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Thread: Adafruit_ST7735 SPI with DMA

  1. #1

    Adafruit_ST7735 SPI with DMA

    I've searched the forum, but haven't been able to find threads mentioning SPI+DMA for the smaller Adafruit_ST7735 TFT.

    It's working well with my Teensy3.2, but I'm looking for a speed boost for whole-screen redraws. This is an example of another TFT/Controller being used with SPI+DMA .. https://github.com/marekburiak/ILI93...LI9341_due.cpp

    Could someone please point me in the right direction for how to get started doing this?

    Do I need to grab something like crteensy/DmaSpi, and modify a copy of the teensyduino library?

  2. #2
    Senior Member PaulStoffregen's Avatar
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    On Arduino Due, DMA is the only way to optimize these libs because Due SPI lacks buffers. If you're familiar with Due, you might believe DMA is the only way.

    Teensy 3.2 has a FIFO which integrates with the hardware chip select signals. Generally, these libraries are optimized using the FIFO and hardware chip selects. For many small write, this approach is much better than DMA. For large areas, it works out only slightly faster than DMA.

    DMA still might be useful for building a non-blocking update to a large region of the screen. But to make effective use, you'd need to do other non-SPI stuff while the screen is updating. If you just wait for the screen to update, using the FIFO is simpler and has lower overhead.

  3. #3
    Senior Member sumotoy's Avatar
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    I have a SPI fifo version for TFT7735 here, maybe take a look.

  4. #4
    Quote Originally Posted by PaulStoffregen View Post
    On Arduino Due, DMA is the only way to optimize these libs because Due SPI lacks buffers. If you're familiar with Due, you might believe DMA is the only way.

    Teensy 3.2 has a FIFO which integrates with the hardware chip select signals. Generally, these libraries are optimized using the FIFO and hardware chip selects. For many small write, this approach is much better than DMA. For large areas, it works out only slightly faster than DMA.

    DMA still might be useful for building a non-blocking update to a large region of the screen. But to make effective use, you'd need to do other non-SPI stuff while the screen is updating. If you just wait for the screen to update, using the FIFO is simpler and has lower overhead.
    It's not apparent from GitHub, but did the official Adafruit_ST7735 library get the improvements from your fork? They seem to have incorporated SPI transactions at some point, but I'm not certain how I could check if they take advantage of Teensy3.x FIFOs?

  5. #5
    Quote Originally Posted by sumotoy View Post
    I have a SPI fifo version for TFT7735 here, maybe take a look.
    How does your library take advantage of the Teensy SPI FIFOs?

  6. #6
    Senior Member+ KurtE's Avatar
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    SumoToy can answer this a lot better than I can as I have not used the TFT7735, but have done a reasonable amount of hacking on the ILI9341 display, and while testing some stuff for the new beta Teensy board, I played with his library some and know that it uses the same stuff that Paul did for the ILI9341 display.

    You might try looking through the postings on the ILI9341 display if you wish to get complete details, but I will take a shot at it:

    There are a couple of main features with keeping the display going as fast as possible, but it all boils down to keeping data going out on the SPI queue with no delays between the bytes and minimal delays between commands and Paul did a great job on this.

    If you do not have a fifo, your code typically when it want's to output something to the display, spins waiting for the SPI output register to be empty, and then puts a byte out on the queue. This will add some amount of delays. On many processors including the T3.2 on SPI1, you at least have some form of double buffering. It may say it has a queue size of 1 or it may say there are actually two registers, the output register and the shift register... With these you can very often minimize this part of the delay.

    However with T3.2 on SPI (SPI0) the queue is more than just a simple queue for outputting the byte (or word), with it you can also encode information on how this command should change the Chip select pins associated with the SPI device. So again with the ILI9341 display there are two other signals other than Clock/MISO/MOSI, which is CS (Chip Select) and DC (Data/Command).

    When you are talking to the display, you need to Assert the CS pin in order to tell the SPI device the data is for it. With the ILI9341 you also need to say which byte is the start of a new command. With most display code you will see the code does something like:
    <Wait until SPI is empty>,<Assert CS>, <Assert DC>, Output command byte, <wait until SPI is empty>, <Deassert DC>, output data bytes... <wait for SPI empty> <Deassert CS>.

    Which introduces delays between bytes, as we we wait for something to complete, before we can change the other two signals with some form of digitalWrite command... With these drivers, we encode the appropriate changes to the CS/DC pins as part of the command going out to the SPI, which is where the big win is.

    But to do this, you need to make sure CS and DC are on pins that can be controlled by SPI0. These are shown in several places, including the page for the ILI9341 display that PJRC sells: http://www.pjrc.com/store/display_ili9341.html

    Hope that helps.

  7. #7
    Thank you. The SPI write sequencing makes sense and everything you mention about the ILI9341 seems to be applicable to the smaller TFT7735, including the chip-select and data-command pins.

    I'll try and resolve the differences between Paul's branch and the original. They've diverged a bit, but a bunch of the difference is just example code. I might be able to consolidate both branches to get a clean merge..

    Is it worth going through that merge exercise? I think it would mean the teensyduino TFT7735 library would be up-to-date with the current SPI optimisations on Teensy3.2. Or might it be better to bring sumotoy's library into teensyduino as it stands?

  8. #8
    Senior Member sumotoy's Avatar
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    The TFT7735 library uses the same SPI FIFO of ILI9341, it uses the 4 byte buffer for SPI0 and of course is really fast, expecially with small displays.
    With Teensy 3.2, speed overclock and SPI fifo I really reach the ST7735 limits (and probably worked out of his parameters).

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