stmhal: For UART, check that baudrate is within 5% of desired value.

Also includes documentation about minimum baudrate.

Addresses issue #1090.

1 files changed, 7 insertions, 0 deletions

diff --git a/docs/library/pyb.UART.rst b/docs/library/pyb.UART.rst
index 257a9f281..509ca4080 100644
--- a/docs/library/pyb.UART.rst
+++ b/docs/library/pyb.UART.rst
@@ -75,6 +75,13 @@ Methods
      - ``timeout_char`` is the timeout in milliseconds to wait between characters.
      - ``read_buf_len`` is the character length of the read buffer (0 to disable).
 
+   This method will raise an exception if the baudrate could not be set within
+   5% of the desired value.  The minimum baudrate is dictated by the frequency
+   of the bus that the UART is on; UART(1) and UART(6) are APB2, the rest are on
+   APB1.  The default bus frequencies give a minimum baudrate of 1300 for
+   UART(1) and UART(6) and 650 for the others.  Use :func:`pyb.freq <pyb.freq>`
+   to reduce the bus frequencies to get lower baudrates.
+
    *Note:* with parity=None, only 8 and 9 bits are supported.  With parity enabled,
    only 7 and 8 bits are supported.