value via a single GPIO pin.

Listing 3: SPI send section from the MAX6675 library


digitalWrite(_CS_pin,LOW); // Enable device

/* Cycle the clock for dummy bit 15 */
digitalWrite(_SCK_pin,HIGH);
digitalWrite(_SCK_pin,LOW);

/* Read bits 14-3 from MAX6675 for the Temp 
 Loop for each bit reading the value and 
 storing the final value in 'temp' 
 */
for (int bit=11; bit>=0; bit--){
	digitalWrite(_SCK_pin,HIGH);  // Set Clock to HIGH
	value += digitalRead(_SO_pin) << bit;  // Read data and add it to our variable
	digitalWrite(_SCK_pin,LOW);  // Set Clock to LOW
}

/* Read the TC Input inp to check for TC Errors */
digitalWrite(_SCK_pin,HIGH); // Set Clock to HIGH
error_tc = digitalRead(_SO_pin); // Read data

digitalWrite(_SCK_pin,LOW);  // Set Clock to LOW
digitalWrite(_CS_pin, HIGH); // Disable Device

The following traces in figure 5 and figure 6 show the results on the Arduino.

Clock pulses from the Arduino to the MAX6675 modules

Figure 5: Clock pulses from the Arduino to the MAX6675

Single clock pulse from the Arduino to the MAX6675 modules

Figure 6: Single clock pulse from the Arduino to the MAX6675

On the Arduino this code gave a uniform pulse train with the pulse widths measuring approximately 6-7µs. As the Arduino is running at 16Mhz I would like to think this could be optimised a lot more to get something like 120ns if I used some assembly or bypassed the Wiring/Arduino API, but that is an exercise for another blog post. 🙂

Clock pulses from the Raspberry Pi to the MAX6675 modules

Figure 7: Clock pulses from the Raspberry Pi to the MAX6675

Close-up clock pulses from the Raspberry Pi to the MAX6675 modules

Figure 8: Close-up clock pulses from the Raspberry Pi to the MAX6675

The RasPi on the other hand gave a very non-uniform pulse train. As this is synchronous communication, the clock does not need to be uniform, it dictates when the data line is read. The average pulse in this waveform is only 150ns wide which is approximately 48 times faster than the Arduino. Given that the RasPi clock speed of 700MHz is roughly 44 times faster then the Arduino clock speed of 16MHz, this makes sense.

The RasPi would vary any individual pulse length by up to a factor of two (from observation) but did not seem to be effected by increasing the load (multiple ssh sessions and running an openssl speed test). The RasPi would also continue to run the MLMC scrolling display and reading the temperature sensor without issues while another process was also reading the temperature sensor at the same time (single_temp.cpp from first example above). Even while openssl was doing speed testing (cpu usage at 98-99%) the visible operation of the screen was not effected.

Investigating Further

The long delay and changing pulse widths could be investigated further with a couple of short programs.

I could investigate the long delay further by looking at repeating the test with the system under different loads and noting the effect on the width and consistency of the pulses. To vary the workload on the RasPi I could use a tool like stress.

We could also step through many values while keeping the system load constant. This would give us an idea if their was a minimum overhead experienced by the delay function or if their was some sort of error in the calibration.

I will leave these as an exercise for another day as plenty of work has already been done on this sort thing before, although not specifically on the RasPi.

Conclusions

This was a very quick look into porting code from Arduino to Raspberry Pi. I found that it was not so difficult to port some very simple applications, the majority of the code would simply run unchanged.

While the code may compile and run, the actual operation of the code will always need to be checked carefully for timing issues and other unexpected behaviour when switching hardware and using new library implementations.

As these systems were using synchronous serial communication the exact shape of the waveforms did not actually need to be uniform for the system to work.

On review of the MAX6675 data sheet the timings slighty exceed the stated maximum clock freq of 4.3Mhz. So If I was intending to use this further I would introduce a short delay into the MAX6675 clock routine to ensure it complies with the data sheet.

