【BPI-Pico-RP2040 开发板】rp2040的PIO实战-使用circuitPython的PIO功能点亮WS2812

前面大致了解了一些rp2040的PIO外设，以及它的一些硬件细节，接下来我们看一下circuitPython是如何使用PIO功能点亮WS2812的。

以下是源码。

# SPDX-FileCopyrightText: 2021 Scott Shawcroft, written for Adafruit Industries
#
# SPDX-License-Identifier: MIT

import time
import rp2pio
import board
import microcontroller
import adafruit_pioasm

# NeoPixels are 800khz bit streams. We are choosing zeros as <312ns hi, 936 lo>
# and ones as <700 ns hi, 556 ns lo>.
# The first two instructions always run while only one of the two final
# instructions run per bit. We start with the low period because it can be
# longer while waiting for more data.
program = """
.program ws2812
.side_set 1
.wrap_target
bitloop:
   out x 1        side 0 [6]; Drive low. Side-set still takes place before instruction stalls.
   jmp !x do_zero side 1 [3]; Branch on the bit we shifted out previous delay. Drive high.
 do_one:
   jmp  bitloop   side 1 [4]; Continue driving high, for a one (long pulse)
 do_zero:
   nop            side 0 [4]; Or drive low, for a zero (short pulse)
.wrap
"""

assembled = adafruit_pioasm.assemble(program)

# If the board has a designated neopixel, then use it. Otherwise use
# GPIO16 as an arbitrary choice.
if hasattr(board, "NEOPIXEL"):
    NEOPIXEL = board.NEOPIXEL
else:
    NEOPIXEL = microcontroller.pin.GPIO16

sm = rp2pio.StateMachine(
    assembled,
    frequency=12_800_000,  # to get appropriate sub-bit times in PIO program
    first_sideset_pin=NEOPIXEL,
    auto_pull=True,
    out_shift_right=False,
    pull_threshold=8,
)
print("real frequency", sm.frequency)

for i in range(30):
    sm.write(b"\x0a\x00\x00")
    time.sleep(0.1)
    sm.write(b"\x00\x0a\x00")
    time.sleep(0.1)
    sm.write(b"\x00\x00\x0a")
    time.sleep(0.1)
print("writes done")

time.sleep(2)

在引入了所需要的必要依赖库后，程序首先定义了一段汇编代码。这段汇编代码就是PIO状态机的程序。由于PIO状态机的指令空间有限，在设计时就加入了指令执行后等待N个周期的设定（可以节省nop指令占用的空间）。

program = """
.program ws2812
.side_set 1
.wrap_target
bitloop:
   out x 1        side 0 [6]; Drive low. Side-set still takes place before instruction stalls.
   jmp !x do_zero side 1 [3]; Branch on the bit we shifted out previous delay. Drive high.
 do_one:
   jmp  bitloop   side 1 [4]; Continue driving high, for a one (long pulse)
 do_zero:
   nop            side 0 [4]; Or drive low, for a zero (short pulse)
.wrap
"""

然后在接下来的程序里，指定PIO输出的引脚

# If the board has a designated neopixel, then use it. Otherwise use
# GPIO16 as an arbitrary choice.
if hasattr(board, "NEOPIXEL"):
    NEOPIXEL = board.NEOPIXEL
else:
    NEOPIXEL = microcontroller.pin.GPIO16

然后初始化PIO状态机

sm = rp2pio.StateMachine(
    assembled,
    frequency=12_800_000,  # to get appropriate sub-bit times in PIO program
    first_sideset_pin=NEOPIXEL,
    auto_pull=True,
    out_shift_right=False,
    pull_threshold=8,
)
print("real frequency", sm.frequency)

最后则是向状态机写入数据，实现WS2812灯珠的闪烁

for i in range(30):
    sm.write(b"\x0a\x00\x00")
    time.sleep(0.1)
    sm.write(b"\x00\x0a\x00")
    time.sleep(0.1)
    sm.write(b"\x00\x00\x0a")
    time.sleep(0.1)
print("writes done")

至此，板子上的LED灯就按红绿蓝三种颜色快闪的形式点亮啦。