RTIOUnderflow exception when attempting to generate periodic TTL pulse

I wish to use the TTLOut channel on my ARTIQ chassis to generate a periodic TTL pulse signal with frequency 100MHz and the high level ratio 0.25 for each period. I use the following code to complete the task:

`
from artiq.experiment import *
from artiq.language.core import *

class TTL_Demo(EnvExperiment):

    def build(self):
        self.setattr_device("core")
        self.setattr_device("ttl32")
        self.TTLOut = self.ttl32
        self.setattr_argument("Freq", NumberValue(100, scale=1, step=1, ndecimals=0, unit="MHz"))
        self.setattr_argument("Ratio", NumberValue(0.25, scale=1, step=0.01))

    def prepare(self):
	  self.period = 1 / self.Freq  # signal period, unit is us
	  self.high = self.period * self.Ratio  # High level time, unit is us
	  self.low = self.period * (1 - self.Ratio)  # Low level time, unit is us
	  self.h_mu = self.core.seconds_to_mu(self.high * 1e-6)
	  self.l_mu = self.core.seconds_to_mu(self.low * 1e-6)

    @kernel
    def run(self):
	  self.core.reset()
	  self.TTLOut.off()
	  while True:
		# self.core.break_realtime()
		self.TTLOut.pulse_mu(self.h_mu)
		delay_mu(self.l_mu)

Here, ttl32 is a TTLOut channel. However, when I run this code, when the break_realtime() is not applied, there is always an RTIOUnderflow exception occur when calling the function pulse_mu(). And if I apply break_realtime() which I shouldn't, there is no signal output on the oscilloscope monitoring the TTL output signal. How should I avoid this RTIOUnderflow exception and what did I do wrong that causes this exception?