AdaFruit recently released a sweet little TFT display that I was dying to hook up to a netduino: the display features a resolution of 128*160 pixels, is capable of showing 18-bit colors and has a microSD card reader on the back of the breakout board. As usual, Limor wrote a nicely detailed Arduino tutorial showing how to connect the display and how to write sketches to drive it.
The Arduino driver relies on the ability of the Atmega168/368 to toggle digital lines extremely fast, which does not work well on the netduino due to the latency introduced by the .Net Micro Framework: even when configured to use hardware SPI, the Arduino driver constantly toggles a data/command output line, rspin below, which would be unbearably slow on the netduino if the same method were applied.
The netduino has one advantage over the Arduino: it has plenty of RAM. So, instead of toggling I/O lines slowly all the time and using next to zero RAM, the netduino driver allocates a 40K buffer corresponding to the resolution of the display in 12-bit depth colors (16 bits per pixel) and leaves the ST7735 in ‘data’ mode upon initialization.
Drawing always happens on the internal buffer first. Then, whenever the actual display needs refreshing, the display I/O operations are performed using hardware SPI, blasting the entire 40K buffer. It may sound crazy but using this method on the netduino is faster than refreshing a single pixel while toggling an I/O line!
Source code and further discussion at Fabien’s blog.
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