很久之前整过PWM舵机，刚好最近师弟需要，并且网上现有教程不是很完整，就整理一下。方便交流以及后面回顾。

        首先要明确，在这个控制方式中需要用到哪些方面：

1、树莓派与PCA9685之间使用I2C通信

2、PCA9685通讯协议

3、FT2331M舵机性能参数

        一、树莓派与PCA9685 I2C通信

        接线        

        接线方式可如下图所示，参考树莓派4B使用PCA9685控制舵机：

         其中VCC供电为6V.

        开启I2C：

        在终端执行：

sudo raspi-config
Interfacing Options-->I2C-->YES-->Finish 

        保险起见 重启树莓派 终端输入 ：

sudo reboot

        安装相关库文件：

sudo apt-get update
sudo apt-get install python-pip 
sudo pip install RPi.GPIO
sudo apt-get install python-smbus

        开启I2C并且完成树莓派与PCA9685接线之后，可以通过如下命令查看设备是否正常搭载：

ls /dev/*i2c*

        查看I2C搭载设备地址：

pi@raspberrypi:~ $ sudo i2cdetect -y 1  # 默认为I2C1,如果是I2C0.则只需要将1-->0即可

        关于I2C设备的操作，详细可看之前的文章关于树莓派4B两个I2C通道（I2C0、I2C1）的总结 。

        二、PCA9685通讯协议

相关代码我放在文中，并且有具体注释。

from __future__ import division
import logging
import time
import math


# Registers/etc:
PCA9685_ADDRESS    = 0x40
MODE1              = 0x00
MODE2              = 0x01
SUBADR1            = 0x02
SUBADR2            = 0x03
SUBADR3            = 0x04
PRESCALE           = 0xFE
LED0_ON_L          = 0x06
LED0_ON_H          = 0x07
LED0_OFF_L         = 0x08
LED0_OFF_H         = 0x09
ALL_LED_ON_L       = 0xFA
ALL_LED_ON_H       = 0xFB
ALL_LED_OFF_L      = 0xFC
ALL_LED_OFF_H      = 0xFD

# Bits:
RESTART            = 0x80
SLEEP              = 0x10
ALLCALL            = 0x01
INVRT              = 0x10
OUTDRV             = 0x04


logger = logging.getLogger(__name__)


def software_reset(i2c=None, **kwargs):
    """Sends a software reset (SWRST) command to all servo drivers on the bus."""
    # Setup I2C interface for device 0x00 to talk to all of them.
    if i2c is None:
        import Adafruit_GPIO.I2C as I2C
        i2c = I2C
    self._device = i2c.get_i2c_device(0x00, **kwargs)
    self._device.writeRaw8(0x06)  # SWRST


class PCA9685(object):
    """PCA9685 PWM LED/servo controller."""

    def __init__(self, address=PCA9685_ADDRESS, i2c=None, **kwargs):
        """Initialize the PCA9685."""
        # Setup I2C interface for the device.
        if i2c is None:
            import Adafruit_GPIO.I2C as I2C
            i2c = I2C
        self._device = i2c.get_i2c_device(address, **kwargs)
        self.set_all_pwm(0, 0)
        self._device.write8(MODE2, OUTDRV)
        self._device.write8(MODE1, ALLCALL)
        time.sleep(0.005)  # wait for oscillator
        mode1 = self._device.readU8(MODE1)
        mode1 = mode1 & ~SLEEP  # wake up (reset sleep)
        self._device.write8(MODE1, mode1)
        time.sleep(0.005)  # wait for oscillator

    def set_pwm_freq(self, freq_hz):
        """Set the PWM frequency to the provided value in hertz."""
        prescaleval = 25000000.0    # 25MHz
        prescaleval /= 4096.0       # 12-bit
        prescaleval /= float(freq_hz)
        prescaleval -= 1.0
        logger.debug('Setting PWM frequency to {0} Hz'.format(freq_hz))
        logger.debug('Estimated pre-scale: {0}'.format(prescaleval))
        prescale = int(math.floor(prescaleval + 0.5))
        logger.debug('Final pre-scale: {0}'.format(prescale))
        oldmode = self._device.readU8(MODE1);
        newmode = (oldmode & 0x7F) | 0x10    # sleep
        self._device.write8(MODE1, newmode)  # go to sleep
        self._device.write8(PRESCALE, prescale)
        self._device.write8(MODE1, oldmode)
        time.sleep(0.005)
        self._device.write8(MODE1, oldmode | 0x80)

