Help me with RS232 circuit

pk178 · Joined: 10 Mar 2010 Posts: 10

Hi everybody !

I have a pickit, and I've tested RS232 module on that, it's ok, but when I design my printed circuit, it doesn't work. I tried it 3 times Mad

I use max232 (product of maxim) and PIC16F887. I designed circuit according to datasheet, this is my printed circuit :

I don't know where is error ! Please help me ! Thanks so much !

jbmiller · Joined: 07 Oct 2006 Posts: 73 Location: Greensville,Ontario

It would be easier if you sent the schematic instead of the PCB layout.

Ttelmah · Joined: 11 Mar 2010 Posts: 19552

Looking at the PCB though. What is the component left of the PIC, with just four pins?.
Assuming this part isn't doing anything silly, then it looks 'possible'. I'd say the most likely proiblems were:
1) If this is a homemade (not PTH) board, then are you sure the connections to the top tracking are made OK?.
2) Have you verified the PIC is actually running?. Try just toggling a pin in a loop. Does it toggle, and at the right frequency. If it doesn't toggle, are you sure the capacitors used on the crystal are 'right'. Classic mistake would be fitting something like 22nF, instead of 22pF for these, which will normally stop the oscillator running.
3) You have an ICSP header. Are you using LVP?. If not, have you disabled LVP in the fuses?. If not, then the PGM pin, _must_ have a resistor holding it low...
4) Whiskers.

etc. etc...

Best Wishes

jbmiller · Joined: 07 Oct 2006 Posts: 73 Location: Greensville,Ontario

Looks like an onboard 'reset' switch (coneccted to mclr).
Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19552

Yes, easily could be. In which case, add 'check that switch really _is_ going off when released' to likely failure modes....

Best Wishes

pk178 · Joined: 10 Mar 2010 Posts: 10

Ttelmah · Joined: 11 Mar 2010 Posts: 19552

OK.
If a 'simple program' works, the first thing to do, is prove that your oscillator is actually running at the expected frequency. Do the classic "pin high for one second, low for one second" test, and with a scope, or just by eye, verify that the frequency really is 0.5Hz. If not, then the serial won't work...

As a 'comment', look to adding a small capacitor close to the PIC, across the supply. The only smoothing on the supply, is the single electrolytic capacitor above the MAX232. electrolytic capacitors perform badly at high frequencies, and you also want the impedance path to the capacitor from active components like the PIC, and MAX232, to be short. This is why if youo look at a piece of electronics like a PC, you will see several large electrolytics at the supply, but also small polyester, or ceramic capacitors close to almost every IC that is performing any form of switching operation.

Best Wishes