The MLMC module operates perfectly with the timings achieved but we are operating at half the throughput we expected due to the delay function not operating identically to the same function on the Arduino. While this didn’t cause a problem in this simple test it could easily have done so in a slightly more complicated system.

Modular LED Matrix Controller version 1.0

Monday, March 12th, 2012
 An Arduino driving 5 MLMC modules displaying the current temperature

An Arduino driving five MLMC modules displaying the current temperature

I have finally motivated myself to publish my firmware and PCB designs for the MLMC on github. The design is functional and I have a set of five modules chained together sitting on my workbench. They happily display the current temperature via an Arduino and the MAX6675 based Thermocouple board I designed previously.

All the files for the project are currently published on github at https://github.com/brokentoaster/MLMC.

Bottom view of the controller PCBs.

Bottom view of the controller PCBs.

This Modular LED Matrix Controller (MLMC) all started about 3 years ago and you can find all the info and a demonstration video at the project web page.

Once a couple of initial glitches in the system had been worked out the modules turned out to be both reliable and easy to drive.

The power consumption as on average 12mA per module while displaying scrolling text. The current consumption is reasonably nicely distributed about the average without any large peaks in current. Each module’s refresh rate is slightly out of sync with its neighbour due to the differences in each of the AVRs on-board oscillators. This has the side effect that all columns switch on at a slightly time and avoids causing a large spike in the current. While not deliberately designed to act this way it is a benefit of the multiple controller modular design.

Trying to drive 1280 LEDs from just 4 wires was not without its problems, the three main problems are covered in greater detail in later posts but in brief they were as follows:

  • Synchronisation issues caused by jitter, noise and missing bits
  • Clocking out the last word from one module to the next
  • PCB design error – SPI pins are not always the same as the ISP serial programming pins.
  • Component Choice. Not enough memory for desired bit depth or bandwidth to achieve original functionality
  • USI in SPI mode is not the same as hardware SPI

Arduino MP3

Tuesday, July 5th, 2011

ArduinoMP3 (Rev C)

The Arduino MP3 webpage is finally up and the source code for the project is on github.

You can find the web pages at:

http://www.brokentoaster.com/arduinomp3

You can find all the future development at GitHub via this link:

https://github.com/brokentoaster/ArduinoMP3

Dynamic Logging Examples

Monday, June 27th, 2011

As I mentioned in an earlier post, the Butterfly Logger now has a dynamic logging feature. This post elaborates on the benefits and costs of the feature using some data gathered during testing. Some examples of the output while using the feature is shown below in figures 1 and 2. In processing the output from the logger I created a couple of scripts to analyse the data and create graphs. These are included below as listings 1 and 2. While the new feature allows us to log for longer without the risk of missing events, it also costs us in no longer being able to predict how long a logger can be deployed for in the field.

A Graphical Overview

Figure 1: Dynamic logging example.

The two part graph shown above in figure 1 demonstrates the increasing and decreasing of logging frequency as the monitored signal changes more or less quickly. The signal being monitored in this example is the ambient temperature of my garage over a week in March 2011. This was done using the AVR Butterfly’s onboard thermistor.

The upper part of the graph shows the temperature plotted in order sampled as it is stored in the memory of the logger. The lower part of the graph plots the temperature against time. This is done using the date and time as recorded by the logger and using the following lines in gnuplot:

set xdata time
set timefmt "%Y/%m/%d %H:%M:%S"
plot 'dataFile.log' using 1:8 with impulses notitle

The temperature in the lower section is plotted as impulses simply to highlight the change in logging frequency.

Figure 2: Comparison of data plotted against time or sample index.

The above graph in figure 2 shows a much larger sample from the same logging session. In this plot you can again see the differences between the samples recorded in memory and the samples plotted against time.

