Electronics > Microcontrollers

ILI9341 LCD driver, max SPI clock speed?

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Chris C:
I'm using an ILI9341-based LCD module, and the 4-wire SPI interface to communicate with it.  According to the datasheet's spec for this interface, I should be able to write at up to 1s/100ns(twc)=10Mhz, and read at up to 1s/150ns(trc)=6.66Mhz.

However, I am successfully writing and reading at 24Mhz!  Not just for a simple test case, either.  I'm doing complex graphics, mixed reads/writes for alpha blends, both 16-bit and 24-bit pixel writes, and so on - yet haven't seen a single glitch.  (Except those caused when I bump my dodgy old breadboard or jumper wires.  That I'm using such poor connections, and it's still working at this speed when not physically disturbed, make this even more amazing.  The signal integrity must be horrible.)

In the past I've tried communicating with other complex SPI devices at speeds past their ratings, just to see what would happen.  All started showing issues with modest increases, even when run at max Vdd.  Yet I'm reading from this at 360% of its max rated speed!  And I know of cases of others doing the same with their ILI9341-based LCD, as well.

All of which really makes me question whether I've read the spec and converted from ns to Mhz properly.  Did I simply make a stupid mistake?  Here's the relevant page from the datasheet:

madires:
I'm reading the ILI9341's datasheet right now (going to add support to the Transistortester) and I think your calculations are correct. A quick web search lists some blog postings claiming that a 24, 36 and even 48 MHz SPI clock runs fine.

Chris C:
Hmm, odd.  I wonder why they underrated it so much.  Or if individual variation between ICs is incredibly large.

Or perhaps that rating applies only at the lowest core voltage.  In looking over the initial register setup provided with the module, I noticed what appears to be an option to change that voltage, and that it's being set one step short of max.

BTW, if you're talking about the same Transistortester that I think you are, that's an amazing project!  And one I will be benefiting from shortly, if my package ever shows up. :)  I'm familiar with most of the ILI9341, having written my own PIC-based driver for the ILI9341 for it.  Should you run into any difficulties, I may be able to help, and will certainly be willing to try.

DavidMenting:
I was pleasantly surprised by this fact as well. I'm running 16 of them at the same time at a bit over 30 MHz in this art installation:

Chris C:
That's a wonderful zoetrope!  Thanks for sharing it.

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