如何理解 Python_Spidev 的 spi.xfer2 方法? (时序图 位操作)

import spidev, time

spi = spidev.SpiDev ()
spi.open (0,0 )

def analog_read (channel ):
    # !下面这两行不懂!
    r = spi.xfer2 ([1, (8 + channel ) << 4, 0]) 
    adc_out = ((r[1]&3 ) << 8 ) + r[2]  #3 字节?
    return adc_out

while True:
    reading = analog_read (0 )
    voltage = reading * 3.3 / 1024
    temp_c = voltage * 100 - 50
    temp_f = temp_c * 9.0 / 5.0 + 32
    print ("Temp C=%f\t\tTemp f=%f" % (temp_c, temp_f ))
    time.sleep (1 )

open (bus, device )

xfer2 (list of values[, speed_hz, delay_usec, bits_per_word])
#Performs an SPI transaction. Chip-select should be held active 
#between blocks.