Add LowPower library

[maxgerhardt]

My attempt at making a nice low power library, that works for RP2040 and RP2350. Will close #2528, related to #345.

Adds a modified version of pico_sleep that removes some stuff (like STDIO reinit) that we can't just do in this core.

Currently working on RP2040 with a Pico board, work in progress.

Current state:

• with all unused GPIOs set to PULLDOWN: 2.08mA
• disabling input gate (gpio_set_input_enabled(p, false)) : 1.87mA
• using INPUT mode instead: 1.58mA
• disabling SWDCLK, SWDIO pins (magic pin number 30 and 3): 1.51mA
• turning off the brown out detector: 1.48mA
• changing the regulator for the digital voltage from 1.10V dow to 0.8V: 1.35mA
• powering down the USB memory: no effect, 1.35mA
• noticed that I was connected my power meter to VBUS and giving it 3.3V here, so all measurements above were actually at 3.3V
• changed to supplying 5V to VBUS: 1.22mA
• change to supplying 5V to VSYS pin (bypasses diode between VBUS and VSYS): 760µA!! (0.76mA at 5V)

ToDo and notes:

• make management of GPIO pins before sleep and after wakeup easy, provide good defaults for a e.g. Pico board
• GPIO pin leakage minimization is important
• turn off as much digital logic as possible to minimize static power dissipation; in DORMANT mode wiht GPIO wakeup, all clocks are off, so dynamic power dissipation is 0

[maxgerhardt]

Powering it from the actual 3V3 pin with my power meter gets the current to 869µA @ 3.3V, which is less Watt than 750µA @ 5V (3750µW vs 2870µW).. so bypassing the regulator gives good savings, too, if it can be done.

[earlephilhower]

Very neat! Do any clocks or subsystems need to be reinitialized after the sleep modes? I'm thinking of things like USB or UARTs, PWM, DMA, etc.? Or do they just preserve their state and freeze clocks so that they just jump back to life afterwards?

[seamusdemora]

So looking forward to seeing this released :)