derekfountain / pico-overclock

Small piece of research into overclocking a RP pico

pico-overclock

This is a small example of overclocking a Raspberry Pi Pico. I wanted to check that the overclock was working and that my assumptions were correct.

The code contains a loop which pauses for 20 NOP instructions, the raises a GPIO, then pauses 20 more NOPs, then lowers the GPIO, then jumps back to repeat. It compiles to this:

200001ac:	46c0      	nop			; (mov r8, r8)
200001ae:	46c0      	nop			; (mov r8, r8)
200001b0:	46c0      	nop			; (mov r8, r8)
200001b2:	46c0      	nop			; (mov r8, r8)
200001b4:	46c0      	nop			; (mov r8, r8)
200001b6:	46c0      	nop			; (mov r8, r8)
200001b8:	46c0      	nop			; (mov r8, r8)
200001ba:	46c0      	nop			; (mov r8, r8)
200001bc:	46c0      	nop			; (mov r8, r8)
200001be:	46c0      	nop			; (mov r8, r8)
200001c0:	46c0      	nop			; (mov r8, r8)
200001c2:	46c0      	nop			; (mov r8, r8)
200001c4:	46c0      	nop			; (mov r8, r8)
200001c6:	46c0      	nop			; (mov r8, r8)
200001c8:	46c0      	nop			; (mov r8, r8)
200001ca:	46c0      	nop			; (mov r8, r8)
200001cc:	46c0      	nop			; (mov r8, r8)
200001ce:	46c0      	nop			; (mov r8, r8)
200001d0:	46c0      	nop			; (mov r8, r8)
200001d2:	46c0      	nop			; (mov r8, r8)
200001d4:	6162      	str	r2, [r4, #20]
200001d6:	46c0      	nop			; (mov r8, r8)
200001d8:	46c0      	nop			; (mov r8, r8)
200001da:	46c0      	nop			; (mov r8, r8)
200001dc:	46c0      	nop			; (mov r8, r8)
200001de:	46c0      	nop			; (mov r8, r8)
200001e0:	46c0      	nop			; (mov r8, r8)
200001e2:	46c0      	nop			; (mov r8, r8)
200001e4:	46c0      	nop			; (mov r8, r8)
200001e6:	46c0      	nop			; (mov r8, r8)
200001e8:	46c0      	nop			; (mov r8, r8)
200001ea:	46c0      	nop			; (mov r8, r8)
200001ec:	46c0      	nop			; (mov r8, r8)
200001ee:	46c0      	nop			; (mov r8, r8)
200001f0:	46c0      	nop			; (mov r8, r8)
200001f2:	46c0      	nop			; (mov r8, r8)
200001f4:	46c0      	nop			; (mov r8, r8)
200001f6:	46c0      	nop			; (mov r8, r8)
200001f8:	46c0      	nop			; (mov r8, r8)
200001fa:	46c0      	nop			; (mov r8, r8)
200001fc:	46c0      	nop			; (mov r8, r8)
200001fe:	61a2      	str	r2, [r4, #24]
20000200:	e7d4      	b.n	200001ac <main+0x50>

That's 44 machine cycles per iteration, and the GPIO can be seen on a 'scope toggling at a constant frequency.

Here's a 'scope image from a Pico running at the standard 135MHz:

Content	ns per cycle	Cycles	Total Cycles
40 NOPs	7.4074	40	296.296
2 GPIOs	7.4074	2	14.8148
Branch	7.4074	2	14.8148

325.9256ns, 'scope says 326.0ns

...and at 200MHz:

Content	ns per cycle	Cycles	Total Cycles
40 NOPs	5.0000	40	200.000
2 GPIOs	5.0000	2	10.0000
Branch	5.0000	2	10.0000

220.0000ns, 'scope says 219.6ns

...and at 270MHz:

Content	ns per cycle	Cycles	Total Cycles
40 NOPs	3.7037	40	148.148
2 GPIOs	3.7037	2	7.4074
Branch	3.7037	2	7.4074

162.9628ns, 'scope says 163.2ns

...and at 360MHz:

Content	ns per cycle	Cycles	Total Cycles
40 NOPs	2.7777	40	111.108
2 GPIOs	2.7777	2	5.5554
Branch	2.7777	2	5.5554

122.2188ns, 'scope says 122.0ns

So the overclocking appears to work and the timings are as expected.