I am using a ST7735 1.8" screen with an Arduino Nano using the Adafruit library. I understand most of what's going on and I am able to get the screen to work. The question I have is about the /CS chip select. It seems as though the /CS is "required" using the library and not any other way. I've been able to choose another Nano pin to perform this function by declaring the appropriate pin and wiring the screen to it (e.g. digital pin 14) and it works no problem. Since the /CS on the display is active low, a 0 is required to select the chip, all is good. But, when the /CS pin is wired directly to ground (0) and making the screen always selected, the screen does NOT work. Any attempt to use a pin other than the declared pin and manually trying to select the screen via a different pin does not work either. Why is it required to use the declared pin only?
What I'm trying to accomplish is to use a binary code of 3 bits (3 digital pins) and a 74LS138, 3 to 8 line decoder, to be able to select 1 of 8 different SPI modules w/o having to use 8 different Arduino pins to select each one.
I am unable to accomplish this both with Arduino and a PIC18F4550. So, is the issue with the screen and that particular chipset or with the way the SPI protocol is handled?
Could be wrong, but I think you can pass in -1 for Cs pin and then tie it to gnd…
@cmndr-brain SPI is a protocol that requires the synchronization of 4 signal pins, although for your display you may not need the MISO signal. As you have discovered you can use any spare digital pin you just don't tie it to ground and leave it if you want to share between different devices.
The display driver takes care of the synchronization or you could use the SPI library directly to send data to the display. You can split the CS signal however you want providing it stays in time with the other SPI signals.
Here is one of many articles explaining the protocol and a multi drop example, you would normally be dealing with mode 0.
@sumguy, thank you for the response and I've read the article that you have linked. But, in the article it states,
"If a single slave device is used, its SS pin may be fixed to logic low if the slave permits it. With multiple slave devices, a multidrop configuration requires an independent SS signal from the master for each slave device, while a daisy-chain configuration only requires one SS signal."
So, by that, I take it that what I'm trying to accomplish should work but, only if the slave permits. I read that as it seems to be the screen and not the protocol.
I've done a lot of work w/ memory chips and Eproms. I've never had a problem with holding the /CS lines either High or Low. So, this didn't make sense to me why it wouldn't work. After all, it's just a select line.
@KurtE, thank you for the suggestion, I'll do some more experimenting and try that (although I may have tried that already).
@cmndr-brain Interesting and something I intend to play around with when I get time.
Where the modes are listed under mode 0 it says Data is shifted out on falling SCLK, and when SS activates , it does not say while SS is activated.
I am in no way doubting what you say , the above has always been my experience so I look forward to learning something here.
I know a while ago some of us working on some ST7789 needed to allow this as there are some displays where they did not expose the CS pin.
We then added similar support to the teensy specific driver as well
It looks as though I have it working now. It seems that the screen needs to see an initial HIGH to LOW transition for it to work. I now have the chip select wire connected to digital pin 14. After configuring pin 14 to OUTPUT, I have to immediately set it HIGH. Then I manually set pin 14 LOW right before the "tft.initR(INITR_BLACKTAB);" code then HIGH right after. This seems to satisfy the screen. Now I can set pin 14 LOW again right before the actual tft.xxxx commands. At this point it seems I can just leave the manual /CS pin low and the rest of the code works. Although, as my project progresses, I will certainly have to turn it off to select another device. Must be something w/ the ST7735 chip that doesn't allow a GND always selected chip during initialization. Now, it doesn't matter what I set TFT_CS to. I'll assign it to -1 so it doesn't get assigned to a physical pin. Now I can assign 3 digital pins as the /CS pins, feed that 3 bit code to a 74LS138 to decode to any 8 selects. This will be my next experiment as time allows. Progress. I thank you guys for the suggestions. Sometimes it just takes the suggestion of someone else to trigger a thought.