Attiny3224 / MegaTinyCore 2.6.10 / Baud Rate Error on baud rate of 1200 on UART1 TX

[6v6gt-duino]

Attiny3224 / MegaTinyCore 2.6.10 / Baud Rate Error on baud rate of 1200 on UART1 TX . However, it appears OK if the baud rate is higher, say 2400 and above. Has anyone experienced this ?
More details: I need 1200 baud to connect to a peripheral device. I have tried a small sketch writing to the TX pin at 1200 baud and checked with a logic analyzer. The detected baud rate is 70670 instead of 1200.
I won't spend too long on this because I have currently implemented SoftwareSerial on UART1 RX/TX pins as a work around and which is OK for this low baud rate, however a bit of a retrograde step, though, for a chip with two UARTs.

[hmeijdam]

Did you try on lower F_CPU than 20MHZ and that also does not work?
see this 20MHZ is on the edge.

[6v6gt-duino]

Many thanks for your reply. That is indeed the issue. To avoid too much repetition I'll give some detail below.

[jvasileff]

I'm guessing you are using a 20MHz clock? It looks like there is an overflow bug configuring the USART for such a low baud rate. You could try either:

1. using 1221 baud instead (which is the lowest possible baud rate at 20MHz), or
2. lowering the clock to 16MHz.

The bug issue is here is in UART.cpp, where 4 * 20MHz / 1200 is larger than the 16bit register.

(on second thought, maybe I'd consider this a documentation issue rather than a bug)

[6v6gt-duino]

Many thanks for your replies. I have confirmed that it does indeed work if I use a 16MHz clock. It seems I did not craft a good enough google search to find that. Having had it pointed out to me, it does appear to be a documentation error (at least) since 1200 appears to be acceptable at 20MHz based on the link provided at https://github.com/SpenceKonde/megaTinyCore/blob/master/megaavr/extras/Ref_Serial.md#minimums .
Indeed the result of the calculation 4 * 20MHz / 1200 (66,666 DEC) cannot quite be squeezed into uint16_t.
I'm playing with an HT9032 telephony chip which needs 1200 baud but since I don't need a 20MHz clock, I'll simply use the 16MHz work around. I'll review the matter again if I put the application on an ATTiny1614 which, anyway, does not have the second UART so SoftwareSerial would be the only option since I am using the first UART for a debugging interface.

[hmeijdam]

If your application does not require a constant available receiving UART, you could do your debugging via serial.swap(1) and have your debugging printed via the alternate TX pin. When done, you swap back via serial.swap(0)

[6v6gt-duino]

OK. Thanks. I seem to remember using serial.swap() when I used a crystal oscillator on an ATTINY1614, and this uses the UART0 pins, so I'll bear that work around in mind also should it get so far.

I see also that I should familiarize myself better with the available documentation. It seems that issues similar-ish to the one I reported are quite common so I'll add the ones I found here in case it makes these more likely to appear in any appropriate search:

megaTinyCore baud rate calculation error
#1137
#1123
#1029