Data-acquisition system draws less than 10 mW

The data-acquisition system in Fig 1 draws approximately 2.2 mA at 3.6V. It consists of a state machine that converts the synchronous serial data from the A/D-converter chip into a standard baud rate framed with start and stop bits for easy interface with a computer's COM port. The counter IC_1A controls the state machine; IC₂ decodes the states. The first step is to program the A/D converter by shifting four bits into it. The counter IC_1B counts the four bits; these bits then shift from the analog-multiplexer IC₅ into the A/D converter.

The second state of the state machine sends a start bit to the data_out line. The third state shifts out the last eight bits from the 12-bit A/D converter. Counter IC_1B counts the eight bits, and the fourth state of the state machine inserts a stop bit. The sixth state starts another serial frame by inserting a start bit. Then, the circuit clocks the A/D converter eight more times, and the converter shifts out the last four data bits and four zeros. The A/D converter then receives another stop bit and, finally, a chip-select toggle to initiate another conversion cycle.

The logic at gates IC₃ and IC₄ simply determines whether the second counter IC_1B needs to count four bits or eight before proceeding to the next state. Pin 7 of counter IC_1A produces a read operation of the second A/D-converter channel on every other pass of the state machine by toggling the channel-select bit the analog multiplexer shifts into the converter.