June 5, 1997

Audio DSP provides flexible serial I/O

Jerry Horn, Burr-Brown Corp, Tucson, AZ

Embedded-system designers are aware of the difficulties in connecting a µC to a large number of peripherals, such as A/D or D/A converters. An interesting possibility, one that you might overlook but definitely should consider, is to use an audio DSP, such as the DSP56004/7/9 from Motorola (Phoenix). These devices contain two audio serial receivers, three audio serial transmitters, and a serial-peripheral-interface port. You can easily program these various serial inputs and outputs to provide other application-specific inputs and outputs, as well as simple repetitive signals, such as convert or frame-synchronization signals. For example, the DSP56004 proves to be a flexible device for use in an evaluation system for the ADS7833 (Figure 1).

The ADS7833 contains three simultaneous-operation, 12-bit A/D converters and an 8-bit D/A converter. The application needed three receive channels for the A/D converters and one transmit channel for the D/A converter. (This channel also contains configuration bits for the A/D converters.) The application also needed a convert-command signal and a serial-transmit channel for a 16-bit D/A converter. The 16-bit D/A converter provides analog reconstruction for any A/D converter.

Figure 2 shows the overall timing diagram for the communications among the ADS7833, the DSP56004, and the DAC56. Figure 2 does not show all the timing signals, because several are identical to those shown. The DSP56004 allows you to program the basic serial clock from 80 kHz to 3.2 MHz. This range provides for an A/D- and D/A-conversion rate of 5 to 200 kHz. The 40-MHz DSP56004 can handle this data rate in addition to much overhead in processing incoming and outgoing data. Note that the circuit in Figure 1 requires no extra glue logic. If you can accept some minimal glue logic, the circuit could easily accommodate two DAC56s by using the left/right WST signal.

Many other serial A/D and D/A converters are available with serial interfaces similar to those in the ADS7833; you can readily interface those converters using this scheme. One example uses three eight-pin, 12-bit, micropower A/D converters that are available from several manufacturers (Figure 3). In this configuration, you can control as many as four DAC56s with the addition of some simple logic. You can find additional related information in the ADS7833 data sheet (PDS-1235), ADS7816 data sheet (PDS-1355), and DEM-ADS7833 User's Manual (LI-485). The documents are all available on Burr-Brown's Web site, www.burr-brown.com. (DI #2039)

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