    def set_pwm(self, channel, on, off):
        """Sets a single PWM channel."""
        self._device.write8(LED0_ON_L+4*channel, on & 0xFF)
        self._device.write8(LED0_ON_H+4*channel, on >> 8)
        self._device.write8(LED0_OFF_L+4*channel, off & 0xFF)
        self._device.write8(LED0_OFF_H+4*channel, off >> 8)

    def set_all_pwm(self, on, off):
        """Sets all PWM channels."""
        self._device.write8(ALL_LED_ON_L, on & 0xFF)
        self._device.write8(ALL_LED_ON_H, on >> 8)
        self._device.write8(ALL_LED_OFF_L, off & 0xFF)
        self._device.write8(ALL_LED_OFF_H, off >> 8)
    
    def cmd(self, channel, Position):
        '''
        已经进行数据转换，position范围0-210度
        '''
        '''
        PCA9685是一种PWM控制器，可以产生从0到4096之间的脉宽值，对应于PWM信号的高电平时间
        PWM频率被设置为60Hz,周期的时间为 1 / 60 秒，即约 16.67 毫秒（16666 微秒）
        servo_min = 150  # Min pulse length out of 4096
        servo_max = 600  # Max pulse length out of 4096
        servo_min 的实际脉宽时间为 (150 / 4096) * 16666 微秒 ≈ 610 微秒
        servo_max 的实际脉宽时间为 (600 / 4096) * 16666 微秒 ≈ 2440 微秒
        Ft2331舵机参数为0~240°对应500~2500us
        '''
        servo_min = int(500 * 60 / 1000000 * 4096)  # Min pulse length out of 4096
        servo_max = int(2500 * 60 / 1000000 * 4096)  # Max pulse length out of 4096
        
        Position = int(Position / (240 - 0) * (servo_max - servo_min) + servo_min)  # 240这个数值需对每个舵机进行标定，有可能需要增大，有可能需要减小
        
        self.set_pwm(channel, 0, Position)



pwm = PCA9685()
pwm.set_pwm_freq(60)
channel = 0

while True:
    
    pwm.cmd(channel, 0)
    time.sleep(1)
    
    pwm.cmd(channel, 90)
    time.sleep(1)
    
    pwm.cmd(channel, 180)
    time.sleep(1)
    
    pwm.cmd(channel, 90)
    time.sleep(1)

        三、FT233M1舵机参数

 舵机控制参数如下所示：

        从旋转角度那一栏可知，舵机由0-240°，对应周期时间为500-2500us。 

所以在第二节中的协议中，控制函数cmd(self, channel, Position)中存在（同样和通信频率有关），如下代码：

'''
        PCA9685是一种PWM控制器，可以产生从0到4096之间的脉宽值，对应于PWM信号的高电平时间
        PWM频率被设置为60Hz,周期的时间为 1 / 60 秒，即约 16.67 毫秒（16666 微秒）
        servo_min = 150  # Min pulse length out of 4096
        servo_max = 600  # Max pulse length out of 4096
        servo_min 的实际脉宽时间为 (150 / 4096) * 16666 微秒 ≈ 610 微秒
        servo_max 的实际脉宽时间为 (600 / 4096) * 16666 微秒 ≈ 2440 微秒
        Ft2331舵机参数为0~240°对应500~2500us
        '''
        servo_min = int(500 * 60 / 1000000 * 4096)  # Min pulse length out of 4096
        servo_max = int(2500 * 60 / 1000000 * 4096)  # Max pulse length out of 4096
        
        Position = int(Position / (240 - 0) * (servo_max - servo_min) + servo_min)  # 240这个数值需对每个舵机进行标定，有可能需要增大，有可能需要减小