樹莓派筆記RaspberryPi -(7) LED 呼吸燈 (PWM 控制)

PWM=Pulse Width Modulation,

通過調整數位脈衝的週期以及在週期中的寬度占比, 來模擬類比信號輸出

像是控制馬達轉速, 舵機角度, 燈號強度...等

目前樹莓派有幾種作法

一是通過軟體控制, 優點是每隻GPIO腳都可以發出, 數量不受限

缺點是占用CPU計算資源, 太多PWM有可能會延遲

另外是寬度占比只能設定0~100, 較不精準

二是硬體控制,

脈衝非常準確, 寬度可以設定0~1,000,000

但在樹莓派上一次只能產生兩組, BCM 12/18共用通道0, BCM 13/19共用通道1

三是DMA控制, 介於兩者之間

通常由擴充版提供, 精準度依板子不同而異

以GrovePi+來說脈衝寬度可以設定0~255的範圍, 僅D3/D5/D6可提供

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軟體控制PWM程式碼

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import RPi.GPIO as GPIO
import time

LedPin = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(LedPin, GPIO.OUT)   # Set pin mode as output

p = GPIO.PWM(LedPin, 1000)     # set Frequece to 1KHz
p.start(0)                     # Start PWM output, Duty Cycle = 0

try:
        while True:
                for dc in range(0, 101, 5):   # Increase duty cycle: 0~100
                        p.ChangeDutyCycle(dc)     # Change duty cycle
                        time.sleep(0.05)
                time.sleep(1)
                for dc in range(100, -1, -5): # Decrease duty cycle: 100~0
                        p.ChangeDutyCycle(dc)
                        time.sleep(0.05)
                time.sleep(1)
except KeyboardInterrupt:
        p.stop()
        GPIO.cleanup()

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硬體PWM程式碼, 必須先安裝pigpio庫, 並執行sudo pigpiod

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import pigpio
import time

PWM_LED_PIN = 18
PWM_FREQ = 1000
 
pi = pigpio.pi()
 
try:
    while True:
        for i in range(0, 50, 1):
            pi.hardware_PWM(PWM_LED_PIN, PWM_FREQ, i*20000)
            time.sleep(0.1)
        for i in range(49, -1, -1):
            pi.hardware_PWM(PWM_LED_PIN, PWM_FREQ, i*20000)
            time.sleep(0.1)
except KeyboardInterrupt:
    pi.set_mode(PWM_LED_PIN, pigpio.INPUT)

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GrovePi+ PWM程式碼

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import grovepi
import time

led = 5 # Connect the Grove LED to digital port D5
grovepi.pinMode(led,"OUTPUT")

try:
    while True:
        for i in range(0, 256, 15): #Increment brightness for next iteration
            grovepi.analogWrite(led,i)  # Give PWM output to LED
            time.sleep(.3)
        for i in range(255, -1, -15):
            grovepi.analogWrite(led,i)  # Give PWM output to LED
            time.sleep(.3)

except KeyboardInterrupt:
    grovepi.analogWrite(led,0)

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