Analog Devices SHARC ADSP-214 Series Hardware Reference Manual page 519

Serial Ports
With companding enabled, interfacing the serial port to a codec requires
little additional programming effort. If companding is not selected, two
formats are available for received data words of fewer than 32 bits—one
that fills unused MSBs with zeros, and another that sign-extends the MSB
into the unused bits.
ADSP-214xx SHARC Processor Hardware Reference 10-61
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