You will notice how the samples actually recorded only ever differ by a fixed amount. This is evident through the constant slope of the upper section of the graph. This constant slope, either increasing or decreasing, is an artefact of the logging threshold value. The threshold value is set at compile time in the software.

The software also has a timeout parameter that establishes the maximum logging interval. If no timeout is set the system will only log when the threshold has been exceeded, however if the parameter has been set then the logger will record a sample after a fixed period time (even if the sample threshold has not been exceeded).

The minimum interval is specified by the sample period parameter as used when logging in the standard mode. The timeout value is simply specified as a multiple of this interval.

Figure 3: Dynamic resolution of the data expressed as number of samples per hour.

Figure 3 is yet further analysis on the same data from the previous two graphs. This plot shows the logging resolution over time. The resolution is expressed as the number of samples recorded per hour. This figure was calculated by processing the data and counting the number of samples recorded that hour. Where no data is recorded for a given hour a zero is recorded. A Perl script to produce this data from the logger output is shown below in listing 2.

This example gives us a good metric for measuring the effectiveness of dynamic logging. If you examine the graph you can see that, at its peak rate, the logger is recording at a resolution of 30 samples per hour. The logging interval for this session was set to once per second, giving us a maximum sample rate of 3600 samples per hour. As the rate is dynamic it makes sense to also look at the average rate over the whole period, which is just over 2 samples per hour. If we compare the average rate to the peak rate we can gauge the efficiency introduced by the dynamic logging feature.

TABLE 1: Numerical summary
Number of hours monitored: 168
Number of samples recorded: 350
Minimum sample rate: none
Maximum sample rate: 3600 samples per hour
Average sample rate: 2.1 samples per hour
Samples needed if using peak rate: 5040
Samples needed if using maximum rate: 604,000

Conclusions

The benefits of dynamic logging

By looking at the number of samples needed at the peak sample rate and comparing this to the number of samples used, we can calculate the theoretical saving in redundant samples. If we were to capture the same data using standard methods we would have used over 14 times more samples. As our storage space is limited, if we were using a standard technique we could only log for a much shorter time.

This reduction in space used can be taken advantage of in two ways. If the temperature characteristics in the garage remain unchanged the logger could record for 3 years without filling up the flash. This is much much more than possible using the standard technique. Another way to utilise this extra space would be to log more sensors over the same period as possible with standard logging.

Other than storage savings the major benefit of the dynamic sample rate is that dramatic events are not missed. If we had wanted to conserve storage space using standard methods we would risk the possibility of sudden changes in the data being overlooked by a much slower sample rate.
Using a dynamic rate we get the benefits of a higher sample rate without the storage cost.

The costs of dynamic logging

There are a couple of disadvantages to this dynamic sample rate technique. One of these is the reduction in resolution in our sensors. This is due to the threshold introduced before a sample is taken. The resolution of the ADC has effectively been reduced from 1024 to approximately 340 levels. For our example this reduces the temperature to a resolution of 0.1°C per level in the temperature range we are looking at. (< Due to the non-linear nature of the thermistor, the effective temperature resolution will change of over the range of the sensor >)

The logger will also consume more power due to being out of sleep mode more often. The logger needs to leave sleep mode often to check if it should be recording a sample to flash. Compared to just reading of the sensorsWriting to flash is by far the most power hungry activity of the logger when c, because of this I don’t anticipate the extra power requirements to be too great, although I have not actually measured the impact.

When using a fixed sample rate technique you can calculate the exact length of time a logger will be able to log for prior to deployment. With the dynamic sample rate you can no longer know how long you can log for when deploying a logger. To mitigate this issue you could either make a prediction based on prior knowledge or develop a method to alert the user when the memory is approaching capacity.

Using data previously gathered in an environment you can predict how long it is reasonably likely to log for. Using the example data used I would calculate the expected logging capacity based on 700 samples a week (< Doubling the average sample rate we saw in the data simply for reasons of contingency. >). Using this value would allow logging for something like 80 weeks or so. This is of course still 7 times longer than standard logging would allow. These types of calculations could potentially be incorporated to the firmware to make using dynamic logging more useful.

