Secondary micro-controller

First, the micro-controller was used to second the Raspberry Pi as piloting the WS2812-like LEDs and acquiring some analog values (potentiometer). Finally I decided to give him all the polling tasks (pilot the TM1638s, pilot the RGB Leds, the TM1637s, and probably a matrix keyboard) the Raspberry cannot perform (ok, I know the raspberry can control the LED through the SPI, if programing is well done, since the timing are very tight).

Choice of the micro-controller

I just needed a SPI connection with the Raspberry Pi, few ADCs (to “read” the potentiometer), and a lot of IOs.

I have listed all the micro-controllers I had in stock, and I’ve chose the ATtiny88, because, among all the micro-controllers I have in stock, this one has enough IOs, run fast enough (8MHz without external quartz, up to 10MHz) and has enough RAM/Flash (I am sure that 1Kb is enough, but I wanted some extra space, “just in case”).

Of course, any AVR or PIC (or any other micro-controller) you have in stock, or any Arduino-based board would be ok (At first, I choosed a ATtiny631, but finally, I moved the connection to the TM1638 and TM1637 boards from the RPi to the micro-controller, and the 631 did not have enough IOs).

As a bonus, I can directly program it through the Raspberry Pi, through the SPI (and that’s very convenient!).

Program it through the Raspberry Pi

This is done following these two tutorials (here and here).

The idea is to use the SPI (SPI_MISO, SPI_MOSI and SPI_CLK) of the raspberry Pi, plus one IO (for the reset) to program the AVR using its In-System Programmable feature. And the SPI will also be used to connect the two devices.

1. install avrdure on the Raspberry

sudo apt-get install avrdude

1. configure it according to our IO setting. Add (or uncomment) the file /etc/avrdude.conf as:

programmer
  id    = "linuxgpio";
  desc  = "Use the Linux sysfs interface to bitbang GPIO lines";
  type  = "linuxgpio";
  reset = 25;
  sck   = 11;
  mosi  = 10;
  miso  = 9;
;

You can choose any other IO for the reset (#25 was arbitrary choice).

1. test if everything works with

sudo avrdude -c linuxgpio -p attiny88 -v

And you should have something like this

avrdude: Version 6.1, compiled on Jul  7 2015 at 10:29:47
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "/etc/avrdude.conf"
         User configuration file is "/root/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : unknown
         Using Programmer              : linuxgpio
         AVR Part                      : ATtiny88
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no         64    4      0  3600  3600 0xff 0xff
           flash         65     6    64    0 yes      8192   64    128  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

         Programmer Type : linuxgpio
         Description     : Use the Linux sysfs interface to bitbang GPIO lines
         Pin assignment  : /sys/class/gpio/gpio{n}
           RESET   =  25
           SCK     =  11
           MOSI    =  10
           MISO    =  9

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e9311
avrdude: safemode: lfuse reads as 6E
avrdude: safemode: hfuse reads as DF
avrdude: safemode: efuse reads as 7

avrdude: safemode: lfuse reads as 6E
avrdude: safemode: hfuse reads as DF
avrdude: safemode: efuse reads as 7
avrdude: safemode: Fuses OK (E:07, H:DF, L:6E)

avrdude done.  Thank you.

(I had to check with another IO conf to be sure that it was working because I couldn’t believe it worked on the 1st attempt !)

1. Configure the fuses of the ATTiny (in my case, I want to use the 8MHz intern oscillator, without scaling, so the fuses are 0xE2DFFF) Go to http://eleccelerator.com/fusecalc/fusecalc.php?chip=attiny88 (or equivalent) to compute your fuses, and use avrdude:

   sudo avrdude -c linuxgpio -p attiny88 -U lfuse:w:0xEE:m 	-U hfuse:w:0xDF:m   
   

Eventually, if you have the error

Can't export GPIO 25, already exported/busy?: Device or resource busy
avrdude done.  Thank you.

