I don't suppose there's any way to investigate that without an oscilloscope? I'm keeping an eye out for one, but it seems like even used, good ones cost well over $300.

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"The key is not to let the hardware sense any fear." - Radical Brad

My current set up uses 11 pins for the LCD interface. Inevitably, in Ben Eater circles, people want to free up some pins by switching the LCD into 4-bit mode, which is an unholy pain. I was thinking that I have a whole unused shift register right there on the 6522, and I ought to be able to spit out data from the CB2 pin, latch it with a 74HC595, and have the LCD read from that, thus gaining 7 pins at the cost of one logic chip. (I still need a few pins for control signals). However, it's not working. At all. Maybe it's my relative inexperience, but I find the 6522 VIA data sheet to be unhelpfully laconic when it comes to using the shift register. Before doing any deeper troubleshooting, I just wanted to double-check that my understanding of how the 6522 works is correct.

I think that's all I should need to do to start spewing bits out of CB2; Although the data sheet says only - "In mode 6, the shift rate is controlled by the phi2 system clock (Figure 27)." - examining Figure 27 seems to indicate that I should also be getting a clock output on CB1 equal to phi2 * 1/2. My little multimeter has a clock setting that detects the system clock on the bus just fine, but when I attach it to CB1 it doesn't blink. What am I missing?

_________________
"The key is not to let the hardware sense any fear." - Radical Brad

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks Garth! Just to double check, bit 2 of the ACR needs to be 0, because if it is 1, it will enable latching on Port B, which expects to use CB1 and CB2 as handshake lines, rather than serial clock and data?



I do have a strobe for the 595 later on, in the subroutine for sending a byte to the LCD; I wanted to make sure my basic conception of 6522 shift register operation wasn't flawed before I looked any deeper. I'll go make sure that bit 2 of the ACR is 0 before I do anything else.

_________________
"The key is not to let the hardware sense any fear." - Radical Brad

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Oooh, right. Logic error! Thanks for spotting that.

Also, on the subject of "read the fine print," turns out the 595 timing is all high-going, but the 6522 SR is low-going. I added an inverter, and everyone is much happier now. All the right LEDs are lighting up in my simplified testing circuit. Time to go back to the LCD!

_________________
"The key is not to let the hardware sense any fear." - Radical Brad

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Studying your schematic, I think that you never check the busy status of the LCD. I'm guessing that means you use software timers in your FORTH code to make sure you wait long enough between bytes. Do I have that right?

Also, looking at the way you have the latch clock connected, I suspect you wait until all 8 bits are shifted into the 595, then use one pulse on PORTB7 to latch the whole byte at once. That is a cool idea. I tied those two together so that the output register was always one bit behind the shift register, as described in the datasheet. I think I have been overcomplicating things. I'll have another go at it this this afternoon.

_________________
"The key is not to let the hardware sense any fear." - Radical Brad

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Which pins on the VIA are being used in SR mode to drive the CLK, DAT, and LAT inputs on the '595, please? Can the CLK pin be re-tasked to output a '0' or a '1' after shifting 8-bits into the '595 shift-register?

I've used two or three pins to drive a 74HC595 or 74HC164 and an HD44780 type LCD display in 8-bit mode. Basically, one line is used to both latch the '595 outputs and strobe the LCD 'E' input and the CLK line is re-tasked to drive the LCD 'RS' (Register Select) input after loading 8-bits into the '595 shift-register. I'm just not sure how you might re-task the CLK line in VIA SR mode. An Arduino demo' is attached...

Good luck. Have fun...

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Perhaps you could just use CB2 plus two port pins and bit-bang the driver and enjoy using the LCD 8-bit interface mode? Also, there may be an advantage using the 74HC164 over the 74HC595. That is, the ability to load 8-bits of data onto the outputs without sending it to the LCD which might come in handy for driving column or row lines on a multiplexed keypad or display (???).

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

There are also serial LCD backpacks that might be driven by a single VIA pin (bit-banged serial). I created one long ago using an 8-pin 12F683 PIC (see below) but a more up-to-date version might use an 8-pin 16F15313 ($ 1.03 @ Mouser) which can be programmed using a relatively inexpensive Arduino Nano clone.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?