I/O signal conditioning question

Try just a 1K pullup resistor to the Vcc of the PIC.

John

How do you know it is only 2.4V? What is the LCD display Vdd? What is the MPU Vdd? Which port are you using for the data bits? Most of the port pins use TTL-like levels when you use 5V Vdd, but PORTC ohn that part has ST inputs, and requires 0.8Vdd to see a logic 1. Do you actually need to read the BUSY signal or can you just use timing? Most LCD controller chips are CMOS and have outputs that swing from 0V to VDD.

If I was to make a WAG as to what is wrong, I'd guess that you have misunderstood the controller datasheet and the BUSY output is open-drain, and thus requires a pullup resistor (something like 10K) to your Vdd.

That's weird. I've always read the busy flag and every LCD I've ever used (at least 5 completely different models), they've all worked ok. Are you sure the output is only 2.4V? What Vcc are you using for the LCD? Are you setting the whole PORT to input mode, or only the busy pin? You are toggling the R/W line correctly, right? 2.4V sounds suspicously like an oscillating pin, have you looked with a scope?

LCD Specs say Vdd +2.4v. Vdd is +5v. Measured on o'scope as 2.4v, so Vdd

+2.4 may be a typo for Vss +2.4. MPU Vdd is +5v, same as LCD. Data bits to LCD are on port D. I've set TRISE:4 (PSPMODE) to 1, hoping to get PORTD to be TTL inputs but it still won't see the BUSY signal. Please pardon my limited electronics knowledge, If I set PSPMODE, with a 10K pull-up resistor from Vdd to PORTD:7, the "low" level bumps up to about 2V. With PSPMODE cleared (Schmidt Trigger inputs), the pull-up does not affect the "low", but raises the "hi" a bit.(2.4v to 2.5v). Is this the expected behaviour?

Yes, I could ignore the busy signal and just burn cycles to get it to work, but I want to do it the right way.

With the busy signal connected with the piezo speaker, I sent the display

40 million characters without dropping one. Using timing, I see a character out-of-place every 5000.

The display is a S28297 made by IEE (Industrial Electronic Engineers) Inc. from Van Nuys CA. The spec. sheet is dated 1983.

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Near N39 40.750 W84 10.408

Thanks for the reply.

The 2.4v measurement is from the scope. I'm actually only setting the one pin to input mode to read the busy signal. I just now tried setting all 8 to input and it's the same. How do I tell if it's oscillating? It looks like a (sort-of) square wave...

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Near N39 40.750 W84 10.408

IIRC, you must set R/~W, reset RS, *and then* pulse E; it's more than setting one pin. The bits from the LCD module will be valid shortly after the rising edge of E (see data sheet for timing). If you're in four-bit mode, do this twice for each byte to be read. You may also want pull-ups if the minimum logic "1" from the module is less than the guaranteed logic threshold for your processor.

Hi Rich - By "setting one pin" above I meant that I was only changing one pin (B7/BUSY) to input mode when I was ready to read the LCD's BUSY/READY state. I'm pretty sure I have the bit flip protocol and timing right: The display runs like a champ when my PIC can see the busy signal.

I tried a 10k pull-up. The peaks went from 2.4 to 2.5v, and the PIC still couldn't see it.

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Near N39 40.750 W84 10.408

how about a Tristate buffer ?

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

getting back to your first post. what about a common emitter to drive the PIC input, use a R from the busy line to drive the transistor into saturation which will pull down the PIC input. there will be no signal leaking back on your display information that way and you'll get your amplified logic signal, inverted how ever. collector to PIC input, emitter to common. base via a resistor to the busy line.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5