LCD controlling with comparators

Hi all,

I've (half-)built a "clock" using a cheap LCD display and a couple of PICs. Currently, it's very OTT with pics, since I don't have any shift registers or anything to control the 40 pins on the LCD, so I've got a PIC for each digit, 8 input pins, and 7 output pins (7 segments per digit). When the 8th input pin goes high, the other 7 are copied to the output. This way, 4 digits can be driven from one PIC with 11 output pins (7 for the segment, and one linked to each "active" input for each digit).

What's the best (least components etc.) to do this in "the real world"? I was thinking of addressable shift registers or something to replace the PICs, but then it occured to me that it's also possible with a comparator... Wire all the segments together, and use a comparator so only when I "clock" the correct 7 comparators, would the outputs be carried across to the LCD. I'm not sure of the relative costs of shift registers/comparators - how would you do it?

--
Danny

Sounds like your driving your lcd with dc, it wont last long.

An lcd controller chip, find 1 to mach your display.

the

each

What kind of LCD do you have? You may need to be careful with how you drive the digits.

world"?

There's a bunch of ways this can be done, but yours wins the Rube Goldberg award. ;-) You should multiplex the digits from just one pic. You can use PORTB to connect to all the segments in parallel and then turn on the anode (or cathode whichever it is) from PORTA pins. You put the segments for the first digit on PORTB and then drive PORTA, 0 low (or high) to illuminate the first digit. You then turn off the PORTA,0 pin and put the data values for the second digit onto PORTB, then turn on PORTA, 1. Cycle thru PORTA, 2 and PORTA, 3 then back to PORTA, 0.

If you setup say a timer interrupt to occur every 10mS or so, you can switch digits in the ISR and leave them till the next interrupt 10mS later. That would update the entire display every 40mS or 25 times per second. Your eyes will never know the difference and it will only need

11 i/o pins.

You could also use a shift register(s) in order to use less i/o pins.

--
You have what's called a "static" display, and the pins marked "com"
are connected to the backplane, which is the transparent electrode
which forms one side of the capacitor comprising each segment and the
backplane.  MOST static LCD displays aren't designed to be driven by
DC, and what will happen is that the ITO will plate out of either the
backplane or the segment and migrate to the opposite electrode, with
the eventual result that the capacitor will be destroyed and the digit
will become unreadable.  The proper way to drive static LCDs is with
square wave AC; the backplane and the segment being driven in phase
when the segment isn't supposed to be displayed, and out of phase when
it is, like this for the segment to be off:

         __    __    __    __    __    __
SEGOFF__|  |__|  |__|  |__|  |__|  |__|  

         __    __    __    __    __    __
BP    __|  |__|  |__|  |__|  |__|  |__|   


and like this for it to be on:

      __    __    __    __    __    __    
SEGON   |__|  |__|  |__|  |__|  |__|  |__  

         __    __    __    __    __    __
BP    __|  |__|  |__|  |__|  |__|  |__|


This task _can_ be accomplished with a µC and a longish shift register
By EXORring the backplane and the segments at about 30Hz., but it's
often done with a device like a National MM5483 so all you have to do
is shift data into it and strobe the output registers without any
extra software overhead. 

http://cache.national.com/ds/MM/MM5483.pdf    

If you've got a 3-1/2 digit seven-segment display, that's 23 segments,
and the 5483 can drive 32 segments and a backplane, so that leaves you
with nine extra segment drivers for decimal points, annunciators, or
the colon if you're doing a clock.  Best of all, Digi-Key's got them
in 40-pin DIP packages for about five bucks.

Whoops?

heh. I meant without using something like that - let's say I'd invented my own 13 segment display for a petrol gauge on my car or something! :-P

--
Danny

57-0160 from rapidelec.co.uk

It seems to be available in a 4-pin option, but I ordered the "31/2" assuming it meant 21 pins on each side, but it only has 40 (no idea what the "4" flava is then!)

