Motorola DSP56012 - Manual (Page 215)

Digital Audio Transmitter
DAX Functional Overview
MOTOROLA DSP56012 User’s Manual 8-5
8.3 DAX FUNCTIONAL OVERVIEW
The DAX consists of:
• Audio Data Register A and Audio Data Register B (XADRA and XADRB), one
for each channel
• Audio Data Buffer (XADBUF)
• Non-Audio Data Buffer (XNADBUF)
• Audio and non-audio Data Shift Register (XADSR)
• Control Register (XCTR)
• Status Register (XSTR)
• Parity Generator (PRTYG)
• Preamble generator
• Biphase encoder
• Clock multiplexer
• Control state machine
One frame of audio data and non-audio data (stored in XADRA/XADRB and XCTR,
respectively) is transferred to the XADSR (for Channel A) and to the
XADBUF/XNADBUF registers (for Channel B) at the beginning of a frame
transmission. This is called an “upload.” At this time the DAX Audio Data register
Empty (XADE) flag is set, and, if DAX interrupt is enabled, an interrupt request is
sent to the DSP core. The interrupt handling routine then stores the next frame of
audio data in the XADRA/XADRB and the non-audio data bits in the XCTR.
At the beginning of a frame transmission, one of the 8-bit Channel A preambles
(Z-preamble for the first sub-frame in a block, or X-preamble otherwise) is generated
in the preamble generator, and then shifted out to the ADO pin in the first eight time
slots. The preamble is generated in biphase mark format. The twenty-four audio and
three non-audio data bits in the XADSR are shifted out to the biphase encoder, which
shifts them out through the ADO pin in the biphase mark format in the next fifty-four
time slots. The parity generator calculates an even parity over the 27 bits of audio and
non-audio data, and then outputs the result through the biphase encoder to the ADO
pin at the last two time slots. This is the end of the first (Channel A) sub-frame
transmission.