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K61 - UART4 - transmitted character being "echoed" onto the receive pin

Question asked by gorakk on Apr 12, 2013
Latest reply on Feb 21, 2014 by georgerouse

Using CodeWarrior 10.3 and MQX 4.0.  K61FN1M0VJ12


I have a simple test program running - it transmits an "A" every 400ms on UART4.  The output looks good - either by looking at it with a scope or viewing on a terminal program on a PC. But, checking RX4 with a scope shows the same transmitted character is present there.


Our BSP and program use the UARTs in interrupt mode.


We have eliminated the possibility that the echo is happening externally to the K61. A bare metal program written with the Keil tools does not have this problem - characters transmitted on TX4 do not show up on RX4. Also, the trace connecting RX4 to the rest of the system was cut as a test as well.


There is a register setting that can put the UART into a loopback mode but register UART4_C1 is 0x00 and this has been verified using the debugger.


I'm not seeing this behavior on the two other UARTs being used - UART2 and UART5.  I've compared UART4 and UART5 registers that have anything to do with UARTs but have not found any major differences.


Does anyone have any suggestions on where else to look?