我用小车追踪例子里的程序改写，想在小车追到目标停止时触发回调函数和串口通信发送1，一直跑不通，百思不得其解，求大佬帮忙看看该如何改写。

import sensor, image, time
import car
from pid import PID
from pyb import UART
import json
sensor.reset()
sensor.set_pixformat(sensor.RGB565)
sensor.set_framesize(sensor.QQVGA)
sensor.skip_frames(10)
sensor.set_auto_whitebal(False)
timer = pyb.Timer(4, freq=1)

uart = UART(3, 19200)

clock = time.clock()
green_threshold   = (76, 96, -110, -30, 8, 66)
size_threshold = 2000
x_pid = PID(p=0.5, i=1, imax=100)
h_pid = PID(p=0.05, i=0.1, imax=50)
def find_max(blobs):
	max_size=0
	for blob in blobs:
		if blob[2]*blob[3] > max_size:
			max_blob=blob
			max_size = blob[2]*blob[3]
	return max_blob
   
def timer_callback(timer):

while(True):
	clock.tick()
	img = sensor.snapshot()
	blobs = img.find_apriltags()
	if blobs:
		max_blob = find_max(blobs)
		x_error = max_blob[6]-img.width()/2
		h_error = max_blob[2]*max_blob[3]-size_threshold
		print("x error: ", x_error)
		img.draw_rectangle(max_blob[0:4])
		img.draw_cross(int(max_blob[6]),int(max_blob[7]))
		x_output=x_pid.get_pid(x_error,1)
		h_output=h_pid.get_pid(h_error,1)
		print("h_output",h_output)
		car.run(-h_output,-h_output)
	else:
		car.run(50,50)
    if size_threshold = 2000:
        output_str=json.dumps([1])
        uart.write(output_str+'\n')
        timer.callback(timer_callback)
         
    pyb.delay(8000)

H7如何用io口定时器模拟输出pwm信号呀，pwm口不够用了想用其他io口来控制舵机该如何解决？