Using RB6 and RB7 as EUSART while in debug

JerryR · Joined: 07 Feb 2008 Posts: 167

I usually stay clear of using debug/programming resources as general IO, but I'm against the wall here on a new design.

I've connected a single channel Maxim RS-232 level translator (MAX3311E) to RB6 and RB7 with the intention of using the EUSART on a PIC16C1784 microcontroller. A PICKIT3 and an ICD3 refuses to debug with the MAXIM device in-circuit, even with the MAX3311E in shutdown. I can program the PIC and it runs fine, just will not debug. Yes, the dreaded "ICD3Err0040: The target device is not ready for debugging. Please check your configuration bit settings and program the device before proceeding."

I can't imagine why the debugger can't share these lines.

I also have a RealICE, but have not tried it as yet?

I'm using MPLAB with CCS C PCWH v5.021.

Any suggestions group? I appreciate your help!

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The MAX3311E data sheet says on page 5:

JerryR · Joined: 07 Feb 2008 Posts: 167

Thanks PCM. I'll research this tomorrow and get back.

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

try this-
on the RX input of the MAXIM - add a 2n3904 as an emitter follower with collector tied to 5v and emitter on the RX input

the floating base is your TX send-to pin.
if you really have to -add a 100k or less pull resistor UP from the base to +5v

i bet you it works....

JerryR · Joined: 07 Feb 2008 Posts: 167

Hi asmboy:

Thanks for the reply. I am going to isolate the rx and may try your suggestion later today.

Regards

JerryR · Joined: 07 Feb 2008 Posts: 167

PCM: Good call! Missed the effects of the pull-up on the RX input line. Isolated the RX output and debugging is happy now.

Asmboy: I may try your fix. Another pass on the PCB.

Great asset you two. Design review via forum board.

Regards

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

Figure 2 on the data sheet shows schematically what PCM was talking about.

in the case of RS-232 TTL i would expect a WEAK pull UP to be the default input state - so the what MAXIM has done strikes me as ODD - though it might have something to do with their data_valid monitor feature/bug ;-))

i am betting you can find out by just hacking a TO-92 leaded version of the transistor onto your present PCB to find out.

JerryR · Joined: 07 Feb 2008 Posts: 167

Asmboy:

I'm guessing MAXIM wanted to maintain the RX in a predictable state by pulling the RX in low. Kind of surprised me too!

Best regards and, again, thanks for your interest.

Ttelmah · Joined: 11 Mar 2010 Posts: 19518

They are following the RS232 standard, so having 'off' reported as 'break'.