PI2Media PI2AES
Hardware Reference Manual – P0.1 - 3/27/2019
PAGE 8
TXO
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AES3/SPDIF encoded digital audio output
Table 3 – RPi to WM8804 Connections
4.1.1
WM8804 SPDIF TRANSMITTER NOTES
1. The WM8804 is controlled using the I2C port. Refer to the WM8804
documentation and Pi 2 Design supplied driver code for more detail.
2. The PI2AES is designed to operate the WM8804 in Master mode. In this
mode the WM8804 receives its master clock from XIN and drives MCLK
(unused), BCLK and LRCLK to the Pi. Serial Data in is received from Pi.
3. Pi GPIO’s 6 and 13 are used to select the desired clock input. GPIO 6 and
13 are pulled low by default. CAUTION - Do not select more than one
clock at a time!
4.2
DUAL HIGH RESOLUTION NDK CLOCKS
A pair of NDK NZ2520SD Ultra-Low Noise clocks provide the critical timing signals for
the WM8804. They are selected via Raspberry PI GPIO5 (22.5792Mhz) and GPIO6
(24.576Mhz).
4.3
RS-422 DIFFERENTIAL TRANSMITTER
An RS-422 Differential Transmitter accepts the SPDIF output from the WM8804 and
sends it as a balanced pair to the 1:1 Isolation Transformer and then to the XLR
connector.
4.4
I2S PARALLEL BUFFER
A 74FCT125 CMOS Buffer is used to drive the I2S signals (MCLK, BCKL, LRCLK and
SDO) to the I2S Expansion Header. This device is designed to drive the I2S Bus short
distances (<12 inches).
4.5
I2S DIFFERENTIAL DRIVER
A DS2063 Quad Differential LVDS Driver takes the I2S Bus and converts each signal
into a balanced differential signal. This is then connected to the HDMI connector. This
device is designed to drive the I2S Bus up to 3 meters.
4.6
AUDIO DATA RATE AND POWER LED’S
A triple stack LED is used to provide visual indication of: Power On (Green), Audio Data
Rate of 48/96/192Khz (Yellow) and Audio Data Rate of 44.1/88.2/176.4Khz (Red).
