Temperature Sensor Has One Digital Input and One Digital Output
The MAX6629-MAX6632 reduces complexity by not requiring the processor to write data. It requires only that you drive the clock and chip-select lines, so a simpler burst of clock pulses will read the temperature data.
The circuit of Figure 1 makes reading the data simpler still by deriving the chip select signal from the clock. Communication is achieved with a single digital input and a single digital output.
Clock pulses on the normally high SCK line drive active-low CS low, so devices of the MAX6629-MAX6632 family let you retrieve digital data by simply applying a burst of clock pulses. (In a normal configuration, the three interface lines active-low CS, SO, and SCK function as shown in Figure 2.) The 2-line approach of Figure 1, on the other hand, requires that the SCK rest state (quiescent state) be high, which in turn calls for a slight modification of the Figure 2 timing diagram.
Figure 1. By eliminating the need for a logic line to the active-low CS input, this circuit allows you to simply clock temperature data from the sensor device.
Figure 2. A serial-interface timing diagram for the Figure 1 temp sensor illustrates the function of active-low CS. It enables the interface when low, then suspends temperature-to-digital conversions until it returns to a level higher than 0.7VCC.
When SCK initially goes low, it pulls active-low CS low via the Schottky diode D1. You should set the R1-C1 time constant so the active-low CS line rises no higher than 0.3VCC between clock pulses (i.e., 1.5V for a 5V supply in Figure 1). An oscilloscope display (Figure 3) shows the slow rise time of active-low CS. To allow time for the sensor IC to perform its next temperature-to-digital conversion, active-low CS should remain above 0.7VCC (3.5V in Figure 1) for at least 0.5 seconds between temperature-read operations.
Figure 3. This scope display from Figure 1 shows how the presence of SCK pulses (top trace) generates a active-low CS signal (lower trace).
A similar version of this article appeared in the March 4, 2002 issue of Electronic Design magazine.