Thursday, September 25, 2014

Using the Seeed CAN-BUS shield with an Arduino Mega

The Seeed Studio CAN-BUS shield is designed specifically to be used with an Arduino Uno but with a simple modification you can use it with several other Arduino boards. I sourced this info from a few different forums and thought I would write up a complete post on exactly how to do this. The Seeed website had some info on how to use the shield with a Mega but their info was not complete and did not work for me. In this post I am going to show how to use the Seeed CAN-BUS shield with an Arduino Mega 2560.

The CAN-BUS shield incompatibility with Arduino boards other than the Uno arises from where Seeed chose to access the SPI pins. They used pins 11,12,13 to access SPI on the Uno. On the Mega the SPI pins are 50,51,52. A better design would have been to use the SPI pins on the ICSP header which is consistent on the Uno, Mega, Due and Leonardo but such is life and we have to work with what is available right now. Seeed's site does mention they are working on a 1.1 version of the board that moves SPI to the ICSP header.

Here is an Arduino Uno with the SPI pins labeled
Arduino Uno SPI pins

And here is an Arduino Mega 2560 with the SPI pins labeled
Arduino Mega 2560 SPI pins

Here is what the Seeed Studio CAN-BUS Shield looks like if you don't already own one
Seeed Studio CAN-BUS Shield v1.0

The mod

Here is how to modify the CAN-BUS Shield to work with a Mega. The basic overview of the mod is that we need to change the SPI pins that the CAN-BUS Shield is using. There are two ways to do this modification. One one is reversible and the other is more permanent. First the reversible method.

The reversible method is to bend three pins on the CAN-BUS Shield like this:

When you plug the shield into the Mega it will look like this:

With bent pins I was able to bend them back use the shield with an Uno again.

Now you need to use three jumper wires to connect the SPI pins on the shield to the SPI pins on the Arduino board. In my example I connected the wires to pins 50,51,52 on the Arduino because I only had male-male jumper wires. If you have male-female jumper wires you can connect to the ICSP header instead.

Jumper wire connections
Mega Pin Shield Pin SPI Desc
50 12 MISO
51 11 MOSI
52 13 SCK

Here is what it looks like with the jumper wires installed

I would recommend you use coryjfowler's MCP2515 library instead of the Seeed's because his fork of the library has a configureable SS pin but Seeed's version will work with this mod because they hard coded the SS pin as digital pin 10.

I then loaded up one of the examples and the shield started up correctly.

If you want to make this modification more permanent you can simply snip off the three pins instead of bending them.

I found some male-female jumper wires at Fry's Electronics yesterday. Here is what it looks like when using the ICSP headers

ICSP Pin Shield Pin SPI Desc
1 12 MISO
4 11 MOSI
3 13 SCK


[Updated 2014-09-29] - Added photos of shield with the pins snipped off. Also added photo showing the male-female jumpers with the ICSP header.

[Updated 2014-10-23] - Added table with pinouts for connecting the shield to the ICSP header.

Friday, September 19, 2014

Trouble installing PyCrypto on Mac OSX 10.9

This afternoon I was trying to install the PyCrypto Python library on my Mac running OSX 10.9.4 with the built in Python. To install the library I was using the command 'sudo pip install pycrypto' but it kept failing when trying to compile parts of the library with the C compiler. This is the error I was getting:

checking for gcc... gcc

checking whether the C compiler works... no

configure: error: in `/private/tmp/pip_build_root/pycrypto':

configure: error: C compiler cannot create executables

See `config.log' for more details

Grrr... I searched around and read a bunch of stackoverflow questions but it seemed like I had everything in place that I needed to compile this library. Xcode was installed, I have the Command Line Tools installed, I have a recent version of pip installed. Why won't the C compiler work??

Well the answer turned out to be something really dumb. I recently updated Xcode but I had not launched the Xcode application since it was updated. Turns out the command line C compiler won't compile anything until you accept the license agreement for the updated Xcode. I opened the Xcode gui and clicked Accept. After that I was able to install PyCrypto. I also found you can accept the agreement from the command line using the command 'sudo xcodebuild -license' but you have to page through all the text and type agree.

