请教MicroPython中使用软件SPI出错

from machine import Pin, SPI
import utime as time
def pin_num(pin_index):
    return (ord(pin_index[1]) - ord('A')) * 16 + int(pin_index[2:])
clk = Pin(("clk", pin_num("PB3")), Pin.OUT_PP)
mosi = Pin(("mosi", pin_num("PB4")), Pin.OUT_PP)
spi = SPI(-1, 100000, polarity = 1, phase = 1, bits = 8, firstbit = 0, sck = clk, mosi = mosi, miso = None)
spi.write(b'h')