7-bit serial data not working?

benoitstjean · Posted: Tue Oct 23, 2012 11:43 am

PIC: 24HJ128GP306
CCS: 4.132

I was just wondering if anyone had ever tried to send 7-bit data through a PIC24 UART. The code below is supposed to configure my SERIAL_STREAM UART to be 7 bits, then I send 0x81, then the SERIAL STREAM is set as 8-bits then I send 0x18.

What I would expect would be that 0x81 be sent as 0x01 since the high-bit (bit 7) would be ignored since I'm sending 7-bit data but that's not the case...

0x18 remains as-is since it's sent as 8-bit.

I connected a logic analyzer to my TX line and when I send the code below, I see 0x81 and 0x18 as the result of the captured data.

benoitstjean · Posted: Tue Oct 23, 2012 12:24 pm

Ok, so I guess I was mislead again by CCS's documentation.

Based on the PIC documentation, it seems that it's only 8-bit and 9-bit data on the PIC...

But CCS's documentation shows, under #USE RS232:

BITS =X Where x is 5-9 (5-7 may not be used with the SCI)

Ttelmah · Joined: 11 Mar 2010 Posts: 19369

You can send 7bit data, if you enable parity, or use a software UART. The compiler calculates the parity and generates this as the eighth bit.

Best Wishes

drh · Posted: Wed Oct 24, 2012 7:55 am

The documentation should have said:
"Bits 5-7 only available with FORCE_SW".
_________________
David