Arduino at 11.0592MHz. Modify Timer0, delayMicroseconds()

Question

I am using the ATMega328P at 11.0592MHz with the Arduino environment. I recognized that the delayMicroseconds() function is about 27% too fast. The reason is, that the code in wiring.c assumes that the clock peed is now 8MHz.

Now I try to fix it. I found different posts but I am not sure what is the most Arduino compatible way. What can you recommend?

1. Multiply the variable "us" in wiring.c with 1.27 if the F_CPU is 11.0592MHz? Easy but only affects delayMicroseconds and not millis(), micros(), delay(), etc.
2. Change the prescale factor maybe from 64 to ???

Other ideas or guides?

Thank you in advance.

Felix

Answer 1

The Arduino way would be to make a custom PLATFORM for your board with that of the new F_CPU speed. The core libraries should carry this F_CPU through.

This is easy enough, by making a boards.txt file with your differences. Where the location of the file (same between the two) is different between the two current IDE's. In the case of Arduino IDE 1.0.5:

C:\Users\mflaga\Documents\Arduino\hardware\myArduino11MgHz\boards.txt

and in the case of 1.5.5:

C:\Users\mflaga\Documents\Arduino\hardware\myArduino11MgHz\avr\boards.txt

Where in your case the sketch directory would be different.

# See: http://code.google.com/p/arduino/wiki/Platforms

##############################################################

myArduino11MgHz.name=Arduino 11MgHz

myArduino11MgHz.vid.0=0x2341
myArduino11MgHz.pid.0=0x0043
myArduino11MgHz.vid.1=0x2341
myArduino11MgHz.pid.1=0x0001

myArduino11MgHz.upload.tool=avrdude
myArduino11MgHz.upload.protocol=arduino
myArduino11MgHz.upload.maximum_size=32256
myArduino11MgHz.upload.maximum_data_size=2048
myArduino11MgHz.upload.speed=115200

myArduino11MgHz.bootloader.tool=avrdude
myArduino11MgHz.bootloader.low_fuses=0xFF
myArduino11MgHz.bootloader.high_fuses=0xDE
myArduino11MgHz.bootloader.extended_fuses=0x05
myArduino11MgHz.bootloader.unlock_bits=0x3F
myArduino11MgHz.bootloader.lock_bits=0x0F
myArduino11MgHz.bootloader.file=optiboot/optiboot_atmega328.hex

myArduino11MgHz.build.mcu=atmega328p
myArduino11MgHz.build.f_cpu=11059200L
myArduino11MgHz.build.board=AVR_myArduino11MgHz
myArduino11MgHz.build.core=arduino:arduino
myArduino11MgHz.build.variant=arduino:standard

##############################################################

Here is a 3rd party GUI Editor of the board.txt.

Disclaimer. The above does compile and should work fine. Where I have not actually tested and loaded into a unit.

---

That said, I would expect the bootloader's F_CPU not to match. There are three possible solutions to this.

First; the optiboot loader should have a corresponding target with the below deviation:

\arduino-1.5.5\hardware\arduino\avr\bootloaders\optiboot\Makefile

myArduino11MgHz: AVR_FREQ = 11059200L

Second; Change the boards.txt upload speed to match the change of =115200*(11059200/16000000).

myArduino11MgHz.upload.speed=79626

This being a non typical baud rate, either avrdude or the serial port may support it.

Third; Don't use the bootload, by uploading via the ISP programmer.

---

I have likewise planned to make a board with a different F_CPU, but only actually made custom boards, with the same speed. I would be curious to know if the above actually works, in your case.