Electronic Design

Read Temperature With One Digital Output And One Digital Input

Before the MAX6629/30/31/32 series of temperature sensors were available, all digital-output temperature sensors were I2C (alternatively called SMBus) sensors that had to be written to for addressing, before they were read. Prior SPI sensors also required writing. Because the MAX66xx series sensors are read-only, data can be obtained by simply clocking them. But there's one tiny complication—they use a chip-select line.

You can get around the need for an additional logic line to drive the chip-select line with the circuit shown in Figure 1. Only one digital output and one digital input are necessary to communicate with a MAX66xx device by simply clocking it to retrieve the digital data.

Normally, serial-interface temperature sensors require three interface lines: --CS, SO, and SCK, the functions of which are depicted in the timing diagram in Figure 2. If only two lines are available, Clock and Data, this circuit makes it possible to communicate with the device without a separate --CS line. This method requires that the "rest" or quiescent state of the clock be high, modifying the above timing diagram accordingly.

The circuit operates by generating a --CS signal from the clock. When the clock initially goes low, it pulls down the --CS line via Schottky diode D1. Set the time constant of R1 and C1 so that the --CS line rises to no higher than 0.3 × VCC between clock pulses. The resultant slow rise of --CS must be greater than 0.7 × VCC for at least 0.5 s between reads to let the devices perform temperature-to-digital conversions.

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