maximum interrupt frequency? GPIO vs GPIOTE

Question

Hi folks, 
 What's the maximum interrupt frequency the nRF51822 can support? I am looking to do an interrupt burst of about 4000 interrupts at a rate 2MHZ or 4MHz. In the ISR, I just want to read 8 GPIO input lines simultaneously. 
 Is this possible? Would I be better served with raw GPIO vs GPIOTE? I am thinking the GPIOTE driver will add some overhead? 
 I will be using a s110 soft device, but i could turn that off during this phase if it helps. 
 Thoughts? 
 Thanks 
 -ujwal

RK · Accepted Answer

You're not going to get near that. 2MHz is 8 cycles (clock is 16MHz). The interrupt latency of the Cortex-M0 is 16 cycles, that's 16 to stack and start the handler, and 16 to return from it afterwards (ignoring tail chaining). So even if your ISR did nothing at all your maximum interrupt frequency is every 32 cycles (22 with tail chaining) so 1/2 MHz. 
 Even 'just reading 8 GPIO lines simultaneously' would require loading a register with the address, (2 cycles), loading the data (2 cycles) plus, if you wanted to store it, another 4 cycles plus probably another 6 to increment the store-to address and test if you're done. That's 14. You can't just be reading it, you must have something, however minor, to do with the data. 
 At 4MHz you have 4 cycles, which isn't enough time to do anything at all. 
 With a piece of hand-crafted assembler you might, just, be able to read your 8 GPIOs plus the clock line (the one I assume you're hoping to use as an interrupt), detect it going low-high and store the rest of the data at .. possibly 1MHz .. just. 
 And all this is entirely incompatible with any softdevice or any other thing going on on the chip at all.

maximum interrupt frequency? GPIO vs GPIOTE

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