Scripts

As promised above I have included the scripts used to produce the data and graphs. They might be useful to anyone getting started with using Gnuplot, BASH and Perl to automate graphing and analysis.

#!/bin/sh

# script to plot logging results temperature against time.
# also plot temperature samples for comparison and analysis of
# the dynamic logging system.

#assume arg 1 is name of file with ^M's ^D's and non data lines already removed.

maxtemp=30

# look at number of points per hour
#cut -d\  -f2 ${1} | cut -d: -f1 | uniq -c > ${1}_res

# TODO: need to pad out hours with zero points
# DONE: use pad.pl to calculate number of samples per hour and pad any zeros
# 	Does not check for an entire day with out samples though.
cat ${1} | ./pad.pl > ${1}_res

gnuplot << EOF
set terminal png size 1024,768 enhanced font "/Library/Fonts/Microsoft/Arial,12"
set output "$1_resolution.png"
set origin 0,0
set grid
set yrange [0:35]
set title 'Logging resolution over time'
set ylabel 'No. of samples per hour'
set xlabel 'Hour of sampling' 
plot '${1}_res' u 2 w i  not

set output "${1}_comparison.png"
set multiplot
set origin 0,0.5
set size 1,0.5
set xdata 
set yrange [0:${maxtemp}]
set title 'Ambient Temperature'
set xlabel 'Sample'
set ylabel 'Temperature (°C)'
plot '$1' u 8 w l not

set origin 0,0
set size 1.0,0.5
set xdata time
set timefmt "%Y/%m/%d %H:%M:%S"
set ylabel 'Temperature (°C)'
set yrange [0:${maxtemp}]
set xlabel 'Time'
set title ''
set format x "%d %b"
plot '$1' u 1:8 w l not

set output "${1}_impules.png"
set multiplot
set origin 0,0.5
set size 1,0.5
set xdata 
set yrange [0:${maxtemp}]
set title '150 Samples of Ambient Temperature'
set xlabel 'Sample'
set ylabel 'Temperature (°C)'
plot '< head -150 $1' u 8 w l not

set origin 0,0
set size 1.0,0.5
set xdata time
set timefmt "%Y/%m/%d %H:%M:%S"
set ylabel 'Temperature (°C)'
set yrange [0:${maxtemp}]
set xlabel 'Time'
set title ''
set format x "%d %b"
plot '< head -150 $1' u 1:8 w i not
EOF

open $1_resolution.png
open $1_comparison.png
open $1_impules.png

Listing 1: BASH script to process the data into graphs.

#!/usr/bin/perl -w

use strict;
# simple script to process temperature logs and show number of samples per hour

# my old script did this...
## look at number of points per hour
##  cut -d\  -f2 ${1} | cut -d: -f1 | uniq -c > ${1}_res
# .. but that didn't account for hours with no samples at all.

# my variables...
my @lines;
my $line;
my $hour;
my $previous_hour;
my $first = 1;
my $count = 0;
my @fields;
my @time;

# read in all lines from stdin
chomp(@lines = <STDIN>); 

#process each line in turn
foreach $line (@lines) {
		
		# extract the hour value from the time, ignoring the date.
		@fields = split /\s+/,$line;
		@time = split /:/,$fields[1];
		$hour = $time[0];
		
		# set up the previous_hour for the first line to enable the count
		if ($first == 1){
			$previous_hour = $hour;
			$first = 0;
		}
		
		if ($hour == $previous_hour){
			#increment the count of samples for this hour 
			$count++;

		} else{
			#  print our total and move on to the next hour
			print "$previous_hour \t $count \n";
			$count = 0;
			$previous_hour++;
			$previous_hour %= 24;
			
			# check for non consecutive hours and pad out with zero totals
			# assuming their hasn't been a total day without samples
			while ($previous_hour != $hour){
				print "$previous_hour \t $count \n";	
				$previous_hour++;
				$previous_hour %= 24;
			}
			
			# remember to count this first new sample in our totals 
			$count=1;
		}	
}

# done

Listing 2: PERL script to process the data logger output and calculate the number of samples per hour.

