LCD will not start

[mister_e]

What happen if you tie the PORTB.3 to gnd via 10K (or less)?

Did you tried to enable the BOD (brown out voltage) and disable the LVP (Low voltage programming) mode?

If nothing work, maybe, you could add a little capacitor (0.1-1uF) between MCLR and GND... maybe.

Schematic?

[btaylor]

Hi Mister-e.

The PIC always starts. I know this because I have solenoids and switches and the switches always control the solenoids correctly. I think that rules out BOD or LVP issues. The 2 x 16 LCD always starts with the top row all black boxes and the lower row blank. When it starts properly, the LCD clears in about 500 mS and shortly afterwards the expected characters appear. When it does not start, the top line remains black and nothing appears on the lower line.

I have tried 100 nf, 1uf and 10 uF as the MCLR capacitor with 10k to +5. Again the PIC always starts.

The big 12 V 10 amp power supply is relatively slow to start and turning it on/off at the wall shows the LCD problem much more often than leaving the power supply on all the time and make/break the power line to the processor board. A scope shows the +5V rise time as 20-100 mS for on/off at the wall and under 1 mS for make/break to the 12 volt feed to the processor board. dV/dT is clearly part of the problem.

A web search has turned up www.densitron.com and they have a discussion on timing aspects plus 4 bit and 8 bit modes. They make much of the tight timing restrictions between the Enable, RegisterSelect, data transitions and Read/Write lines and say this is a common problem with faster processors. sadly their app note is shot with typos and they do not define the pin functions (should RS be high or low during write? - they show it as both)

I have the distinct feeling that PBP may be marginal with some brands of LCD at higher crystal speeds.

Cheers
Brian

[mister_e]

try...

Start:
    Pause 1000
    LCDOUT $FE,1,"First"
    Pause 1000
    FLAGS=0
    LCDOUT $FE,1,"Second"
    pause 1000
    FLAGS=0
    LCDOUT $FE,1,"Third"
Loop:
    Goto Loop

What happen?

I heard that some LCD may need up to 2 second to start correctly... but i never had one in my hand

[btaylor]

Hi mister-e.

Your code, after fixing the start/loop and flag issues, does not work on power up. It does work after a MCLR reset but so do all my attempts.

I am trying now to implement the full reset sequence in code.

[paul borgmeier]

this might help - untried by me

EDIT: My uploads do not appear to work?? - I put the file here as well
http://www.cruxanalysis.com/temp/softwarereset.pdf

[Darrel Taylor]

Brian,

Which model LCD do you have from Densitron? They list 20 diffeent 2x16's.

How long is the cable to the LCD?

I looked at a couple of the desitron datasheets, they seem to be within the spec's for PBP's LCDOUT timing.

And, RS can be either high or low during a write. It would be 0 for a command such as Clear Screen, or 1 to write data to memory.
<br>

[btaylor]

Hi Darrel,
I do not have any Densitron LCDs, The LCDs I am using (I have 3) have data sheets viewable at
https://secure4.vivid-design.com.au/...ed/QP-5515.pdf
They are Chinese as are most things these days. The supplied documentation is pretty sparse. I only mentioned Densitron as they were the first appnotes I found with any discussion of setup lines and handshake timings.

I have tried all three LCDs and they all behave the same. I have tried cable lengths from 35 to 400 mm - no difference. The LCD will work most times after a direct MCLR reset. That is, power up, wait a few seconds then manually ground MCLR.

I tried the following code which sets up the interface with Enable already high, waits a short while then takes Enable low for each of the data states described in the manual. I think I have these correct. This code clears the 'all black' LCD cells shortly after startup but then leaves the LCD bank - as though it is turned off in software despite my having the LCD ON bits enabled (or so I think). I have tried a range of PAUSE times to no avail.

I also tried setting up the interface with Enable low, strobing Enable high for 100 uSecs then moving on to the next setup state. No joy there either.

'attention
for a = 0 to 1 ' call attention mode several times
portd = %00010011 ' attention request
pause 1
portd = %00000011 'drop ENABLE
pause 5
next a

'set 4 bit mode
portd = %00010010 'set 4 bit mode
pause 5
portd = %00000010 ' drop enable
pause 5

'set function
portd = %00010010 'first nibble
pause 5 'allow setup time
portd = %00000010
pause 5
portd = %00011000 'select 2 lines 5x7 character set
pause 5 'allow setup time
portd= %00001000 'low ENABLE
pause 5

'display OFF
portd = %00010000 'first nibble
pause 5 'allow setup time
portd = %00000000 'low ENABLE
pause 5
portd = %00011000 '
pause 5 'allow setup time
portd = %00001000 'low ENABLE
pause 5

'display ON
portd = %00010000 'first nibble
pause 5 'allow setup time
portd = %00000000 'low ENABLE
pause 5
portd = %00010001 '
pause 5 'allow setup time
portd = %00000001 '
pause 5

'set entry mode
portd = %00010000 'first nibble
pause 5 'allow setup time
portd = %00000000
pause 5
portd = %00010110 '
pause 5 'allow setup time
portd = %00000110
pause 5

'display ON
portd = %00010000 'first nibble
pause 5 'allow setup time
portd = %00000000
pause 5
portd = %00010001 '
pause 5 'allow setup time
portd = %00000001
pause 5

I will try with another brand of LCD I have at home tonight.

Driving me crazy.
Brian