Program ATTiny85 running at 16 KILOhertz

Question

I set the fuses of my attiny85 so it uses the internal 128khz clock, prescaled so it is 16kHz. I set the CKOUT fuse too so I can check the clock using an oscilloscope on pin 3 and yes it is 16kHz.

Now my problem is I have no option in the Arduino IDE to compile my code for 16kHz clock speed. The slowest I have is 128khz, which causes the "blink" sketch to run for 8 seconds up, 8 seconds low on my attiny instead of 1 second.

I tried with "attiny" from David A. Mellis using https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json (Slowest : 1Mhz) and "ATTinyCore" from Spence Konde using http://drazzy.com/package_drazzy.com_index.json (Slowest : 128khz).

Does someone know how I can set my Arduino IDE to compile the code for an attiny85 using the 16khz internal clock ?

(Please, no question "why use a 16khz clock ?". The point is, this chip may be set to 16khz clock, so it should be possible to program it to use such clock speed)

Thank you.

Answer 1

The Arduino core you've referenced (and pretty much all other AVR Arduino cores too) specify the clock frequency that a particular selected board is running at in the boards.txt file. The platform.txt file then retrieves that value and creates the F_CPU macro with the clock frequency in hertz as a value.

For the ATTinyCore by Spence Konde, the ATTiny85 board definition is the attinyx5 section. There, you can either add a new clock selection menu item or modify the settings of an existing one, e.g.

https://github.com/SpenceKonde/ATTinyCore/blob/f5eabc8fb97e8fd72f1782a5e97345729e42ec47/avr/boards.txt#L409-L412

attinyx5.menu.clock.128internal=128 kHz (internal WDT)
attinyx5.menu.clock.128internal.bootloader.low_fuses=0xC4
attinyx5.menu.clock.128internal.build.f_cpu=128000L
attinyx5.menu.clock.128internal.build.clocksource=3

The file is locally under C:\Users\<user>\AppData\Local\Arduino15\packages\ATTinyCore\hardware\avr\1.5.2.

There, f_cpu should be modified to 16000L for 16KHz, the low_fuses (lfuse) setting should be set to 0x24 and the text "128 kHz" should be adapted too. This is working per the comment on the question.

On a final note, you should create an issue at the core's repo to have a 16kHz added by default.

Also, you might find it easier to work with PlatformIO (+VSCode) instead of the Arduino IDE. A getting started page is here. In PlatformIO, instead of having to modify the internal boards.txt file, you can create a project configuration file (platformio.ini) in which you specify the same thing setting.

[env:attiny85]
platform = atmelavr
board = attiny85
framework = arduino
board_build.f_cpu = 16000L
; for upload settings, see 
; https://docs.platformio.org/en/latest/platforms/atmelavr.html#upload-using-programmer