Butterfly Logger Firmware 0.31A

Tuesday, May 3rd, 2011

I’ve just completed another release of firmware for the AVR Butterfly Logger project. It can be download it at the project site on Sourceforge.

New Features

The main feature of this update is dynamic logging. I’ll post some examples of this in action soon but for now I’ll just give you a basic overview.

Dynamic logging means that the period between samples is altered dynamically at runtime depending on the state of the system being monitored. If the system is undergoing rapid change then many samples will be taken automatically. If the system is stable then very few samples will be taken at that time. This has the same effect as sampling continuously at a high sample rate but removing any repeated or redundant data except that the repeated and redundant data is simply never recorded.

When using the dynamic logging feature you will need to log time and date to enable you to be able to properly reconstruct the data later once downloaded.

The dynamic logging feature also has an optional timeout value so that if no change has been read from the ADC for a large number of log attempts then a log is forced to ensure a minimum resolution such as 1 sample every hour is maintained.

Currently the dynamic logging feature only monitors a single ADC channel. I have plans to extend it to monitor other sensors and also multiple sensors simultaneously in future releases.

Setup and options for the dynamic logging features are currently maintained in the file main.h. There are some comments in the file to explain the options available.

To save on program space the dynamic logging parameters can be implemented as a compile time option. If more flexibility is wanted the features can also be implemented with the parameters as runtime changeable options that can be altered via the serial port interface.

Bug Fixes

This version also includes a fix for an issue with updating the LCD readings after the flash was full.

Butterfly Logger Firmware 0.30C (bugfix)

Saturday, January 22nd, 2011

Last week I released another firmware update for the AVR Butterfly Logger. This was a bugfix to the thermistor routines for calculating negative temperatures from the onboard temperature sensor.

You can download the source code from SourceForge.

The thermistor routines in the logger project were adapted from Martin Thomas’ port of the Atmel AVR Butterfly example code. This routine compares the ADC input from the thermistor sensor to a table of known readings. The table () contains a single entry for each degree celsius from -15°C to +60°C. The routine would simply step through the table until the ADC reading exceeded ( negative temperature coefficient (NTC) device. This means as the temperature increases the resistance drops. In our electrical configuration of the sensor this also applies to the voltage seen at the ADC input.>) the value in the table. The position in the table is then given as the temperature in degrees celsius.

When first developing the data logger in 2004 and 2005 I quickly wrote an interpolation routine that would extend the integer value given in the original with an extra two decimal places. After finishing writing the code I reviewed both the code and the corresponding output. At the time of that brief review everything appeared to make sense. As the original project had other more accurate temperature sensors available, the operation of the onboard temperature sensor was not critical. The onboard temperature sensor was instead intended as a fast secondary sensor for live feedback to the user via the LCD and so the code was not widely tested. Whoops.

This December (being the coldest on record in the UK for the last 100 years) showed up some of the flaws in my code. After some prompting from a couple of users of the project I set aside some time to investigate further. My investigation involved writing a test routine to simulate all possible sensor readings and then evaluating the results. This was an incredibly simple exercise, just a few lines of code and then plotting the results with Gnuplot. The output from the function is shown below in Figure 1.

Figure 1: Thermistor routines test output

This figure shows the full-scale results for the original and fixed routines along with the data points given in the original source. As you can see the interpolated line appears to follow the datapoints quite nicely until it drops below zero degrees.

