STM32G0: failed assertion trying to set USART4 baud rate

[scgtrp]

I'm using revision 9545471 and targeting a STM32G070RBT6, which has a USART4.

When I call usart_set_baudrate(USART4, 100000) the device crashes due to an assertion failure at:

(gdb) bt
#0  cm3_assert_failed () at ../../cm3/assert.c:24
#1  0x08001964 in rcc_get_usart_clk_freq (usart=usart@entry=1073761280) at rcc.c:583
#2  0x08001be6 in usart_set_baudrate (usart=1073761280, baud=100000) at ../common/usart_common_all.c:55

That function doesn't know about USART4, because it's also missing from RCC_CCIPR (RM0454 p163):

I think what's going on there, based on this bit of the clock tree (RM0454 p124):


and this table in the USART docs (RM0454 p777):

is that this only works on "full" USARTs and not "basic" ones. It's unclear to me where the basic USARTs get their clock from.

(This also suggests it shouldn't even work on USART3 on the G070, but the previous revision of the board I'm working on used USART3 and it seemed to work fine, possibly by accident.)

[scgtrp]

For anyone who shows up here searching: I'm now successfully working around this by setting USART4_BRR directly, calculated using the APB clock, e.g.

USART4_BRR = rcc_clock_config[RCC_CLOCK_CONFIG_HSI_PLL_64MHZ].apb_frequency / 100000;

I think this means that adding "else just return the APB frequency" as a fallback case to rcc_get_usart_clk_freq() in place of the cm3_assert_not_reached() call that's there now would fix this, but I'm not certain enough that that's always correct to submit it as a pull request.