difficulties connecting axe132 to displaytech 204a

pheonixfire

New Member
G'Day, I've connected a picaxe axe132 to a displaytech 204a lcd display from rs components, the problem is when
I power it up nothing shows on the display.

I've only altered the firmware on the chip so it knows its dealing with a 20 character a line display

I have checked in case it's shorting or bridging from my soldering, but everything appears fine

Hopefully I can get some advice from someone with a little more experience in this area then I currently do


Any help is appreciated
pheonixfre
 

hippy

Technical Support
Staff member
Have you tried adjusting the contrast pot ?

Some displays may be quite sensitive to supply voltage, may need 5V and using 3 x AA batteries my not be enough.

Things should work with the firmware set to 16 characters so perhaps worth trying the default firmware in case there has been some mistake made when editing.

Displays do have differing pinouts so how they connect to the AXE132 may differ. Do you have a link to the display datasheet which shows its pinout and a photograph to show how the display is connected to the AXE132 ?

One test you can try is to remove the 18M2 from the AXE132 then power up the board. A single line of 5x8 blocks across the display is a good indication the power connections and contrast are both okay.
 

Jeremy Harris

Senior Member
One potential gotcha is that the pins are often the other way around on some displays, and IIRC, 20 character displays have the pins in the opposite orientation to 16 character displays.

This means that pin 1 can be the opposite end, needing the AXE132 to be rotated to get the pin alignment OK. 16 x 2 displays often have the pins at the top of the display, so when the AXE132 board is fitted the programming socket points inwards, whereas 20 x 4 displays often have the pins at the bottom, so the AXE132 has to be reversed and fitted with the programming socket overhanging the end of the display board.

From looking at the RS display you have it looks as if this has the pins at the top like the 16 x 2 displays from Rev Ed, not like the 20 x 4 displays. So the first thing to check would be that you have the AXE132 mounted with the programming socket pointing inwards and not overhanging the display module.

The other thing to check is that you have the contrast pot turned right down towards 0V. If this is, say, halfway, then the display will often look blank, as the contrast pin often needs to be near to 0V to get adequate contrast. I've often left the contrast pot out on home made boards and just wired the display contrast pin direct to 0V and it usually works OK.
 
Last edited:

hippy

Technical Support
Staff member
Do you have a photo which shows how the display connections join to the AXE132 connections ?

The LCD connection order seems to be : VSS, VDD ... DB7, LED+, LED-

The AXE132 connection order is I believe : LED+, LED-, VSS, VDD ... DB7

Depending on exactly how connected there may be a mismatch in connections.
 

nick12ab

Senior Member
Here's a link to the lcd datasheet as requested

http://cdn.displaytech-us.com/sites/default/files/display-data-sheet/204A series-v21.pdf

View attachment 20148

any help is appreciated
pheonixfire
The AXE133 PCB appears too far to the left and the backlight pins on the LCD module are not visible in the photograph which suggests that the AXE133 PCB is connected incorrectly to the LCD module. When using a module where the backlight pins are near the data bus pins instead of the power pins, the AXE133 PCB needs to be connected as shown below:

IMG_20160826_145441886.jpg
 

pheonixfire

New Member
If I'm understanding what your asking correctly (fair chance I'm not) you want a picture from the
other side of it

lcd picture.png

any help is appreciated
pheonixfire
 

hippy

Technical Support
Staff member
Perfect photo, just what was wanted.

It does appear to me that the AXE132 is misaligned to the LCD display connections, the AXE132 needs shifting 'two holes over' in the direction of the yellow arrow in the image below, and as per nick12ab's post #6 ...

Added: The pin I labelled VDD in the diagram below should be VSS.
 

Attachments

Last edited:

pheonixfire

New Member
thanks for the help it is now working

lcd picture 2.png
It's a bit messy, but I didn't have a spare header strip

all I need to do now is work out how to turn on the back light

thanks again for the help
pheonixfire
 

neiltechspec

Senior Member
This is what I do with 20X4 LCD / OLED to stop the driver board sticking out beyond the end of the display.

Upper display is LCD with backlight on C.2, by the addition of a transistor & SMT resistor.

Lower display is OLED. Oops, no it isn't OLED, it's a blue LCD with permanently on back light.

On both, the AXE driver board is either hot glued or double sided taped to the display.

PICAXE LCD.jpg

Neil
 
Last edited:

nick12ab

Senior Member
all I need to do now is work out how to turn on the back light
A transistor and some other components need to be fitted next to the programming jack, see my blog for details.

Note that although I used a BC556B, you can use any PNP transistor with the same pinout, but you'll need to check that it has sufficient gain and/or adjust the base resistor according to the current requirement of your backlight.
 

pheonixfire

New Member
okay that makes sense, where I'm getting lost is turning on the backlight in the software of
the axe132.

the firmware says :
control outputs C.2, C.1, C.0 (via lower 3 bits of X)
; So, if using a backlit LCD with the active low transistor driver
; on output C.2, then 255,%000 switches backlight on and 255,%100 switches off
but I'm a bit lost on whether I need to send that from the control picaxe or whether I can set
it in the axe132 firmware(somewhere)

any help is appreciated
pheonixfire
 

neiltechspec

Senior Member
Use it just like you would for a usual display command -

(with a PNP BC557 driver)

serout lcd,baud,(255,%00000000) 'backlight on

serout lcd,baud,(128,"hello") 'display hello on first line

pause 1000

serout lcd,baud,(255,%00000100) 'backlight off

Neil
 
Top