At this point the linear interpolation is way-off and producing mostly garbage (you can see this clearly in the close-up below in Figure 2)

Figure 2: Close-up of the thermistor routines test output

Looking at the close-up in Figure 2 you can see the awful job the routine does when dealing with negative temperatures. If you look very closely at the positive temperatures you will also note a small error in that calculation as well. After plotting the output and going over the code it was very easy to spot the couple of errors. After the corrections the graph of the output (The green line in Figures 1 and 2) intersects perfectly with the data points given in the source code. If it were not for visual inspection of the results (via the graph) I don’t think I would have noticed this second error at all.

Further changes were also made to the routine so that when the readings are outside the range specified by the data table then they will record an over or under temperature error. This is signified in the log as +++.++ or ---.-- for over temperature or under temperature respectively. On the LCD, +++.+C or ---.-C is displayed. Previously the routine had just returned the maximum or minimum temperature from the table.

It had been at the back of my mind that there was something not quite right about that bit of code for some time. It was so simple in the end to write a routine to test the function with all possible inputs that I could kick myself for not doing it earlier. By plotting the results on a graph it became obvious how the code was misbehaving. The key lessons I’ve learned from this it is that a little testing goes a long way and that some simple visualisation can really help you understand a bug quickly.

More information about the AVR Butterfly Logger project can be found at the project website.

Any questions can be asked in the project forums.

Butterfly Logger Firmware 0.30

Monday, October 25th, 2010

In an unprecedented move I’ve released another version of the firmware for the AVR Butterfly Logger within a few months of a previous release. This was a minor change just adding MAX6675 based J and K thermocouple support.

You can download the source code from sourceforge, with the changes from the previous version are listed below.

Changes summary:
- MAX6675 K-Type Thermocouple logging (adjustable averaging)
- MAX6675 available on 'T' via the USART.
- MAX6675 available on LCD via 'THERMOCOUPLE' Menu. 

The new library has configurable support for averaging readings but each individual reading takes 200ms so an 8 point average will take 1.6s to complete. If you use this averaging feature please make sure that your logging interval is long enough to ensure enough time for the complete reading to finish. While thermocouples are capable of measuring wide ranges of temperature both positive and negative the MAX6675 is limited to returning readings between 0°C and +1024°C. If the chip detects an open thermocouple then the system will log ‘-1’ as the temperature.

The MAX6675 connects to the AVR Butterfly via the SPI bus. This bus is also used by the onboard Dataflash and the Kionix acceleration sensors (when in use). As this bus is already available the only additional pins required is a single chip select (CS). The library currently only supports a single thermocouple input but the code could easily be expanded to suport multiple MAX6675s requiring a single CS line for each additional chip and thermocouple.

The SPI bus is available on PORTB and also via the ISP connector. An example breakout PCB for the MAX6675 was published here. Default wiring from the this PCB to the AVR Butterfly Logger is given below:

PCB MAX6675 Butterfly J403 (ISP) J405 (USI)
1 1 GND GND 6 4
2 7 MISO PB3 1
3 6 CS PE6 3
4 5 SCK PB1 3
5 4 VCC VCC 2

More information about the data logger project can be found at the project website.

Any questions can be asked in the project forums.

Butterfly Logger Firmware 0.29

Sunday, September 19th, 2010

I’ve finally managed to get another release of the Butterfly Logger firmware done. I was supposed to have released a lot of this a couple of years ago but it got lost in the ether somewhere.

Without any further ado the source code is here, with the changes from the previous version are listed below.

Changes summary:
- Added  'j' and 'J' commands to read SHT75 Humidity and Temp.
- Fixed the SHT75 clock waveform (now it looks  symetrical )
- Added ENABLE_SHT_CALCS option to print/store real world values for SHT75
- Tuned vref for more accurate results. (Battery voltage and dependant SHT75 calculations)
- Added Kionix accelerometer logging
- Fixed vref ( done by matthias.weisser )
- Make file uses no-inline-small-functions ( done by matthias.weisser )
- Direction ADCs are now refered to as Auxilliary ADCs

More information about the data logger project can be found at the project website.

Any questions can be asked in the project forums, here.

