Could hsersetup command cause a change to the states of the HSEROUT/HSERIN pins

[John Chris]

Hi there,

I have a brief question. I'm trying to determine the source of NUL characters transmitted spuriously in between RS-232 level transmissions from a 28X2 (...25k22 operating at 3.3 V) whose hardware serial lines are taken through an analog switch and the MAX3244 transceiver. Note that I can open all switches so that HSEROUT and HSERIN are not connected, and I can disable the MAX3244 with an output pin.

The sequence of events is as follows:

- starting from slot 0
- receive transmission from PC
- echo back to PC
- run appropriate slot
- in another slot:
- switch serial interface for appropriate device:
- hsersetup command, switch to a different baud rate
- send i2c communication to analog switch, this switches the HSEROUT and HSERIN of PIC from one pair of RXOUT/TXIN on transciever to another, ultimately switching PIC serial communications from the PC to another device.
- power up device, receive characters in background unit a prescribed eol character received.
<strt>
- switch serial interface for PC
- hsersetup command, baud rate back to original
- i2c communication to switch to select channel
- subject to error checking ...
<end>
- transmit command to PC
- interrupt power to device
- run slot 0.
- back in slot 0

The NUL transmission ocurrs at some point between <strt> and <end>

Do we expect the HSEROUT line, for instance, to change states / be driven discontinuously when the `hsersetup' command is issued? When the baud rate is changed? Could this result in the transmission of a null character?

Any suggestions are greatly appreciated.

Chris

 

[John Chris]

I should note that after power-cycling the PIC or program download, I almost always see a NUL. If I repeat the above with a period of less than 5-6 s I don't see the NUL, period greater than 5-6 s, I observe a NUL.