Book Analog Interfacing to Embedded Microprocessors
sor, or for coder/decoder. In general, a codec (either type) will have two-way
operation: it can turn analog signals into digital and vice-versa, or it can
convert to and from some compression standard.
The National Semiconductor LM4540 is an audio codec intended to imple-
ment the sound system in a personal computer. It contains an internal 18-bit
ADC and an 18-bit DAC. It also includes much of the audio-processing cir-
cuitry needed for 3D PC sound. The LM4540 uses a serial interface to com-
municate with its host processor.
The National TP3504 is a telecom-type codec, and includes ADC, DAC,
filtering, and companding circuitry. The TP3504 also has a serial interface.
42 Analog Interfacing to Embedded Microprocessors
Interrupt Rate
The MAX151 can perform a conversion every 3.3 ms, or 300,000 conversions
per second. Even a 33 MHz processor operating at one instruction per
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