ATtiny 2 series RTC prescaler value incorrect, millis() and delay() run 32X faster than nominal.

[burnsb]

Line 144 of timers.h:

#define _RTC_PRESCALE_VALUE (0x05)

Should be:

#define _RTC_PRESCALE_VALUE (0x28)

This affects line 367 of wiring.c

RTC.CTRLA = (RTC_RUNSTDBY_bm|RTC_RTCEN_bm|_RTC_PRESCALE_VALUE);//fire it up, prescale by 32.

Per pg 274 of the data sheet:

So, the divide by 32 prescaler is 0x05, but it is shifted 3 bits.

To test this, under the tools menu in the Arduino IDE, set millis()/micros() Timer to RTC:

Using the stock blink example, the led will flicker very fast. Adding the following to setup corrects the prescaler value and blink rate:

while(RTC.STATUS);
RTC.CTRLA |= (0x28);

See pull request #1289

[burnsb]

been using the 1626 successfully for my project but since last week I am getting problems with Serial + code not executing.

I have a working device soldered last week which I can even reflash. Devices from today are all spamming nonsense over Serial + code not executing. ICs should be from the same batch since I have been using the same reel.

Currently on Win10, Arduino 1.8.13. Also tested on Ubuntu 2404, 1.8.13, clean Arduino + megaTinyCore install. Let me know how I can help find a diff between the two hardware versions if that's the cause.

I think you may have responded to the wrong issue as this is about the prescaler settings for the RTC (Real Time Clock) that are used as a non-default source for millis()/delay().

As for general debug advice: reduce the problem to the minimum example. In other words, find the smallest chunk of code that will compile that shows the problem. Then, if you can, figure out why it doesn't work. Otherwise, you now have a small example that you can use to ask for advice. (Probably want to ask in discussions, not issues.)

I would think that it's most likely that something you changed broke serial. Go back to an older version of your code and see if it still works. You can pull the hardware rev of the chip through the SIGROW, but that's outside the scope of this discussion.

[unsurv]

deleted for clarity, my bad

[erebusnz]

Confirmed on an ATtiny202 (0-series) with 2.6.11 and millis timer = RTC - so this isn't just limited to the 2-series.

Symptom was a firmware 2 Hz LED blink (millis() & 0x1FF) appeared solidly lit. A scope on the pin showed a 50% square at 62.5 Hz - exactly 32× the expected 1.95 Hz.

The compiled output confirms the root cause described above. With _RTC_PRESCALE_VALUE = 0x05 unshifted, init writes:

ldi r24, 0x85        ; RUNSTDBY | (stray bit 2) | RTCEN
sts 0x0140, r24      ; RTC.CTRLA — PRESCALER field (bits 6:3) = 0x0 = DIV1

so the RTC ticks at the full 32.768 kHz instead of 1.024 kHz. The intended value is 0xA9 (RTC_RUNSTDBY_bm | RTC_PRESCALER_DIV32_gc | RTC_RTCEN_bm).

Workaround worked using #1289 in setup():

Verified as 1.95 Hz after flashing.