F2803x F28035 0 Wait States in RAM with Singel Cycle Instruction Not adding up

tom prohaszka

I am running a simple test to see how fast a GPIO can toggle. I configured the F28035 for 60MHz operation (confirmed via XCLOCK OUT). I ran the loop shown below, and also show the single cycle assembly instruction being executed.. To avoid Pipeline effects, I execute over 8 GPIO Toggles in a row. The best toggle rate I get is 117nS. I tried this with and without ENPROT (write/read protection) enabled. So, based on what I'm seeing, it appears the GPIO can toggle no faster than the length of the pipeline from RAM. Am I missing something?

while(1) { GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1; GpioDataRegs.GPATOGGLE.bit.GPIO19=1;
}

ASSEMBLY CODE:

GpioDataRegs.GPATOGGLE.bit.GPIO19=1;

0x008023: 1A070008 OR @0x7, #0x0008

333 GpioDataRegs.GPATOGGLE.bit.GPIO19=1;

0x008025: 1A070008 OR @0x7, #0x0008

334 GpioDataRegs.GPATOGGLE.bit.GPIO19=1;

0x008027: 1A070008 OR @0x7, #0x0008

335 GpioDataRegs.GPATOGGLE.bit.GPIO19=1;

0x008029: 1A070008 OR @0x7, #0x0008

over 15 years ago

0 Tim King61612 over 15 years ago

Expert 2355 points

One of the issues here is that using GPATOGGLE is what is called a Read-Modify-Write command (as it reads the register, modifies the register, and writes it as well). This takes one pipelined cycle however using it back to back does cause pipeline stalls which will slow you down by at least one pipeline per run. Thus, your multiple toggles do not prevent pipeline effects. See

http://processors.wiki.ti.com/index.php/General_Purpose_IO_%28GPIO%29_FAQ_for_C2000#Q:_How_can_I_toggle_a_GPIO_pin_as_fast_as_possible_in_C.3F

for reference.

On the other hand it should not be as low as 8 cycles for a repeated pin toggle. Have a go at placing NOPs in between each TOGGLE and see if it goes at 15MHz (one toggle every 33ns).

Tim

0 tom prohaszka over 15 years ago in reply to Tim King61612

Expert 1240 points

Thanks for the feedback. The best I was seeing was 120ns. So there is some discrepancy and I'm not sure where it is.

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F2803x F28035 0 Wait States in RAM with Singel Cycle Instruction Not adding up