Hey everyone. I recently built a 6502 computer using ben eater's videos. I got everything working on the breadboard with ram, rom, 6522 and 16x2 LCD so I figured I'd try to do something on my own. I bought a 128x64 LCD that uses an ST7920 chip and due to the fact that I've only got 5 pins left on the VIA chip I figured I'd try out bit banging SPI to communicate with it. I found this post from a while back viewtopic.php?p=45555 where Dr Jefyll shows some methods for bit banging SPI using the VIA and I've been trying to use some of that code. I'm pretty new to assembly and electronics so I decided I would try to just get something working first then try to optimize from there.

From what I can tell from reading the data sheet (https://www.lcd-module.de/eng/pdf/zubeh ... hinese.pdf) each data frame consists of 3 bytes:
1.) Sync bit - 5 1's followed by RW, RS and an additional zero to complete the byte (last 3 are 0's in my case)
2.) Most significant nibble followed by 4 0's
3.) Least significant nibble followed by 4 0's

I have my LCD hooked up like this:
VCC -> Connect to 5V
GND -> Connect to GND
RS -> Connect to 5V
RW (MOSI) -> VIA PA0
E (CLOCK) -> VIA PA1
PSB -> Connect to GND
RST -> Connect to 5V
A (or BLA) -> Connect to 5V
K (or BLK) -> Connect to GND

Here is the code I'm using to send a SPI byte:


and then I'm using it like this:



The data sheet said to wait 72 microseconds between commands, so I've added this delay code. Most likely overkill but it is what it is.



I'm expecting to see a cursor or something, but there is no change in the display at all. I'm not really sure what to do to debug this. Do you guys see anything wrong in my code?

Hi,

I think you are confusing some things.

When using SPI you need to know the following:
MOSI (Master Out Slave In) is the bit output from CPU to the LCD, MISO (Master IN Slave Out) is the bit input from the LCD to the CPU. CLK is the shift clock output to the display, CS is the chipselect for the LCD SPI port.
Bitbanging SPI isn't so difficult, here a code example (it's 6809 assembly, but must easy be portable to 6502 assembly).
This code is for sending 16 bit to a 7 segment display array controlled by SPI. PPORT is the I/O port I use (A simple 74574 latch). PORTBUF is a variable where the last status of pport is stored. D is a double accumulator containing A and B, in 6502 assembly you probably use psha and pshb for saving both accumulators.


I hope this will help you. Unfortunataly I am not so in 6502 assembly yet. For the LCD probably you need to set CS to zero while sending the bits and rise it to one when all bits are send.

_________________
Greetings,

Nick

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Thanks for the response. Could you possibly clarify on what I seem to be missing. Reading through your post, I feel like that was already my understanding. Is there something that I have wired incorrectly? This particular LCD only takes input in SPI which is why I’ve only hooked up MOSI and the clock. Did you see something wrong with the logic in my code?

Thanks.

Is this correct? The code is expecting the clock on bit 0, not bit 1.

Clear needs a 1.6ms delay. Normally you'd read the status to see when it's ready for another command, but it looks like serial is write-only. You'll have to assume the worst case delays.

The flowcharts include a much longer (40ms) power-on delay, and different delays after each of the initialisation commands. This isn't an HD44780, but it looks like the interface is compatible with it. When I've had to deal with HD44780's I've always followed the flowcharts to the letter, and never had any trouble. Trying to take short-cuts has usually gone wrong.



This will disable the cursor, so you shouldn't be expecting to see one. Change it to 00001110 (or 00001111 for blinking).

Good catch on the wiring. I actually do have the clock connected to bit 0 on port a and MOSI connected to bit 1 on port A. That was a typo on my part. I suppose I should look closer into the delay. I'm not really sure what is the best way to delay for exactly 40ms, 1.6ms and 72 microseconds. I guess I'll need to do some calculations on my clock speed and cycle counts and come up with some amount of work that takes that amount of time. Is that how that is normally done?

Looks like you're correct about disabling the cursor. I'll try the byte you've provided tonight and see if there is any change.

My SPI display has in SPI the following lines: CS, CLK and MOSI. Without CS the display won't work. I assumed that it is the same as on your display. The delays in write only mode are needed as John West told already.
I hope after changing the instruction given by him and using the given delays it will work.

_________________
Greetings,

Nick

Welcome, instantaphex. Yes, normally for a delay you use a loop which counts down, or two nested loops can be better for a longer delay. Maybe this thread will help. (There's a wealth of experience in previous threads, so be sure to search.)

Just tried this and saw a blinking cursor... thank you!! I've literally been at this like an idiot for 4 nights like an idiot writing the first bit of code for this machine that wasn't modified from a tutorial, desoldering headers from random things I had lying around, poking around with a multimeter and pulling my hair out that it didn't work. Looks like my delay code is long enough for the worst case scenario for the commands I've used so far. I really appreciate your help.

Glad it's working! And yes, your delay will be more than comfortably adequate, to the point of being far too slow for displaying graphics (somewhere over 300,000 cycles if I'm counting them right, so depending on the clock speed it'll be tens or hundreds of milliseconds). But that's OK. It's better to be slow but working than fast but not. It's a lot easier to make improvements when you can test each change.