Using thermocouples with the Reprap Mendel

Monday, September 6th, 2010

Using thermocouples with the Reprap Mendel is pretty straight forward, although I did overcome a couple of issues in getting it working.

A known issue with the MAX6675 Arduino/Reprap code

The MAX6675 chip can take up to 220ms to take a reading from the thermocouple and the reading begins automatically after the previous read. If another read is attempted before the current reading has been completed then the chip returns the previous value and restartsthe internal analog to digital the conversion. This means that if you read from the chip every 150ms or so you will always get the same first reading returned again and again.

The arduino library code needs a 220ms delay between multiple reads to allow for the MAX6675 chip to take another reading. In the current state the library simply reads the same value repeatedly. Luckily it is a known issue documented on the project’s site and is not hard to fix.

The taking of a single reading rather than taking a number of readings and averaging will contribute to a high noise level in the thermocouple readings when using this software.

Discovering the problem

When testing the PCB with the library code everything works fine. The arduino returns a reading every second and everything looks good.

When using the Reprap firmware with the thermocouple code it always returned the first value read from the chip (usually 24°C or so). This lead to things getting pretty hot as the extruder would turn the heater to full duty cycle while recording no change in temperature.

I first looked at the SPI lines on the scope to see if there was excessive noise or some other issue. I also compared this side by side with the ‘working’ arduino system. The scope traces are shown below. The reference channels (R1 and R2) are the reprap firmware and the bottom traces (1 and 2) are the arduino. I noticed nothing wrong with the traces, they both looked very good.

Reprap code compared to aduino library code communicating to the MAX6675.

It was here however that I noticed that the arduino code calls another read to the chip immediately after the preceding read. After a bit more investigation I noticed that the library software calls the temperature reading function 5 times consecutively and always returns 5 identical results. A quick read of the data sheet confirmed the typical conversion time to be 170ms with a maximum of 220ms. The arduino library starts a conversion just 20μs after finishing reading the last. Reading the data sheet further also confirmed the action of restarting a conversion every time the chip is read.

I monitored the lines on a scope and found that the Reprap software was indeed reading the chip every 136ms. To fix this I simply changed SLOW_CLOCK in configuration.h to 8089 from 5000. Based on the current delay giving 136ms, this gives 220ms delay between calls to the thermocouple chip. After making this change to the firmware the thermocouple now works perfectly.

Time between calls to the MAX6675 thermocouple chip using the standard firmware on my reprap.

Fixing the problem

A more robust fix to the reprap extruder firmware is shown below, but I have not tested this code yet. This simply exits the temperature reading method if enough time has not elapsed between calls to the function. The advantage of this is that the main loop can run as fast as it needs to without impacting on the operation of the thermocouple sensors.

#ifdef MAX6675_THERMOCOUPLE
#define MAX6675_CONVERSION_TIME 220 // number of milliseconds between valid temperature readings

  int value = 0;
  byte error_tc;
  
  static unsigned long last_read_time=0; // number of milliseconds since power on at last MAX6675 read 
  unsigned long time_since_last_read=0;  // number of milliseconds since last read of MAX6675 

  /* check the chip is ready to produce a new sample */
  time_since_last_read=millis()-last_read_time;

  /* only read from the chip if a conversion has been completed */
  if(time_since_last_read > MAX6675_CONVERSION_TIME){
    
    digitalWrite(TC_0, 0); // Enable device
  
    /* Cycle the clock for dummy bit 15 */
    digitalWrite(SCK,1);
    digitalWrite(SCK,0);
  
    /* Read bits 14-3 from MAX6675 for the Temp
     	 Loop for each bit reading the value 
     */
    for (int i=11; i>=0; i--)
    {
      digitalWrite(SCK,1);  // Set Clock to HIGH
      value += digitalRead(SO) << i;  // Read data and add it to our variable
      digitalWrite(SCK,0);  // Set Clock to LOW
    }
  
