def tick(timer):
    global N1, N2, N12, flag
    X = N1
    N12 = N1 - N2
    N2 = X
    flag = 1
        
tim = Timer()
tim.init(freq=20, mode=Timer.PERIODIC, callback=tick)

t = -1
dt = 1/20
kv = 65535 / 13.4   # convert volts to pwm
kw = 0.16*pi        # convert counts to rad/sec

fwd.duty_u16(0)
rev.duty_u16(0)

while(t < 2):
    while(flag == 0):
        pass
    flag = 0
    if(t < 0):
        V = 0
    else:
        V = 10
    fwd.duty_u16(int(V*kv))
    rev.duty_u16(0)
    Speed = N12*kw
    print('{: 7.2f}'.format(t),'{: 7.4f}'.format(Speed))
    t += dt
 
print('Stop')
fwd.duty_u16(0)
rev.duty_u16(0)