Python on bare metal

Fast & efficient MicroPython

Transcript

1. Python on bare metal
   fast & efficient code with MicroPython
   Jessica Greene @sleepypioneer
   

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2. MicroPython
    A complete reimplementation of Python
    Designed to be efficient with resources
    Designed to run bare metal
   Includes:
    A compiler, runtime & familiar REPL
    Support for basic libraries
    Extra modules to control hardware
   

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3. PyBoard, ESP32, ESP8266 & more...
   

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4. Challenges of working on microcontrollers
   BATTERY POWERED VERY LITTLE MEMORY
   (RAM, ROM)
   NORMALLY REQUIRE
   LOW LEVEL LANGUAGE
   

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5. How does MicroPython solve the issue of having a low
   amount of RAM?
   Interned strings, most already in ROM
   Small integers stuffed in a pointer to avoid using heap (trick from lisp)
   Optimised method calls to not use heap
   Garbage collection
   

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6. Normal python uses
   megabytes RAM if
   not 100s
   MicroPython can run
   with only 8 kilobytes
   & it runs without an
   OS, so on bare
   metal
   Python Vs MicroPython
   

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7. How you can impact
   this with your code?
   

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8. Tips: Memory allocation
   Doesn’t use heap Uses Heap
   Expressions Importing
   If, while, for and try statements Defining functions & classes
   Local variables Assigning global variables for the first time
   Small integer arithmetic Creating data structures
   Inplace operations on existing data structures
   Calling functions/ methods with positional or
   keyword args
   

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9. Tips: CPU time
   USE FUNCTIONS TO
   AVOID GLOBAL
   SCOPES
   USE LOCAL
   VARIABLES
   CACHE MODULE
   FUNCTIONS &
   OBJECT METHODS
   AS LOCALS
   PREFER LONGER
   EXPRESSIONS NOT
   SPLIT UP ONES
   RUNTIME IS FAST!
   E.G. STR.STARTSWITH
   

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10. Tips: RAM usage
    DON'T USE HEAP
    WHEN POSSIBLE
    SHORTER VARIABLE
    NAMES, REUSE
    THEM; EG X,Y,I, LEN,
    VAR
    DON'T USE * OR **
    ARGS
    SCRIPT
    MINIFICATION
    ULTIMATE SOLUTION:
    FREEZE SCRIPTS INTO
    THE FIRMWARE (AT
    COMPILE TIME)
    

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11. Code example
    

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12. LED Blink script
    import time, machine
    led = machine.Pin('LED_BLUE')
    for i in range(N):
    led.on()
    led.off()
    50kHz
    @dpgeorge
    

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13. Inside a function - removes global variables 60kHz
    import time, machine
    led = machine.Pin('LED_BLUE')
    def blink_simple(n):
    for i in range(n):
    led.on()
    led.off()
    @dpgeorge
    

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14. Preload methods & range() optimisation
    def blink_preload(n):
    on = led.on # returns a bound method
    off = led.off
    r = range(n) # optimise the range
    for i in r:
    on()
    off()
    180kHz
    @dpgeorge
    

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15. Loop unrolling
    def blink_preload_unrolled8(n):
    n //= 8
    on = led.on
    off = led.off
    r = range(n)
    for i in r:
    on()
    off()
    on()
    off()
    ....
    220kHz
    @dpgeorge
    

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16. Native mode
    @micropython.native
    def blink_preload_unrolled8_native(n:int):
    n //= 4
    on = led.on
    off = led.off
    r = range(n)
    for i in r:
    on()
    off()
    on()
    off()
    ....
    290kHz
    @dpgeorge
    

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17. Viper mode
    @micropython.viper
    def blink_unrolled8_viper(n:int):
    n //= 8
    p = ptr16(stm.GPIOB + stm.GPIO_BSRR)
    for i in range(n):
    p[0] = 1 << 4 # high
    p[1] = 1 << 4 # low
    p[0] = 1 << 4 # high
    p[1] = 1 << 4 # low
    ....
    12890kHz
    @dpgeorge
    

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18. In assembler
    @micropython.asm_thumb
    def blink_asm(r0):
    lsr(r0, r0, 3)
    movwt(r1, stm.GPIOB + stm.GPIO_BSRR)
    mov(r2, 1 << 4)
    label(loop)
    strh(r2, [r1, 0]) # high
    strh(r2, [r1, 2]) # low
    strh(r2, [r1, 0]) # high
    strh(r2, [r1, 2]) # low
    ....
    27359.25kHz
    @dpgeorge
    

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19. Takeaways
    ○ If your code runs faster it can sleep
    for longer and that saves battery!
    ○ ROM everything that you can
    ○ Use runtime methods!
    ○ Local not global variables!
    

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20. THANK YOU!
    www.github.com/sleepypioneer/fast_and_efficient_micropython
    www.micropython.org
    www.github.com/micropython
    

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