Support for STK500 devices with STK500_XTAL != 7372800

[Ho-Ro]

The stk500 v1 (as well as the stk500 v2) uses the constant xtal frequency (7372800 Hz) of the original Atmel STK500 HW when calculating the timer parameters Parm_STK_OSC_PSCALE and Parm_STK_OSC_CMATCH from the terminal command fosc to set the external clock for targets without their own clock source.
These two parameters are direct register values for an 8-bit timer of an ATmega, (e.g. TCCR2B and OCR2A for m328p).
When using an STK500 compatible programmer with a different clock (e.g. based on an Arduino Nano with 16 MHz), the resulting frequency is much too high.

My current solution is to change this line in stk500.c from

#define STK500_XTAL 7372800U

to

#define STK500_XTAL 16000000U 

before building :)

I don't see a straightforward concept at the moment how to provide the correct frequency for avrdude, unless you introduce a new programmer parameter, e.g. xtal.
I am posting this here for discussion anyway, maybe you can find a simple future solution that fits into your concept.

Thx, Martin

[mcuee]

I think it is okay to have -x xtal for stk500v1 and stk500v2.

[mcuee]

@Ho-Ro

Maybe you want to create a PR to add xtal to avrdude stk500v1/stk500v2.

[MCUdude]

I'd like to hear what @stefanrueger thinks, but I can probably create a PR that allows for -x xtal

[Ho-Ro]

I already started to deep dive into stk500v1 (stk500.c), there are also some minor errors in -x handling that I also can fix.
stk500v2 will come next.

[stefanrueger]

Using a -x xtal=... options sounds like a good solution for this problem for the affected programmers.

[Ho-Ro]

main...Ho-Ro:avrdude:stk500-x_xtal

This is work in progress, step 1 fixed some minor bugs sleeping in the code (it seems that these extra options are not heavily used).

Unfortunately my editor kate automatically removes trailing spaces even when told to do not, so there are some format changes. If you don't like this, I can rework - finally I found the solution.

Step 2 will be the implementation of -x xtal=nnn for stk500.c before I go on with v2.

EDIT: HOLD on, the more I test, the more bugs creep in, e.g. -xfosc=100k works, but -xfosc=100000 sets fosc=0.000
I will fix this together with step2 - just pushed to my repo.
Good night!

[MCUdude]

Great start! We probably add support for xtal "units" as well, just like we can with fosc (you can most likely just copy-paste the code).

-x xtal=16000000
-x xtal=16MHz (or Mhz)
-x xtal=16M
-x xtal=1600k
-x xtal=1600kHz (or khz)

[Ho-Ro]

(you can most likely just copy-paste the code).

I did - and during testing I discovered some issues how we call it in the job - not bug, not error...

-x xtal=16MHz (or Mhz)

or even mhz (shudder)

but -x xtal=1234567Hz should also work, I will add this. - wenn schon, denn schon - how we say in German. :)

[mcuee]

• Support for STK500 v1 and v2 devices with STK500_XTAL != 7372800 #1566 has been merged.