You can add 25 (or the corresponding GPIO pin)in the file /sys/class/gpio/unexport with:

echo 25 > /sys/class/gpio/unexport

1. write the code, compile it and send it with avrdude to the ATtiny !!

avr-gcc -c -mmcu=attiny88 toto.c -DF_CPU=8000000UL -Os
avr-gcc -mmcu=attiny88 -o toto.elf toto.o
avr-objcopy -j .text -j .data -O ihex toto.elf toto.hex

(there is a dedicated Makefile for this in the src/AVR/ folder)

1. Warning ! After each use of avrdude (of the SPI bus), I need to reinitalize the SPI with

   sudo rmmod spi_bcm2835
   sudo modprobe spi_bcm2835
   

   this is already included in the Makefile provided.

Polling and timing

The ATtiny need to do some polling to detect some changes in the switches/potentiometers, through the TM1638 boards. I also wanted to make the RGB LEDs (and the LED displays?) blinking.

For that purpose, I need to prepare some periodic tasks:

• polling of the analog inputs (potentiometers)
• polling to get data from the TM1638
• make the RGB LED blink

I need to read the inputs (switches, not push button) at leat 10 times a second (rought approximation), and each “task” take between few cycles (run the ADC) and less than a millisecond (pilot the RGB leds or talk with the TMs). So I have decided to set a period of 3.90625ms (1/256 of a second), and to count in which cycle we are (with the 8-bit variable ncycle):

• when (ncycle&3) == 0: (every 15.625ms) acquire ADC3, read 8TM1 (1st TM1638)
• when (ncycle&3) == 1: (every 15.625ms) acquire ADC4, read 8TM2 (2nd TM1638)
• when (ncycle&3) == 2: (every 15.625ms) acquire ADC5, read 8TM3 (3rd TM1638)
• when (ncycle&3) == 3: (every 15.625ms) acquire ADC2 (and check if the switches connected to ADC2 have changed), read 8TM4 (4th TM1638)
• when (ncyccle&63) == 63: (every 250ms) update the RGB leds if necessary (according to blinking)

So TIME1 is configured to generate interruption every 31250 ticks, with a prescaler of 1/1 (Frequency of the ATtiny: 8MHz, see this AVR timer calculator):

	TCCR1B = (1U<<CS10) | (1U<<WGM12);     /* prescaler = 1/1 and Clear Timer on Compare match (CTC) T*/
	TCCR1C = (1U<<FOC1A);    /* channel A */
	OCR1A = 31250;              /* 31250 ticks @ 1MHz -> 3.90625ms (1/256 s) */
	TIMSK1 = (1U<<OCIE1A);      /* set interrupt on Compare channel A */

For the RGB Leds, we need another variable blinkCycle to count the blink cycle (the 2 MSB of ncycle required to get 250ms are not enough). Four bits is enough, it gives 16 different cycles. So, for each RGB Led, a 16-bit word encodes the blinking scheme (each bit indicates if the LED is on/off in the corresponding blinking cycle). See the APA106 documentation for the blinking details (how to encode blink pattern, how to compute when to send data, and when it is not necessary, etc.)

Communication between RPi and micro-controller (protocol)

The communication between the Raspberry Pi is “simple”. It is done through the SPI, plus one line (GPIO 24 of the Raspberry Pi and PC0 of the ATtiny) that is used by the micro-controller when it wants to send some data (IRQ-like).

RPi -> ATtiny

The RPi send a first byte, that indicates the command. This command may be followed by some data (up to 8 bytes). The following table sums up the commands:

ATtiny -> RPi

When the ATtiny wants to tell something (ie “some button has changed”), it first initiates the communication by putting RPi_IO24 up (and then down). Then, it can send its information (when the RPi send then No operation command (byte 0)). The ATtiny first send a 1-byte header, and then the data (0, 1 or 2 bytes ). The following table sums up the header:

Example:

• 0b10000000 when the RPi must shut down
• 0b00011010 when the TMx8 index 2 has changed (1 byte will follow: the corresponding data)
• 0b00010100 when the Potentionmeter index 0 has changed (1 byte will follow: the value of this potentiometer)
• 0b00111101 when the Potentiometer index 1 and the TMx8 index 1 has changed (so 2 bytes will follow, first the TMx8 data and then the potentiometer)