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stm32/common: Implement handling for setting the baud rate correctly when in 8x oversampling mode

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dragonmux
2024-04-24 08:41:58 +01:00
committed by Piotr Esden-Tempski
parent 9480f493b9
commit 2d15b12ff2
3 changed files with 25 additions and 1 deletions
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20
lib/stm32/common/usart_common_all.c
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@@ -71,7 +71,27 @@ void usart_set_baudrate(uint32_t usart, uint32_t baud)
}
#endif
#ifdef USART_CR1_OVER8
if (USART_CR1(usart) & USART_CR1_OVER8) {
/*
* When using 8x oversampling instead of 16x, the calculation works slightly differently.
* We do the same main calculation as for 16x, but with the clock rate effectively doubled.
* This keeps accuracy up and gives us the best possible divider. However, to set BRR,
* we have to do some shenanigans - specifically, USARTDIV[15:4] = BRR[15:4], but
* UARTDIV[3:0] = BRR[2:0] << 1 and we are required to keep BRR[3] as 0.
*/
const uint32_t divider = ((2 * clock) + (baud / 2)) / baud;
USART_BRR(usart) = (divider & USART_BRR_UPPER_MASK) | ((divider & USART_BRR_LOWER_MASK) >> 1U);
} else {
/*
* Modified version of the formula from the datasheets that tries to improve
* the accuracy of the calculated dividers by introducing a rounding factor
*/
USART_BRR(usart) = (clock + baud / 2) / baud;
}
#else
USART_BRR(usart) = (clock + baud / 2) / baud;
#endif
}
/*---------------------------------------------------------------------------*/