Keep PIOs running during 'lightsleep()' to help power saving

[mungewell]

In reading the rp2040 datasheet I see that it is possible to keep the PIO functional during the lightsleep() mode; this does require that the SYS_PLL is kept running, but DOES offer significant power saving for projects where the PIOs (rather than the CPU) are performing workload.

This simple blink.py script shows that the PIO are normally stopped during lightsleep().
blink.py.txt

However with these patches to microPython/Pico-SDK, the PIO clocks/blocks(s) are kept running.

20240313_mp_keep_pios_running.patch.txt
20240313_sdk_keep_pios_running.patch.txt

The power saving for this? Running 3.8V into the VSYS pin.

Before patches : 20mA, then 3mA, then 23mA (noting CPU freq is reset to default)
After patches : 20mA, then 8mA, then 20mA

So overall a saving of 12mA, when the CPU does not need to be enabled, but whilst still flashing the LED.

I would suggest that the call to lightsleep() be extended to lightsleep(time_ms, sleep_en0_bitfield, sleep_en1_bitfield).

Edit:
Here's a demo of the 'magic' happening. The Pico+OLED is running at 13mA from 3.8V whilst in the 'power save' mode!
https://youtu.be/YZEwyyycofg?si=Siods6kGUAUzKczd

[mungewell]

Here's a more complicated example, structured in a way closer to my own project's (Pico-Timecode) implementation.

blink_fifo.py.txt

This uses the FIFOs; where the duration of the PIO loop (representing 1 frame of video), and whether the LED should flash, is pushed into the FIFO ahead of when it's actually used. The PIO is timed according to it's clock divider, and 'plays' out the frames in sequence, creating a '1PPS' signal on the LED.

Of note is that the lightsleep() call is of 4 frames duration, and relies on the FIFO being accessed by the PIO whilst the CPU is asleep.

Power savings?

Again with 3.8V into VSYS pin... in the first 10s (regular utime.sleep()) we have 21mA draw, after 10s (lightsleep()) we have 8mA draw.

Average measurements read from my PSU, instantaneous may be different - we expect a pulse every 4 frames as CPU re-fills the FIFOs.

[mungewell]

Updated patches:

20240314_mp_keep_pios_running.patch.txt

20240314_sdk_keep_pios_running.patch.txt

These now allow you to specify which clocks you want to keep active when calling lightsleep().

CLOCKS_SLEEP_EN0_CLK_SYS_PIO1_BITS = 0x00001000
machine.lightsleep(10000, CLOCKS_SLEEP_EN0_CLK_SYS_PIO1_BITS)

Not sure how you can reference the #defines of the SDK from python script....

[mungewell]

Patch suggested for/to Pico-SDK:

raspberrypi/pico-sdk#1668