Audio Out; Connectivity - Digilent Basys MX3 Reference Manual

Basys MX3™ Board Reference Manual

17 Audio Out

The Basys MX3 contains an audio out module, controlled by the PIC32 using the digital signal A_OUT. This digital
signal is PWM controlled to generate multiple values between 0 and 3.3V. A PWM signal is a chain of pulses
generated at 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 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, in this case .33V. Figure 17.2
shows a waveform represented as a PWM signal.
Vdd
Gnd
Pulse Window
The volume of the output audio signal can be adjusted using a thumbwheel potentiometer labeled SPEAKER
VOLUME, situated in the top left corner of the board.
The generated audio out signal is outputted to an onboard speaker through a 1.2W audio power amplifier
(IS31AP4991) and to an onboard headphones connector, labeled LINE OUT. When headphones are connected to
the LINE OUT port, the onboard speaker is automatically muted.

17.2 Connectivity

Table 17.1 shows the details about the A_OUT signal connecting the audio module to the PIC32.
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Figure 17.1. Audio out schematic diagram.
Pulse Width Digital Signal
= 1 / Pulse Frequency (f)
Figure 17.2. Waveform represented as a PWM signal.
Analog Signal (PWMA)
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