Data Word Formats
5-48 ADSP-21368 SHARC Processor Hardware Reference
When companding is enabled, the data in the
RXSPxA buffers is the
right-justified, sign-extended expanded value of the eight received LSBs. A
write to TXSPxA compresses the 32-bit value to eight LSBs (zero-filled to
the width of the transmit word) before it is transmitted. If the 32-bit value
is greater than the 13-bit A-law or 14-bit μ-law maximum, it is automati-
cally compressed to the maximum value.
Since the values in the transmit and receive buffers are actually com-
panded in place, the companding hardware can be used without
transmitting (or receiving) any data, for example during testing or debug-
ging. This operation requires one peripheral clock cycle of overhead, as
described below. For companding to execute properly, program the
SPORT registers prior to loading data values into the SPORT buffers.
To compand data in place without transmitting:
1. Set the SPTRAN bit to 1 in the SPCTLx registers. The SPEN_A and
SPEN_B bits should be =0.
2. Enable companding in the DTYPE field of the SPCTLx transmit con-
trol registers.
3. Write a 32-bit data word to the transmit buffer. Companding is
calculated in this cycle.
4. Wait two cycles. Any instruction not accessing the Tx buffer can be
used to cause this delay. This allows the SPORT companding hard-
ware to reload the transmit buffer with the companded value.
5. Read the 8-bit companded value from the transmit buffer.
To expand data in place, use the same sequence of operations with the
receive buffer instead of the transmit buffer. When expanding data in this
way, set the appropriate serial word length (
SLEN) in the SPCTLx registers.
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