CBarn24050 just suggested I shouldn't be driving it from DC - is this really a big problem? It seems to be working fine, my clock has two digits! Other two can wait till after tea - I've already wired about 60 pins together across 4 PICs...

This sounds like a fantastic idea *but* of the 40 pins on my display, the only ones marked on the datasheet are one for every segment/dp on the screen, plus two "com" (one on each side). I assumed it meant "common" and grounded it, while connecting the other pins to pic outputs, and they drive fine. I assume your method isn't useful for this particular LCD?

Ta

--
Danny

you

I found it, but no datasheet.

what

60

Matrix type LCDs have to be scanned, the pixels can never be allowed to see any DC voltage or they will cook. Your display does not appear to be in this category. Your display is a very fixed purpose display. Your display will only display 3 1/2 digits, that means the first column can't show all the numbers 0 - 9, only 0-1 or 0-3 depending on the display.

You should see the BASIC-52 computer I made with scads of jumpers for the external latches and ram for the 8052. Kinda neat about 3.5" square and runs on a 9V battery, just hook up to the serial port, push the reset button, hit the space bar (or is it return) and it's 1977 again. ;-)

pic.

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put

low

PORTA,0

turn

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this

This appears to be the case. If you don't have access to the cathodes of each individual segment, then you can't multiplex it in the way I described. You should be using a Hitachi type LCD display anyway, or 7 segment LEDs. ;-)

Datasheet came with it (cos it's not on the web):

The table in the bottom right is blown up here:

It'll only do 1 on the first character. I thought maybe it was good for clocks (as I'm doing now), but there's no "decent" am/pm indicator, and it can't do 24hr, so maybe it's even more useless! ;)

heh, at least it does something - this just lights up a few segments, and it's got more wire in it than my entire apartment!!

So, given the LCD, and the available hardware, would you agree that my "Rube Goldberg" clock, is indeed, the best way I could've done it? ;P

The 5 PICs I've used... 4 are just simple registers (cos I don't have any) like this:

Start btfsc PORTB, 5 ; If RB5 is high movf PORTB, W ; Copy input! movwf PORTA ; Move to output GOTO Start ; Start again

END

And the "driver" I haven't done yet. I know in the code above, if RB5 goes low between the first two instructions, it might not work properly, it should copy PORTB into a register, then work on it from there - however, ther's *no way* I'm getting those chips out of those breadboards with all those wires in (around 80 wires so far, and the driver chip isn't there), so it can stay for now ;-)

--
Danny

This is essentially what I did, I've made 4 PICs control the 4 digits (I don't have any shift registers, and couldn't find any on maplin or Rapid

- am I searching for the wrong thing?!)

I considered this, but it was really just something to keep me busy until wheels and op-amps come for my robot. It'll be taken apart soon - I just wanted to check my understanding of assembly programming a little more, and wanted something to do with the bits I have (Even if you can buy bits to do this stuff already!) :-)

That said... Only my driving chip would need the crystal, and I do have

2x4Mhz that I bought a while back (I didn't realise they had internal ones). Since my driving chip has spare pins (the only one that does!!), I might hook it up to that. What's the best way to time something like this - the only delays I've done so far are done by decrementing a register and skipping if 0 - I'm sure there's a nicer way to do it?

Ta

--
Danny

I see (oops)... What I don't understand though, if "com" is for the backplane - what about ground? :-\

--
Danny

Bummer, that's it huh? Unfortunately, that doesn't tell us anything important about how to drive the display other than the pin-out. Maybe you shouldn't purchase any more surplus LCDs from them. Hitachi based displays are the common display for tinkering. You can get them in anything from a 1x8 to a 4 * 80. They are usually as cheap as the one you have, but have much more utility.

to

to

column

for

and

You need to find yourself a nice Hitachi based display. ;-)

for

square

again.

cathodes

or 7

Well if you had to make it out of only the parts you used, then ok. However, if you had a couple of shift registers on hand, you could have used only one PIC.

Out of curiosity, why did you chose PORTA to drive the segments and PORTB to read data in?

properly,

Why do you say that?

