I am using LPC11C14 board with 12Mhz crystal. Most of ASM instructions are 1 cycle, with entering C function takes 3 cycles and exiting takes 3 cycles.
__asm void test () { //3 cycles MOVS R1, #1 //1 cycle SUBS R1, R1, #1 //1 cycle CMP R1, #0 //1 cycle BEQ endf //3 cycles endf BX lr }
So this function should take exactly 12 cycles which means it should take 1us to execute, but if I use oscilloscope and put gpio turn on-off i get around 3-4uS toggle. Although simulator says that for gpio set/clear it takes 10 cycles, it is still missing some 20 cycles.
LPC_GPIO0->DATA |= (1<<0); test (); LPC_GPIO0->DATA &= ~(1<<0); test (); LPC_GPIO0->DATA |= (1<<0);
I guess issue is somewhere in crystal settings or ? I was looking at settings for main clock, but I couldnt figured out whether it is defaulted to IRC oscillator and how to change it to use external crystal.