concept: low power

Author: ysoldak

content/docs/concepts/low-power.md

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+---
+title: "Low Power"
+linkTitle: "Low Power"
+type: "docs"
+weight: 35
+description: >
+  How to consume less power on microcontrollers
+---
+
+Microcontrollers are usually rather non-demanding when it comes to power consumption.
+Couple hudreds mW is the most you may expect a hobby project would use.
+
+Sometimes, even that is not low enough.
+Power your project from a battery, and it'd benefit greatly from consuming less.
+
+Common scenario for such projects is sleeping most of the time, and only rarely briefly wake up to query sensors and execute actions.
+The main caveat here is if not configured properly, microcontroller and peripherals continue to consume power while your program "sleeps".
+
+The standard way to pause execution in Go is to use `time.Sleep()` function that is available in TinyGo too. Depending on the chip and effort put into its support, the efficiency of this function may vary. The only thing certain: this is _not worse_ than busy looping until a certain time has passed.  
+Still, with such high-level function as `time.Sleep()` it is not always possible to disable everything that consumes power as the decisions may depend on the application, presense of peripherals and the board used.
+
+Most microcontrollers have one or more "low power" modes, also known as "deep sleep" and, sometimes, "dormant".  
+In such modes microcontroller may consume as low as milli- or even micro-Watts.  
+TinyGo gives you low level access to hardware to try and make most of it.
+
+## Implementations
+
+Manufacturers implement different modes, depending of capabilites of their products and name these modes differently too.
+Hence no naming consistency across microcontrollers, unfortunately.
+
+Always consult "Power consumption" part of a respective datasheet to understand capabilities of your hardware.  
+When reading datasheets, keep in mind peripherals that draw power too. These include \[power\] LEDs, various sensors or co-processors usually present on development boards.
+
+Capabilities of different chips:
+- Some very good chips (like the nRF series) can go to a very low power state (just a few µA) with RAM retention and waking up from a low-power RTC.
+- Some limited chips (like the esp32) can't really "sleep" with low power -- they can disable the core and memory and reset after a predefined time losing state stored in memory.
+- Other chips need to use a special feature to go into deep sleep mode.
+
+Especially the distinction between waking up with or without RAM is important, because it greatly affects how you write software.
+
+The common way to wake up from deep sleep is to use [interrupts](compiler-internals/interrupts), such as timer, real-time-clock or an interrupt on a pin change.
+
+### RP2040
+
+Minimal achievable power consumption is 0.18mA in "Dormant" and 0.39mA in "Sleep" modes.  
+See [RP2040 datasheet](https://datasheets.raspberrypi.com/rp2040/rp2040-datasheet.pdf#page=621), section 5.7 Power Consumption.
+
+### nRF Family
+
+[nRF series chips](https://www.nordicsemi.com/Products) are exceptionally efficient and is recommended choice for low-power projects.  
+Even with [BLE broadcasting enabled](https://devzone.nordicsemi.com/power/w/opp/2/online-power-profiler-for-bluetooth-le) it can use less power than RP2040 in its lowest power state!
+
+Minimal achievable power consumption is in the area of µA.
+
+TinyGo implements standard `time.Sleep()` very efficiently for nRF chips so you can just use it and have great results.  
+The only thing you may want to tweak is to disable the UART with help of `-serial=none` flag to go even lower than default.
+
+## The Way to Low Power
+
+> Attached [debugger](../guides/debugging) usually blocks the low power state in chip.  
+> The best way to measure when using a debugger is to flash the code, do a power cycle, and then measure.
+
+> The changes discussed in this paragraph shall be made in TinyGo and not in your application code.  
+> These are `runtime` and/or `machine` packages, in files related to your target.
+
+Following steps shall help you achieve low power consumption:
+
+- Put a `for { arm.Asm("wfi") }` loop before anything else is initialized (at the beginning of [Reset_Handler](https://github.com/tinygo-org/tinygo/search?q=%22export+Reset_Handler%22)).  
+  _`wfi` pauses the processor, puts it in "wait for interrupt" state. And when inserted before any initialisation, this ensures no internal subsystem is running consuming power._
+- Measure the current, this should be the lowest consumption as advertised for the chip.
+- If it is higher than that, investigate why (it could be a power LED, regulator, some clock that's enabled by the bootloader, maybe other things).  
+  _Usually this ranges somewhere between 1µA and 100µA, depending on the low-power capabilities of the chip. Don't continue until you really do reach the lowest possible power consumption._
+- Slowly move the loop to later in the code, until you move it into the main function.
+- There are probably things initialized by default that consume current, like UART, PLLs, different clocks. Disable them before continuing, or investigate why they consume power.