How about this:

Start btfss PORTB, 5 ; If RB5 is low goto Start ; then no data is available movf PORTB, W ; Otherwise, copy input! movwf PORTA ; Move to output GOTO Start ; Start again END

In cases like this, it's ok to just check the port pin without making a shadow copy of the whole register. It's output pins that you need to be concerned about the read-modify-write issues.

have

That's right. ;-)

does!!),

Er um, not really. There are other ways to do long delays, but I'm not sure that I'd say they were nicer. ;-) You would probably want to use a timer and associated interrupt routine to keep accurate track of elapsed intervals (like seconds for example). That way, your main level code could go about it's business without worrying about updating the seconds ticker at the right time.

Yeah - I didn't buy it for any purpose really, was just interested in seeing how it worked - something extra for debugging output and was fairly cheap :)

That's why I programmed 4 pics to work as I understood shift registers would (I didn't consider serial-in), it's the main driving pic that'll be the clever stuff :-) (if I get a clock working quickly, I might add time set / alarm buttons, like most alarm clocks :))

For parallel data, I needed 7 outputs (7 segments) and 8 inputs (7 segments plus clock/latch (what's the right word?))

My programmer doesn't like turning MCLR off (internal), so that left me with 7 I/O pins on one port. I used the same pin on the other port as the clock/latch, so I could just copy (other 7 bits would be in the right place). Copying over MCLR doesn't matter since it's ignored.

I didn't have enough room on my breadboard, so I had to fudge it slightly... I put two chips upside down, with the bottom left 4 pins in the same rows as the top right 4 of another pic. Then I wired those 4 inputs up the other way, since all intputs are driven from the same 7 outputs of the driving pic, with just the clock (RB5) being wired seperately.

Thinking about it, if I clock with two instructions, eg:

; Set output pins for char 1 ; Clock digit 1 ; Clock digit 1 off ; Set output pins for char 2 ; Clock digit 2 ; Clock digit 2 off

But my worry was, if after your first insutrction (which was RB5 is high), the driving pic sends RB5 low and then gears the outputs for the next character. By the time you get to the next instruction (moving PORTB to W) I might've changed the data. It's what we like to call "random failure"!

Since it may take two instructions to change the output, it's not a problem, *BUT* since the 4 "shift registers" are running on internal clocks, I don't know how accurate they are, and the driving pic could potentially clock RB5 and change the 7 output pins in between - ?

What I don't like, is the reading a value, satisfying a condition, then using a "new" copy of the data, which might not satisfy the criteria. It's assuming that the data doesn't change between instructions, which, given the data is provided externally, is entirely possible! :(

--
Danny

:)

I've not used them before, but I'm sure even I can manage this one... Connect the crystal to two pins and set the config to use it as per the datasheet!

That sounds a bit better - so I can get a routine to run every second accurately, without having to subtract one from my "waitAsecond" routine every time I add a new instruction? ;-)

Btw, is 4Mhz 4 million instructions per second? Guess I need to make sure my code (eventually checking for time set buttons, alarm etc.) doesn't run for more than this time, else my timer interupt will interupt itself, and then I'll probably get stack overflows (or improper functioning at least!)

--
Danny

Maybe

based

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have

buttons,

I'm bad about not finishing my projects all the way. I breadboard them up, tinker with them till they work and then get bored and move on to something else. I'm going to solder up my clock listening project since I actually have some use for that. ;-)

On your slave PICs you might consider it to be a "chip enable" pin, or even a "read".

me

in

The reason I ask is that the RA4 pin is an open collector when used as output. This means it needs a pullup resistor when used as an output pin. Therefore, you'll see allot of it being used as an input pin whenever possible. It's also useful for talking to Dallas 1-wire devices. ;-)

RB5

the

You have to give time for the slave PIC to "see" the data. If your slave PIC has a worst case timing of say 10 cycles to recognize its "enable" pin and retreive the data, then the master PIC has to allow at least that much time every time it presents a digit. Then you wont have random failures.

making a

to be

then

which,

I'm not sure what you mean by "new" copy of the data.

the

Don't forget the caps to ground, or it wont start. It's only recently that I started using the internal osc. It's actually getting to be fairly usable as the new parts come along. A 16F628 I'm playing with right now is only fast by about 3.5% according to my 15 year old frequency counter. This is actually convenient for me since I can use Timer0 with a prescaler of 4 and get interrupts at almost exactly 1mS intervals (instead of 1.024mS) without having to reload it every interrupt.

