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Book Analog Interfacing to Embedded Microprocessors

on a 33 MHz 80960. This is time that could be spent executing code. Serial Interfaces SPI/Microwire SPI is a serial interface that uses a clock, chip select, data in, and data out bits. Data is read from a serial ADC a bit at a time (Figure 2.14). Each device on the SPI bus requires a separate chip select (-CS) signal. 36 Analog Interfacing to Embedded Microprocessors Figure 2.14 SPI bus. The Maxim MAX1242 is a typical SPI ADC. The MAX1242 is a 10-bit suc- cessive approximation ADC with an internal S/H, in an 8-pin package. Figure 2.15 shows the MAX1242 interface timing. The falling edge of -CS starts a conversion, which takes a maximum of 7.5 ms. When -CS goes low, the MAX1242 drives its data output pin low. After the conversion is complete, the MAX1242 drives the data output pin high. The processor can then read the data a bit at a time by toggling the clock line and monitoring the MAX1242 data output pin.