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Topic: Arduino Due USART SPI is very, very slow? (Read 95 times) previous topic - next topic


Hi everyone,

I'm currently trying to read samples from an SD card to send them to an external DAC using the Arduino Due. The problems are:
- The built in SPI doesn't work with both the SD card and DAC, I've tried lots of different circuit configurations and software solutions nothing helped!
- So I tried using the USART SPI example, made by D. Lloyd, but my code is soooo slowww! It takes more than 5 ms to write to one of the DACs, I'm currently using an MCP4922 IC. The main culprits are the 2 loops I use between each byte, and before setting the CS high. This DAC needs a 2 byte word before CS goes high.

I can't find what is wrong with my code and I need a sampling rate of at least about 44 kHz, I'm setting the SPI CLK to 10.5 MHz, but that does nothing.



Apr 21, 2018, 05:34 am Last Edit: Apr 21, 2018, 05:42 am by ard_newbie
An example of read/write SD card via SPI, this thread reply #4:


and this with SDFat:


For an example of external DAC monitoring, Google DUE DAC SPI and you will find a .pdf tutorial

Why don't you use your buitlin DACs ?


Apr 21, 2018, 07:46 pm Last Edit: Apr 21, 2018, 07:46 pm by danny92

The SD card works with the internal DACs, if I don't use the external SPI DAC.
And the external SPI DAC (MCP4922) works without the SD card, I'm able to generate a sinewave for example.

I can't use an external DAC at the same time as the SD card I've tried it several ways , and I think it will never work, because there is interference at the SPI interface, and the SD stops reading and I get only noise on the DACs.

I'm currently using those 12 bit DACs but I will upgrade them to 16 bit DACs. I need an alternative to the SPI bus that allow a bitrate higher than 4 Mbit/s. I don't understand why I'm setting the Baud Rate Generator Register to 10.5 MHz (MCK/8) and I have to wait more than 3 ms? The problem is I need to send 2 bytes to the DAC and not one. Do you have any idea of what is causing that? And what I can do to solve this problem.



Ok, the problem was the divider, I was setting it to very large value.
Now I'm getting about 10-12 microseconds, per each dac write. I think it is a bit slow, is it possible to improve it to about 5 or 6 microseconds? Maybe with a faster digital write.


 The SPI USART is now working fine with a SCK of about 16.8 MHz, the write to dac functions are taking about 5-6 microseconds.

I also need to test the USART1, I could use it to read from an external ADC. Another option I have is to use the same USART SPI (USART0), once I'm defining the CS "manually".


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