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routine

Well, it's not quite that straightforward. You usually count a bunch of short intervals (like milliseconds) and then do something important when

1000 have gone by. A common housekeeping interrupt loop time is 10mS. Every 100 interrupts you could add one to a seconds counter. Timer2 is real good at this kind of stuff, but I like to use it for other things. Timer0 is kinda weak (only 8 bits), but it's handy. ;-)

No, it's 1 million instructions per second. The clock is divided by 4 as every instruction cycle consists of 4 Q states. 4MHz is a nice clock speed because every (single cycle) instruction takes 1uS to execute.

improper

When an interrupt occurs, the GIE flag is cleared. This prevents the scenario you described. When the ISR returns, it uses a retfie instruction which sets the GIE bit again so that another interrupt can occur. If you forget to clear the flag for the interrupt that you just processed, then you'll be processing it again as soon as you execute the retfie. ;-) This is a common mistake for newbies.

--
There _is_ no hard ground required, it's like this:

INn------------------------------------------+
                                             |
INb----------------------+                   |
                         |                   |
INa---+                  |                   |
      |                  |                   |
      +--A               +--A                +--A
       EXOR Y------+      EXOR Y------+       EXOR Y------+           
INBP>-+--B         |     +--B         |      +--B         | 
      |            |     |            |      |            |
      +------------|-----+------------|------+            |         
      |COM        a|                 b|                  n|
    +-|------------|------------------|-------------------|----+
    | |            |                  |                   |    |
    | |          [SEGa]             [SEGb]              [SEGn] |
    | |            |                  |                   |    |
    | +------------+------------------+---------//--------+    |
    |                      DISPLAY GLASS                       |
    +----------------------------------------------------------+

Where the EXORs are part of the external drive circuitry, the "DISPLAY
GLASS" is the display itself, and SEGa, SEGb, and SEGn represent the
inherent capacitances between the various segment electrodes and the
backplane.

--
OK. One thing I didn't mention is that INBP should be a square wave
and that INa, INb, and INn should be logic level highs or lows; high
to make the corresponding segment visible and low to make it
invisible.  Also, be aware that all this circuitry is internal to the
5483, so all you need to do is set the frequency of its internal RC
oscillator, then treat it like a 32 bit shift register and it'll do
the rest of the work for you.  

If you want to play around with it and do it with 8 bit serial-in
parallel-out shift registers (with output latches) you'll need three
of them plus six quad EXORs.

What's that do?

I'm the same with software... I've written about 30,000 "personal" websites, and none of them ever make it to the web, because by the time I've finished, I've thought of a better way to do it... It's good for learning, but gets me a reputation!! I've decided if I can do this clock properly (replace these 4 pics with cheap shift registers or something) with alarm, I might start using it - just because I can! :)

I meant in a more general sense. From looking at shift registers, they seemed to have clock and latch - clock seemed to be to set data (for serial use), and latch was to move the collected input to the output pins... I guess mine's a combi! ;)

Hmmm, I wasn't aware of this - and everything seems to work without a resistor. What are the implications - what difference would I see in high/low with and without an external resistor?

As well as holding the "enable" pin for 10 cycles, to be safe (if it where 10), I'd also have to hold the "enable" pin low while maintaining the same data to be "safe". Imagine if I hold the enable pin high for

100 cycles, we know my chip has read the data successfully, so I then drop *all* pins down to 0. Since my "shift register" is running very quickly, all it takes is for this moment (when all pins go low) to be between the "is enable pin high" and "move input to output" and 0h would be copied to my output!

Does that make sense?

--
Danny