1.8" vs 2.2" TFT display

[tef]

I'm interested in knowing how the new 2.2" Adafruit TFT display compares with the 1.8" TFT display. Beyond the obvious size and pixel count differences, are there much differences in things like communication speed with the display IC, power consumption, visibility in daylight, limitations of the display IC, or other differences. Any comments would be appreciated. These are both competitively priced displays but I'd like to know about potential non-obvious differentiators.

[adafruit]

they're nearly the same, the 2.2" uses ~60mA instead of ~40mA, both use SPI so are about the same speed. They are TFT so not daylight-readable. much more details are available in the datasheets for the display modules.

[kscharf]

The 2.2" display doesn't require the additional "R/S" or register select address line. Instead the SPI interface must transfer 9 bits instead of 8 and the extra bit is the "R/S" function. This does require that the driver bit bang one additional bit as the SPI hardware must transfer 8 bits by default. I suppose the driver could also do two 8 bit transfers with the first transfer having 7 "don't care" bits set to zero that end up being shifted off the end of the world but would that be any faster? If you use the SD card interface you must then provide the "R/S" signal for that as well as an additional CS line.

[tef]

That's a good point about the register select line. The other implication of this, at least with the current implementation of the Adafruit_HX8340B library, is that writing each data byte also requires an additional function call and return in addition to bit banging of the R/S bit. In the Adafruit_ST7735 library, the low-level data output is done with spiwrite(), which is inline code and thus optimized for speed. Also I note that none of the "Fast" library functions have been implemented for the HX8340B, so that things like fillScreen(), fillRect(), drawRect(), fillCircle(), drawFast*Line(), drawBitmap(), etc. are all going to be slower. I started to count the differences in the number of ATmega instructions for the two display drivers for the most common low-level I/O like drawPixel() [which typically gets called a bazillion times], but decided a better approach is just to time things empirically. Haven't done that yet though.

[pburgess]

Hi tef,

I made some changes to the Adafruit_HX8340B library just a few days ago, which also included adding the 'fast' functions. Check it out, this might partly address the performance concerns.

[tef]

Nice work, pburgess! This definitely speeds up common operations on the 2.2" display.