pelwell
commented
6 years ago Pi Zero W and Pi 3B+ have flow control signals connected to the Bluetooth modem, so they can run at 3000000 baud and flow. Without flow control we limit the baud rate to avoid overflow - ttyAMA0/UART0 has larger FIFOs so can be run at 921600 baud, whereas the less capable ttyS0/UART1 we limit to 460800 baud.
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