pic24fj128ga202 uart issues. [Solved]

smee · Joined: 16 Jan 2014 Posts: 24

Hello, hoping someone can throw me a few suggestions, as i am being tortured by this PIC.

I am using MPLAB 8.92, PCD 5.080.

I don't seem to be able to get the uart to transmit on port PIN_B6 (RP6).
I am using the sdip 28 pin package.

I am trying to talk to a vdrive2. PIC is 3.3v and vdrive2 is 5v.

CTS is on PIN_B5 - and it works correctly.
RTS is on PIN_B7 - and works correctly.
RXD is on PIN_B8 - and works correctly.
TXD is on PIN_B6 - and refuses to work correctly.
I can turn PIN_B6 high and low with output_high/low functions.

RXD and CTS both have pullups to 5v via 4k7, and have set open collector at the PIC end.

But i can't get the uart to toggle the line.
I have tried 2 different chips, both the same.

I don't have a simple program, will do tomorrow.
Any suggestions?

temtronic · Posted: Fri Jan 22, 2021 11:53 am

this ...

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

Understand that the 'open collector' option does not allow the voltage
to go very much above the PIC's supply. It is not designed to allow you
to drive a signal requiring a higher voltage, it is designed to allow wires
to be 'wire ored' together.
The output float option only acts between bytes when using the UART.
To drive the VDRIVE with a 3.3v PIC, you need a 3v to 5v interface for
the TX (and the output handshakes if you use them), and a resistot
divider to feed the signals the other way. Simple 3v to 5v circuit is:

https://www.gadgetronicx.com/logic-level-converter-5v-3-3v/

Or a 5v logic gate with a low Vih specification.

smee · Joined: 16 Jan 2014 Posts: 24

This chip has some serious issues if you ask me, avoid it if you can.

dluu13 · Joined: 28 Sep 2018 Posts: 395 Location: Toronto, ON

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

I think he wants to 'blame the chip', for his own incompetence in not
realising he needs proper circitry to adapt a 5v device to use with a
3.3v chip. I;ve used this chip and it works well.

dluu13 · Joined: 28 Sep 2018 Posts: 395 Location: Toronto, ON

The PIC24FJ128GA204 is my go to when I want to test out some new parts because it comes on the curiosity board, with a header for the mikroe click boards.

dyeatman · Joined: 06 Sep 2003 Posts: 1938 Location: Norman, OK

I have used the '204 and '206 numerous times with no problems.
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Google and Forum Search are some of your best tools!!!!

temtronic · Posted: Sat Jan 23, 2021 2:34 pm

Maybe it's time ( past due ??) to add a 'sticky' on how to properly interface 3 V PICs to 5 volt peripherals ?

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

The answer is the same as for the SD card sticky already there.
(going the other way).
However a more generic one brought a little more up to date, might
well be worth generating.

smee · Joined: 16 Jan 2014 Posts: 24

hi guys,
I solved my problem by swapping my rts and txd pins over,
and it now works as required.

My issue was that the uart could not control the pin.
I had tried uart1,uart2,uart4 all with the same issue on PIN_B6.
Moving over to PIN_B7 made it work as required.
It seemed odd that PIN_B6 would not work.
I hunted around and found that a few people have had issues with
differing modules of the pic24fj128ga202.

Uart issues here
https://electronics.stackexchange.com/questions/192824/pic24fj128ga202-uart2-and-uart3-not-working

and again uart issues and port rb4.

https://electronics.stackexchange.com/questions/211679/pic24fj128ga202-some-i-o-portb-pins-not-behaving-correctly

Another with uart issues
https://www.microchip.com/forums/m910808.aspx

uart issues again
https://electronics.stackexchange.com/questions/208408/rs485-not-working-in-pic24fj128ga202

The common thread with these issues and also with my problem was PIN_B6/RP6.
Then i found this one,
this one finds issues with output compare, uart and other output compare, and actually gets to the bottom of the issue.

https://www.microchip.com/forums/m961027.aspx

At the end of this one it says...

Got the resolution proposal in the tech support system.
I think this is OK as long as CW2 bit 11 is documented in data sheets with
"needs to be programmed for RP6 to function correctly".

It seems that RP6 will not work with PPS modules unless this bit is set.
And as RP6 is one of the few pins that are 5v tolerant, it's an important pin.

I haven't tried this CW2 bit 11 setting, as like i said i shuffled my pins around.

I also had another couple of issues with this chip.
PIN_A4 and PIN_B4, I had to abandon trying to use them as i just couldn't get them to work.
I am using mplab8.92 and ICD3, and the support for the 24fj128ga202 is beta support in mplab8.92.
They are supposed to be inputs according to the datasheet but the Tris bit
is fixed at '0' and they will not do as they are supposed.
I had the OSCIO and SOSC_DIG setting on.

And like RP6 they are 5v tolerant pins so quite important pins.

smee · Joined: 16 Jan 2014 Posts: 24

Maybe it's time ( past due ??) to add a 'sticky' on how to read the first post Very Happy

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

Maybe (more to the point), it is important to actually be correct in what you
write in your first post.

You said that RTS and CTS were both 'working correctly', but in fact the
issue was that the PIC was not being sent a low on CTS, so (of course),
would not send anything.... Sad

A simple test on the board would have shown this wasn't low.

The VDRIVE, though as a module it has a 5v supply, is actually internally
a 3.3v device (check the VNC1 datasheet), so you don't have signalling issues.

temtronic · Posted: Sun Jan 24, 2021 5:43 am

re: The VDRIVE, though as a module it has a 5v supply, is actually internally
a 3.3v device (check the VNC1 datasheet)

hmm, I never really thought about that, but of course makes sense. I'd always considered it 5v as it plugs into a USB slot. Got me wondering if there's a 3v version...though it's have to be 'flash drive' compatible...

going to be even more 'fun' when 1.5v becomes the new peripheral standard.....

smee · Joined: 16 Jan 2014 Posts: 24

this doesnt really serve anyone, but for completeness.

the cts and rts lines did work correctly.
pulling the cts line low by the pic caused
the rts line to be subsequently pulled low,by the vdrive.

and once that had happened rs232 transmission would
also occur, identifying the vdrive, and stopping the
three red flashes.

inserting a usb drive, would also cause more transmissions.

this was all observed by the 4 channell dso attached to the lines.

what didnt happen was the transmission of commands to the vdrive
on the txd pin ( PIN_B6 / RP6 ).

the level of the rts/cts lines didnt matter to my driver at that time.
it would just transmit whatever was in the queue.
that was as uart4, so i recoded to use uart2- same outcome.
uart1 runs rs485 comms, so i changed the pin_select on uart 1 to use
pin_b6, again nothing. and i knew the uart1 was all ok.

spent ages checking registers pps,etc, all were correct as far as i could tell.

changed chip to a new one, put a fputc in an a forever while loop,
so that i should see a stream of back to back 'E' on the transmit pin.
still nothing on the pin, checked transmit buffers/registers -all empty and waiting.
checked that the pin could be toggled with output_low/high - that worked.

changed the pin from b6 to b7, and that fixed it, everything worked on b7.

i had already,had to abandon using pins A4,B4.
B4 had been the rxd pin originally and A4 the rts input but that didnt work either.

i suspect the A4,B4 problems are similar to the B6 issue, but i dont need them at the moment.

this pic is peculiar.