The STM32F437 USART has an automatic flow control feature. My understanding of this is that CTS and RTS will be handled automatically by the MCU. My interest is in RTS. To me it seems that RTS is asserted/de-asserted for every received character. The MCU seems to only have an internal 1 byte buffer, unlike the old STR9 MCUs which has a 12 byte FIFO. This limitation means that's if my MCU is communicating with something like a GSM modem at a high baud rate of 2040800 then the RTS signal will be toggled at a rate of 230400 bits / (8 + 1 + 1) = 23040 bits per character = 1/23040 = 0.000043 secs = ~43uS. This means if the receiver is using interrupts it has to cope with them at a rate of 43uS. This gets worse if an even high baud rate is used. It is unlikely that the receiver will cope with this. Is my understanding correct? If so this seems like very limited feature and is only useful for slow baud rates.