    /* Read the TC Input inp to check for TC Errors */
    digitalWrite(SCK,1); // Set Clock to HIGH
    error_tc = digitalRead(SO); // Read data
    digitalWrite(SCK,0);  // Set Clock to LOW
  
    digitalWrite(TC_0, 1); //Disable Device
  
    last_read_time = millis(); //remember the read time for next time
  
    if(error_tc)
      currentTemperature = 2000;
    else
      currentTemperature = value>>2;
  }
#endif

}

The arduino library can be improved by simply inserting the following at line 55 of the file MAX6675.cpp just before the end of the for loop.

if (i>1) delay(220); // Wait 220ms for next sample to be ready

This will force a 220ms delay when reading the thermocouple multiple times via the read_temp(int samples) function. Alternatively you could use the millis() function to track the last time it was read and automatically delay the read by the appropriate amount.

How to fix LCD contrast problems on the AVR Butterfly

Thursday, June 17th, 2010

I had heard a number of reports of contrast problems on the AVR Butterflies produced over the last year. Unfortunately I haven’t had an opportunity to look into this until now. This week while checking some MP3 player code on a new AVR Butterfly I ran into the problem for myself. Luckily it was a simple fix.

A quick search on AVR Freaks turned up the following solution.

Turns out adding the following to lcd_driver.c fixed the problem:

//updated 2006-10-10, setting LCD drive time to 1150us in FW rev 07,
//instead of previous 300us in FW rev 06. Due to some variations on the LCD
//glass provided to the AVR Butterfly production.
LCDCCR |= (1<<LCDDC2) | (1<<LCDDC1) | (1<<LCDDC0);

I inserted these lines as instructed into the file LCD_driver.c at the end of the LCD_Init() function just after gLCD_Update_Required = FALSE; (this is around line 170 for ButterflyMP3).

The change has fixed the problem on my hardware. Although I have updated the files in the CVS, I have not done a general release for either ButterflyMP3 or Butterfly Logger. You can get the modified files directly from the CVS system, here for ButterflyMP3 and here for Butterfly Logger

In addition to the changes given above in the post at AVR Freaks, the macro around line 45 in the file LCD_driver.h

#define LCD_CONTRAST_LEVEL(level) LCDCCR=(0x0F & level)

should be changed to something like

#define LCD_CONTRAST_LEVEL(level) LCDCCR=(0xF0 & LCDCCR) | (0x0F & level)

This change will ensure that the drive time setting does not get erased when using the macro to set the contrast at some time after initialisation. I have not tested this as none of my firmware sets the contrast after startup.

Why are these changes needed?

It appears that the LCD’s characteristics have changed and it now requires a much longer drive time of 1150μs. The original LCD needed only 330μs. I’m unsure of the effect of this on the overall power consumption other than it will increase the LCD’s portion of it.

If the original 330μs drive time is used, then some of the newer LCDs will be very dim and you may only be able to read them at an angle if at all. If you really want to squeeze a little more battery life out of your AVR Butterfly based project then you could have a go at tweaking this value back to 850μs or even 575μs and check the display for readability.

Heres an explanation of what those bits do from the ATMEGA169PA data sheet on page 248.

• Bits 7:5 – LCDDC2:0: LDC Display Configuration
The LCDDC2:0 bits determine the amount of time the LCD drivers are turned on for each voltage transition on segment and common pins. A short drive time will lead to lower power consumption, but displays with high internal resistance may need longer drive time to achieve satisfactory contrast. Note that the drive time will never be longer than one half prescaled LCD clock period, even if the selected drive time is longer. When using static bias or blanking, drive time will always be one half prescaled LCD clock period.

And the accompanying table from the same datasheet also on page 248.

Table 23-7. LCD Display Configuration

LCDDC2LCDDC1LCDDC0Nominal drive time
000300 µs
00170 µs
010150 µs
011450 µs
100575 µs
101850 µs
1101150 µs
11150% of clkLCD_PS