Arty Z7 Reference Manual
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23
13.1 Pulse-Width Modulation
A pulse-width modulated (PWM) signal is a chain of pulses at some fixed frequency, with
each pulse potentially having a different width. This digital signal can be passed through a
simple low-pass filter that integrates the digital waveform to produce an analog voltage
proportional to the average pulse-width over some interval (the interval is determined by the
3dB cut-off frequency of the low-pass filter and the pulse frequency). For example, if the
pulses are high for an average of 10% of the available pulse period, then an integrator will
produce an analog value that is 10% of the Vdd voltage. Figure 13.1.1 shows a waveform
represented as a PWM signal.
Figure 13.1.1. PWM Waveform.
The PWM signal must be integrated to define an analog voltage. The low-pass filter 3dB
frequency should be an order of magnitude lower than the PWM frequency, so that signal
energy at the PWM frequency is filtered from the signal. For example, if an audio signal must
contain up to 5 KHz of frequency information, then the PWM frequency should be at least 50
KHz (and preferably even higher). In general, in terms of analog signal fidelity, the higher the
PWM frequency, the better. Figure 13.1.2 shows a representation of a PWM integrator
producing an output voltage by integrating the pulse train. Note the steady-state filter output
signal amplitude ratio to Vdd is the same as the pulse-width duty cycle (duty cycle is defined
as pulse-high time divided by pulse-window time).
Figure 13.1.2. PWM Output Voltage.
14 Reset Sources
14.1 Power-on Reset
The Zynq PS supports external power-on reset signals. The power-on reset is the master reset
of the entire chip. This signal resets every register in the device capable of being reset. The
