adcclock-speedmicrocontrollerspi
How can I determine the clock speed requirements of a mCU to manage a given ADC input?
This application is very simple – I just want to take an SPI stream from an ADC into a mCU and spit it out the mCU. No major DSP happening or anything like that on board. [added] The mCU is ultimately there to push the video out a USB port.
In this scenario I have a 40,000 kSPS (40 MSPS) 10bit ADC that I need to push through a 16 bit micro. Do I use a 40MHz micro? Can I use a slower micro?
Except for the overhead it takes for the Start and Stop conditions, acknowledge bits, and device address plus R/W bit, they are the same.
So the more data that is sent per message, the closer the data rate will be to the clock speed. For very short messages (one address byte and one data byte), the data rate will be a little less than 50% of the clock rate. For very long messages, it will be more like 90%.
There was a bug... I was setting the single shot mode for the ADC DMA twice instead of for the TC DMA channel.
dma_channel_set_single_shot(&dmach_adc_res1_conf);
Best Answer
Your question suggests that the SPI is the input from the ADC. However, the ADS825E is a parallel output device. So, is the SPI the output from the microcontroller? I'll presume it is.
You'll always need a faster clock. Even for a RISC controller running 1MIPS/MHz a 40MHz clock will allow you to execute 40M read instructions per second, but you have extra code, at least a jump instruction to create a loop. So at 80MHz you can read, and jump to the read instruction. But that's not all. If you want to shift the read data out through SPI that will need to be clocked as well. At least 10 clock pulses per read sample, to be precise. Unless you're using an asynchronous microcontroller (I don't know if they already exist outside of the lab yet) that means 400MHz. At least.
You can do this with some high end ARM controllers, but if you just want to read and shift out there are better solutions. Oli mentions an FPGA, but a CPLD will probably do.