PiGPIOMEM.jl

Julia interface for Raspberry Pi GPIO using /dev/gpiomem.

/dev/gpiomem provides userspace access to the BCM2835 GPIO control registers (see BCM2835-ARM-Peripherals.pdf p89).

Installation

pkg> add https://github.com/notinaboat/PiGPIOMEM.jl

Simple examples

julia> using PiGPIOMEM

julia> pin7 = GPIOPin(7)
GPIOPin(7)

julia> pin7[]
false

julia> set_pullup(pin7)

julia> pin7[]
true

julia> set_output_mode(pin7)

julia> pin7[] = false

julia> pin7[]
false

julia> set(pin7)

julia> pin7[]
true

julia> clear(pin7)

julia> pin7[]
false

julia> pin17 = GPIOPin(17; output=true)
GPIOPin(17, output=true)

julia> pin17[] = true

julia> clear(pin7, pin17)

julia> pin7[], pin17[]
(false, false)

julia> set(pin7, pin17)

julia> pin7[], pin17[]
(true, true)

Full Documentation

See src/PiGPIOMEM.jl or online help:

julia> using PiGPIOMEM
help?> PiGPIOMEM

Implementation

The implementation aims to minimise pin access overhead. The pin number is bounds checked only once at construction time and register addresses and bit masks are pre-computed.

The resulting compiled pin access functions are very small:

julia> code_native(set, [GPIOPin]; debuginfo=:none)
	.text
	ldr	r1, [r0, #8]
	ldr	r0, [r0, #12]
	str	r1, [r0]
	bx	lr

julia> code_native(clear, [GPIOPin]; debuginfo=:none)
	.text
	ldr	r1, [r0, #8]
	ldr	r0, [r0, #16]
	str	r1, [r0]
	bx	lr

julia> code_native(getindex, [GPIOPin]; debuginfo=:none)
	.text
	ldr	r1, [r0, #8]
	ldr	r0, [r0, #20]
	ldr	r0, [r0]
	ands	r0, r1, r0
	movne	r0, #1
	bx	lr

The following test produces a 9.7MHz square wave on a Raspberry Pi Zero W.

julia> using LLVM

julia> @noinline nop() = LLVM.Interop.@asmcall("nop")

julia> toggle(pin) = while true
    clear(pin)
    nop()
    nop()
    set(pin)
    nop()
    nop()
end
toggle (generic function with 1 method)

julia> toggle(GPIOPin(21, output=true))

Alternatives

• JuliaBerry/PiGPIO.jl -- uses the pigpiod daemon interface.

• notinaboat/PiGPIOC.jl -- ccall wrappers for the pigpio C Interface.

• ronisbr/BaremetalPi.jl -- uses /dev/gpiomem, /dev/spidevX.X, /dev/i2c-X etc.