Friday, September 5, 2014

Experimenting with BeagleBone Black and a userspace TFT library

This post is part of a series of posts I have done about the Adafruit 2.2" TFT display. You can check out those previous posts here and here. In those previous posts I was using this display with an Arduino and ran into issues with the refresh rate. To get a decent refresh rate I had to do some extra coding to determine what data needed updating and then only redraw the sections of the screen that actually needed to change. In this post I am going to take a look at using this display with the BeagleBone Black single board computer. My hope going into this is that the CPU horsepower of the BBB will allow quicker screen redraws which would eliminate the need to selectively redraw sections of the screen.

Since the BBB is a full computer there is a Linux operating system sitting between the code and the hardware. This quite a bit different from an Arduino where you have direct access to the hardware. My BBB came with Angstrom Linux on it but I am more familiar with Ubuntu and the Adafruit tutorials were written assuming you have Ubuntu installed. So I swapped the OS to Ubuntu 14.04 following Adafruit's tutorial. To control the TFT display with a BBB Adafruit created a user space library in Python. To install the library and wire up the display I followed the instructions on The instructions were written for the 2.8 inch display but they worked just fine for my 2.2 inch display. I think these instructions would probably work for most ILI9341 based TFT displays.

The wiring for the 2.2" display is slightly different than the 2.8" display. Here is how I wired it up.

  • BeagleBone Black SCLK pin P9_22 to TFT SCK.
  • BeagleBone Black MOSI pin P9_18 to TFT MOSI.
  • BeagleBone Black CE0 pin P9_17 to TFT CS.
  • BeagleBone Black pin P9_12 to TFT RST.
  • BeagleBone Black pin P9_15 to TFT D/C.
  • BeagleBone Black 3.3V power pin P9_3 to TFT Vin.
  • BeagleBone Black ground pin P9_2 to TFT GND.

Then I ran the sample application to make sure everything worked correctly

Now that we have all that stuff out of the way let's get to the real point of this post, how fast can the BBB update this display! To test it I took one of Adafruit's example scripts that came with the library and modified it so it read the value of an analog pin (P9_40) and then display that value on the TFT screen with a custom font. I connected a 10k pot to the analog pin so I could adjust the value.

The first time I ran the test script I ran into problem with the bottom the text being clipped off. Turns out it was a bug with a Python library. I detailed the fix for that here.

In the script I also recorded a time stamp before drawing the TFT screen and immediately after to measure how long it took to actually draw the screen.

Here is a video of the testing

If you made it to the end of the video you saw that it was taking approximately 0.6 seconds for the draw command to complete but the screen itself appears to update nearly instantly. This is a huge improvement compared to when I tested this display with an Arduino. On the Arduino you could see the screen slowly painting the image onto the screen.

Here is a video of the same exact test with an Arduino:

I'm not sure what is taking 0.6 seconds with the BBB. The screen redraws seem to happen faster than that. Maybe it's the data transfer over SPI plus the screen redraw (Update: See this post to learn how to speed up the redraws). In any case I am quite pleased with it. Another advantage of using the BBB is I'm able to use any TrueType font to display text. The library on the Arduino only had one font which became very blocky when using large characters. As you saw in the first video I used an Arial font which looked very smooth.

I originally wanted to use this TFT display for my speedometer project but abandoned it for a 7 segment display because the refresh rate was so slow with the Arduino. Based on these BBB results I think I'm going to make a v2.0 digital speedometer using both an Arduino and a BBB. The BBB isn't great at doing real time things like counting pulses so I think I will continue to use the Arduino to count the VSS pulses and then feed the speed data into the BBB over I2C or CAN. I'm already brainstorming other data to display on the screen...

Oh here is the python code I used to test this display if you are interested:

Helpful Links

[Updated 2014-09-05]
This is in reference to my statement about about the BBB not being great at real time tasks. I've been doing some reading and the BBB does have a hardware counter on it but it looks like you have to be a NASA engineer to get it to work. This post discusses the PRU on the BBB. When he started talking about having to write Assembly code I tuned out. I'll just use an Arduino to do the accurate pulse counting, thanks.

[Updated 2014-10-04]
Added Fritzing wiring diagram and pin outs.

[Updated 2014-10-08]
Added link to post that shows how to speed up display redraws.