Can I disable Interrupts on a BBB for a short duration (0.5ms)?

Question

I am trying to write a small driver program on a Beaglebone Black that needs to send a signal with timings like this:

I need to send 360 bits of information. I'm wondering if I can turn off all interrupts on the board for a duration of 500µs while I send the signal. I have no idea if I can just turn off all the interrupts like that. Searches have been unkind to me so far. Any ideas how I might achieve this? I do have some prototypes in assembly language for the signal, but I'm pretty sure its being broken by interrupts.

So for example, I'm hoping I could have something like this:

disable_irq();
/* asm code to send my bytes */
reenable_irq();

What would the bodies of disable_irq() and reenable_irq() look like?

Answer 1

The calls you would want to use are local_irq_disable() and local_irq_enable() to disable & enable IRQs locally on the current CPU. This also has the effect of disabling all preemption on the CPU.

Now lets talk about your general approach. If I understand you correctly, you'd like to bit bang your protocol over a GPIO with timing accurate to < 1/3 us.

This will be a challenge. Tests show that the Beaglebone black GPIO toggle frequency is going to max out at ~2.78MHz writing directly to the SoC IO registers in kernel mode (~0.18 us minimum pulse width).

So, although this might be achievable by the thinnest of margins by writing atomic code in kernel space, I propose another concept:

Implement your custom serial protocol on the SPI bus.

Why?

The SPI bus can be clocked up to 48MHz on the Beaglebone Black, its buffered and can be used with the DMA engine. Therefore, you don't have to worry about disabling interrupts and monopolizing your CPU for this one interface. With a timing resolution of ~0.021us (@ 48MHz), you should be able to achieve your timing needs with an acceptable margin of error.

With the bus configured for Single Channel Continuous Transfer Transmit-Only Master mode and 30-bit word length (2 30-bit words for each bit of your protocol):

To write a '0' with your protocol, you'd write the 2 word sequence - 17 '1's followed by 43 '0's - on SPI (@48MHz).

To write a '1' with your protocol, you'd write the 2 word sequence - 43 '1's followed by 17 '0's - on SPI (@48MHz).