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RA8875 Not Asserting DISP Signal for 7" TFT
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RA8875 Not Asserting DISP Signal for 7" TFT

by clroche1 on Tue Jul 07, 2020 5:49 pm

Hi all:
I've been trying to use the Adafruit RA8875 controller with the Adafruit 7" TFT display. I'm using a PIC24 to drive the controller so I meticulously converted the INIT code in the .cpp Library to PIC assy but could not get the display to turn on. After hours of troubleshooting, I resorted to probing the signals on the 40 pin FFC connector and discovered that the DISP signal on pin 31 (DISPLAY ON/OFF) is not tied to VCC (or being driven to VCC). When I manually drive pin 31 HIGH (100 Ohm resistor to 3.3V) it magically works (FYI, I issue the command for REG x01 to turn it ON but it has no effect on the DISP signal). I can't find any signal out of the RA8875 IC itself for this function and I found a document that indicates this must be 'tied' HIGH. So the question is...Is the RA8875 board supposed to tie this signal to 3.3V or control it? . Thanks in advance.

clroche1
 
Posts: 3
Joined: Tue Jul 07, 2020 5:28 pm

Re: RA8875 Not Asserting DISP Signal for 7" TFT

by clroche1 on Wed Jul 08, 2020 4:45 pm

Replying to self in order to update the post:
The RA8875 data sheet generally ignores the DISP signal except in one place: Page 78/178 shows a schematic that connects GPIO0 to a TFT signal labelled DISP_OFF. Of course there are no signals named GPIO0 or DISP_OFF but presumably they mean GPO0 and DISP. In any case I probed the Adafruit circuit board and there is no connection from the GPO0 pin (pin 32 on the IC) to pin 31 on the 40 pin FFC. The GPI/GPO pins are associated with the key scanning feature in the data sheet, so if applicable, the GPO0 pin would need to be set in the key scan register group.

Also, I mentioned that turning the display ON/OFF via register 01 had no effect. Actually that's not true. Since I got it working with my jumper, turning it OFF apparently kills the TFT timing signals because it definitely goes dark. It's just not being done via the DISP signal.

Bottom line is this seems to be an omission on the Adafruit circuit board (Hold DISP HIGH). But how could this be the case though as other users would have encountered this same problem?

clroche1
 
Posts: 3
Joined: Tue Jul 07, 2020 5:28 pm

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