The ole Cape

from time import sleep

# relevant registers
MODE1       = 0x00
MODE2       = 0x01
LED         = 0x06
ALL_LED     = 0xFA
PRE_SCALE   = 0xFE

class Pca9685:
    def __init__( self, bus, addr ):
        self.addr = 0b10000000 | addr
        self.bus = bus
        self.write_reg( MODE1, 1 << 5 )  # initialize MODE1 register
        sleep( 500e-6 )  # wait 500us to allow oscillator to power up

    def read_reg( self, reg )
        assert reg in range( 0, 256 )
        return self.bus.read_byte_data( self.addr, reg )

    def write_reg( self, reg, value ):
        assert reg in range( 0, 256 )
        return self.bus.write_byte_data( self.addr, reg, value )

    def read_regs( self, reg, count ):
        assert reg in range( 0, 256 )
        assert count in range( 1, 257-reg )
        return self.bus.read_i2c_block_data( self.addr, reg, count )

    def write_regs( self, reg, values ):
        assert reg in range( 0, 256 )
        return self.bus.write_i2c_block_data( self.addr, reg, values )

    def get_pwm( self, output ):
        assert output in range( 0, 16 )
        reg = LED + 4 * output

        [ on_l, on_h, off_l, off_h ] = self.read_regs( reg, 4 )
        on = on_l | on_h << 8
        off = off_l | off_h << 8

        phase = on
        duty = ( off - on ) & 0xfff
        if off & 0x1000:
            duty = 0
        elif on & 0x1000:
            duty = 4096

        return ( duty, phase )

    def set_pwm( self, output, duty, phase=0 ):
        assert duty in range( 0, 4097 )
        assert phase in range( 0, 4096 )

        if output == 'all':
            reg = ALL_LED
        else:
            assert output in range( 0, 16 )
            reg = LED + 4 * output

        on = phase
        off = ( duty + phase ) & 0xfff
        if duty == 0:
            off |= 0x1000
        elif duty == 4096:
            on |= 0x1000

        on_l = on & 0xff
        on_h = on >> 8
        off_l = off & 0xff
        off_h = off >> 8
        self.write_regs( reg, [ on_l, on_h, off_l, off_h ] )