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ATTiny co-processor on CM Panel POE

This article explains in detail the functions performed by the Microchip ATTiny88 co-processor present on the CM-Panel POE

Microchip ATTiny88 in an AVR RISC-based CMOS 8-bit microcontroller with:

  • 8 KB ISP Flash memory
  • 6 4B EEPROM
  • 512B SRAM
  • 28 general purpose I/O lines

More info are available on the Microchip ATTiny88 page at

This processor is programmed in-factory to perform the following functions:

  • Measure each 1s the on-board voltages and display a report on the serial debug port
  • Manage the correct shutdown in case of power failure to avoid the microSD curruption
  • Indicate the state of charge of the supercap using the RGB led
  • Manage the state of display power supply circuits
  • Send a signal via a GPIO line to shutdown Linux

The firmware used is this:

This article explains how to compile it and flash the ATTiny with your own version.

Signals wired to the co-processor

These are the GPIO lines used by the ATTiny and their respective purposes:

PA0 ADC MeasBacklit
PA2 Main5V
PA3 Flag4
PB0 Flag1 N.C.
PB1 Flag2 N.C.
PB2 Flag3 N.C.
PB6 OUT Disp On Display ON High 27
PB7 OUT KB RGB led blue Low
PC0 ADC Meas1V8
PC1 ADC Meas3V3
PC2 ADC Meas5VExt
PC3 ADC Meas5VInt
PC4 micro_SDA
PC5 micro_SCL
PC7 IN Touch_INT Low
PD0 OUT GOSHDN Shutdown command to CMx module Low 25
PD1 IN SHDNDONE Shutdown done by CMx module High 26
PD2 IN INT1# Interrupt from RTC Low
PD3 IN Pushbutton P2 Button (Start/stop) Low
PD4 OUT KG RGB led green Low
PD5 OUT KR RGB led red Low
PD6 OUT Txdata Debug port TX
PD7 IN Rxdata Debug port RX

Install the Arduino IDE to program the ATTiny88 co-processor

Download and install the Arduino IDE from the official Arduino website. We used Legacy IDE 1.8.19 to write this article.

add the ATtiny support to the Arduino IDE ny navigating to Arduino > Preferences... > Additional Board Manager URLs: and add the following URL for the ATtiny core by Spence Konde (DrAzzy):

Click "OK" to close the Preferences window.

Open the Board Manager by navigating to Tools > Board > Boards Manager....

In the Boards Manager window search for ATtiny and select ATTinyCode by Spence Konde then press then .

Now navigate to Tools > Boards > ATTinyCore > ATtiny48/88 (Optiboot)

Configure the processor settings

  • Tools > Chip > ATTiny88
  • Tools > Clock Source > 4MHz (Internal)

Copy inside the editor this sketch:

then select the menu Sketch > Verify/Compile

Sketch uses 4342 bytes (53%) of program storage space. Maximum is 8192 bytes.
Global variables use 292 bytes (57%) of dynamic memory, leaving 220 bytes for local variables. Maximum is 512 bytes.

then Sketch > Export compiled Binary.

then Sketch > Show sketch folder and check that the file pwr.ino.hex exist.

If you have a Tiny USB programmer you code inject the code directly by hardware to the co-processor using the J4 Prog port on CM-Panel POE.

If not transfer the file pwr.ino.hex to the CM-Panel POE microSH using SSH and program the co-processor using avrdude.

How to inject the code inside the ATTiny88 co-processor

There are some ways to transfer the binary code inside the ATTiny flash memory:

  1. Using the Tiny USB cheap ISP programmer and Arduino IDE
  2. Using the Tiny USB cheap ISP programmer and the CM-Panel itself
  3. Using an USB to serial converter and the Arduino IDE
  4. Using an USB to serial converter and the CM-Panel itself

Methods 3 and 4 are possible because the ATTiny become already programmed with the ATTiny firmware + Optiboot loader
Methods 1 and 2 works also with a fully blank ATTiny

In case of ISP using the CM-Panel itself install avrdude by typing:

$ sudo apt update
$ sudo apt install avrdude

Download the avrdude configuration file from our site by typing:

$ wget   

Download the ATTiny firmware from our site by typing:

$ wget

Insert the Tiny USB programmer between one of the USB port of CM-Panel and the J4 PROG port and type:

$ sudo avrdude -Cavrdude.conf -v -pattiny88 -cusbtiny -e -Uefuse:w:0xfe:m -Uhfuse:w:0b11010111:m -Ulfuse:w:0xE2:m -Uflash:w:pwr.ino.hex:i

or insert an USB to serial adapted between an USB port on CM-Panel POE and the ATTiny debug port available on EXP2 header, open the PROG BYPASS jumper, then type:

$ sudo avrdude -Cavrdude.conf -v -pattiny88 -carduino -P/dev/ttyUSB0 -b19200 -D -Uflash:w:pwr.ino.hex:i

and reset the ATTiny by pressing the reset button near the supercap (Jumpers, headers and push buttons on CM-Panel POE).

ATTiny debug port

The coprocessor has a debug serial port whose signals are exposed on the EXP2 connector as shown in the figure.
These signals are at 3.3 volts with the following serial parameters: 19200 baud,N,8,1 no flow control.


Get the stand alone optiboot binary file:

$ wget

Programming just the Optiboot bootloader width Atmel ICE

$ sudo avrdude -Cavrdude.conf -v -pattiny88 -catmelice_isp -Pusb -e -Uefuse:w:0xfe:m -Uhfuse:w:0b11010111:m -Ulfuse:w:0xE2:m -Uflash:w:optiboot_attiny88_8000000L.hex:i 

Programming just the bootloader with USBTiny

$ sudo avrdude -Cavrdude.conf -v -pattiny88 -cusbtiny -e -Uefuse:w:0xfe:m -Uhfuse:w:0b11010111:m -Ulfuse:w:0xE2:m -Uflash:w:optiboot_attiny88_8000000L.hex:i 



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