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Baud rate changes when debugger connected

Question asked by Martin Zimmerman on Dec 18, 2014
Latest reply on Dec 22, 2014 by Martin Zimmerman

We are seeing an odd behaviour with an SKEAZ128MLK4 Kinetis processor.


The baud rate is configured to 57.6K, and this works when we are connected to the target board with a debugger via the SWD port.

However, if we remove the debugger, or boot / power-cycle the board and let it run the exact same code without the debugger attached, the baud rate becomes 65K rather than 57.6K.


We suspect this is happening to all of the UARTs, not just our debug one, as the system communicates fine with a BLE module @ 14.4K just fine when the debugger is connected, but then fails to talk when booted stand-alone.  We suspect the same change is happening to the baud rate on the BLE UART, bumping it from 14.4K to some other value, which effectively breaks that communications link.


Something within the clock-tree is being influenced by the presence of the JTAG/SWD connection.  (that or the JTAG/SWD connection is initializing some part of the clock tree that the code alone is not).


Any ideas?