How to get faster SPI with Xiao SAMD21?

[NSFW]

I’m using a Xiao SAMD21 to drive an an LCD display (ST7735R controller, 128x160) and it is almost fast enough to get the refresh rate that I want…

I have two questions:

1. What is the default SPI rate for the Xiao SAMD21?
2. Can I make it use a faster SPI data rate?

I’ve tried experimenting with requesting different data rates using SPI.beginTransaction (and even with the legacy setClockDivider API) but with no visible change in data rate.

Since the Xiao is is 48mhz clock and I’ve read that the display can do 15mhz, I am hoping that 12mhz (or /4 divider) would work. But I don’t even know what the current (default) rate is… for all I know it might already be at 12mhz.

If the default is less than 12mhz, I’d like to increase it to 12.

Or, if it’s already 12mhz by default, then that would be good to know.

Thanks!

[msfujino]

This is not SAMD21 data, but the following link may be helpful.
Even if you increase the SPI clock speed, the data transfer speed will not increase significantly if the interval between 1-byte transmissions is large.
If measurement is necessary, you can obtain similar data by uploading the sketch you want to try.

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I knew that there were differences between “mbed” and “non-mbed” in terms of maximum SPI clock frequency (mbed:8MHz non-mbed:32MHz) and port drive mode (mbed:standard drive non-mded:high drive). However, when I used an OLED display at the same 8 MHz, I found that there was a big difference in frame rate. I investigated and found that the main reason is that the 1-byte transfer time is 5.6uS for “non-mbed” and 19.5uS for “mbed”. The data transfer throughput is only 1/3.5. Be careful when using S…

The SPI clock frequency of SAMD21 is 24 MHz by default, but I set 12 MHz to match the 15 MHz specification of ST7735.

[msfujino]

It’s been a while, so I had forgotten about it, but I think the link below will be helpful.

XIAO SPI Speed slower than a UNO

I’m using SPI for a MCP4802 DAC which I tested first with a UNO and have now hooked up to a Seeeduino XIAO expecting a faster speed, given the clock is 4 times as fast but it’s actually a little bit slower! The DAC can handle up to 20MHz which is probably much more than either the XIAO or the UNO can manage. Just uploading 0 and 128 in a loop to get a square wave my scope is showing a miserable 12.7kHz, i.e. two channels of 8 bits uploaded at a rate of 25,400 per second. With the UNO, I can …