Implementing real-time embedded system to sample from five SPI ADCs

Hello,

I would like to implement a real-time embedded Linux system on an AT91SAM ARM processor. Attached to the SPI bus of this processor are 5 Analog-to-Digital converters (ADCs) that can be read using the Linux SPI bus subsystem.

The ADCs are sampled by having the AT91SAM ARM processor toggle a GPIO at a rate of at least 1 KHz. The GPIO is attached to a pin on the ADC. A high voltage level on this pin instructs all of the ADCs to take a sample. The sample is then read over the SPI bus by the embedded system master. This implies that data will be read from each SPI slave device every (1 / 1000 Hz) = 1.0e-3 seconds.

I am wondering if the following could be done with real-time Linux:

(1) I would have to write a Linux kernel module to toggle the GPIO pin. The rate of toggling would be controlled by a high-resolution timer, and would occur with a period of at least 1 kHz.
(2) After the GPIO is brought to a high voltage level, the master must wait for at least 3.7 microseconds before reading from the SPI bus.
(3) Each ADC would be read in sequential order.

The ADC being used is the AD7691 from Analog Devices (http://www.analog.com/static/imported-files/data_sheets/AD7691.pdf).

The data from the ADCs could be used in a real-time control system loop. Can this be done with real-time Linux, or is the latency too high? What is the maximum rate at which I could read data from these ADCs? Could it be 10 kHz or even 100 kHz?

Nicholas

Hi Nicholas,

according to what I understood, you want to use the data coming from your ADCs in a userspace application which implements the control loop.

I think there is a good chance you can then do all the steps (1)-(3) within an userspace application. However, I am not 100% certain about the best solution to implement (2) on this setup. May be anybody else knows something about the maximum sample frequency, but this also depends on how complicated your algorithms are.

Here [1] you can find a good example on how to write a RT userspace application. Using the SPI character devices it should also be possible to talk to your hardware.

Regards,
Andre

[1] https://rt.wiki.kernel.org/index.php/RT_PREEMPT_HOWTO

Thanks, Andre; that is a very good link, and thank you for the information. I am hoping that I can sample from the ADCs at a rate of at least 1 kHz, and I am wondering if it is best to do this in a userspace program or in a kernel driver.

My processing algorithms are not very complicated. If one of the ADC data values is greater than a certain value, then a digital-to-analog converter (DAC) control register is updated. All that I want to do is to collect data at a rate of 1 kHz by toggling a port pin and then reading the data values over SPI.

Nicholas