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Simplify SPI bit-banging implementation

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And add some comments to make stuff clearer
This commit is contained in:
Dave Jones
2016-10-21 22:12:55 +01:00
parent a9f9aa4a3e
commit cbff81f3a1
1 changed files with 11 additions and 4 deletions
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15
gpiozero/pins/spi.py
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@@ -64,6 +64,11 @@ class SPISoftwareBus(SharedMixin, Device):
""" """
result = [] result = []
with self.lock: with self.lock:
# See https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
# (specifically the section "Example of bit-banging the master
# protocol") for a simpler C implementation of this which ignores
# clock polarity, phase, variable word-size, and multiple input
# words
if lsb_first: if lsb_first:
shift = operator.lshift shift = operator.lshift
init_mask = 1 init_mask = 1
@@ -76,13 +81,15 @@ class SPISoftwareBus(SharedMixin, Device):
for _ in range(bits_per_word): for _ in range(bits_per_word):
if self.mosi is not None: if self.mosi is not None:
self.mosi.value = bool(write_word & mask) self.mosi.value = bool(write_word & mask)
# read bit on clock activation
self.clock.on() self.clock.on()
if self.miso is not None and not clock_phase: if not clock_phase:
if self.miso.value: if self.miso is not None and self.miso.value:
read_word |= mask read_word |= mask
# read bit on clock deactivation
self.clock.off() self.clock.off()
if self.miso is not None and clock_phase: if clock_phase:
if self.miso.value: if self.miso is not None and self.miso.value:
read_word |= mask read_word |= mask
mask = shift(mask, 1) mask = shift(mask, 1)
result.append(read_word) result.append(read_word)