Voltage level problem

chancaar · Joined: 23 Mar 2004 Posts: 6 Location: Belgium

Hello,
I'm using a 16F877 to communicate with a RS-232 serial connector. When I measure at the output of the MAX233 (which transformes the level to TTL for the pic) I can detect 0V or 5V, which is perfect! Omce I connect an input of the PIC to the output of the MAX233, The voltage level will not go lower than 2V and I will never detect a "0"... The pen which is connected to the output of the MAX233 is configured as an input so, I don't think I did something wrong!
The pin I use is "RD5/PSP5"
as an input by using: "set_tris_d(0b00100000);" (1 = input, right?)

When I don't connect the pic, The level is perfect: 0V or 5V...

Can somebody help me?
_________________
Yves Willemaers
Belgian student

Ttelmah · Guest

chancaar · Joined: 23 Mar 2004 Posts: 6 Location: Belgium

I define the "d" port like this:
set_tris_d(0b11101111); (D4 as an output, D5 as an input)

I use next statement:
#use rs232(baud=9600,bits=8,parity=N,xmit=PIN_D4,rcv=PIN_D5)

The ground is well connected.
The strange thing is: It works with another pen (PIN_B5)
In the datasheets I can see that this B5-pen is a normal pen without extra "features". The PIN which does't work (which doesn't reach the 0 voltage level) is defined as : RD5/PSP5 (ST/TTL).
_________________
Yves Willemaers
Belgian student

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

1) disconnect pic from MAX233 chip. Tie TX pin on MAX233 to +5V through 10K (or similar value) resistor. Did corresponding pin on MAX233 go to -9V (or whatever value you read on the MAX233's V+ pin)? Tie the TX pin on the MAX233 to ground through the same resistor. Did the output pin swing to +9V? If so then the MAX233 is probably OK

2) Now look only at the PIC. Is your transimit pin able to toggle between 0V and +5V OK? Use an oscilloscope to watch the pin. It should have nice sharp edges during the transition. This is assuming you haven't picked a pin that is really an open-collector output. There are a few of those on the PIC. If that is the case then the edges will look sloppy and slow and the output level may never reach +5V.

3) RJ is right, are you using #fast_io()? You didn't answer him, you only repeated your tris_a() statment. That statement doesn't do you any good unless you also have #fast_io().
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!