AN929 Datasheet, PDF (4/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
Serial Output
Serial output sensors measure temperature and
communicate with the processor via a standard serial
data protocol, such as the SPI™, I²C™ or SMBus
protocols. Silicon IC sensors are available that
integrate the temperature sensor, ADC and tempera-
ture detection logic on a single chip. Thermocouple and
RTD ASICs are available that have an amplifier, ADC
and linearization logic integrated into a single IC. Serial
output sensors offer the advantage of reducing the
system component count, as well as using digital logic
to provide a linear output.
Figure 7 provides an example of the timing diagram of
the SPI serial output of the TC77 silicon IC sensor
where the temperature data is represented by a 13-bit
two’s complement digital word. The SPI serial interface
consists of the Chip Select (CS), Serial Clock (SCK)
and bidirectional Serial Data (SI/O) signals. Communi-
cation with the TC77 is initiated when the CS goes to a
logic ‘0’ and the SI/O then transmits the first bit of data.
The Least Significant Bit (LSB) is equal to 0.0625°C.
The SCK input is provided by the microcontroller and
data is transferred on the rising edge of SCK. Once 13
bits of data have been transmitted, the SI/O line is then
tri-stated.
CS
CLK
1
8
13
SI/O ^HI-Z Sign 2⁷ 2⁶ 2⁵ 2⁴ ₂³ 2² ₂¹ 2⁰ 2^-1 2^-2 2^-3 2^-4
HI-Z
FIGURE 7:
TC77 SPI Serial Output Silicon IC Sensor.
Logic Output Sensors
Logic output sensors are sometimes referred to as
temperature switches because they typically function
as a thermostat to notify the system that a maximum or
minimum temperature limit has been detected.
Figure 8 shows an example of a thermistor and silicon
logic output sensor.
The features of logic output sensors include:
• Logic level output
• Output indicates that the temperature is above (or
below) a preset value
• Available in both push-pull and open-drain
configurations
• Output signal can be either active-low or high
• Either factory or user-programmable temperature
settings
Logic level output sensors are similar to analog output
sensors, except that the output amplifier of the sensor
is a comparator. The comparator circuit sets the switch
point of the sensor through either internal or external
resistors. The output is typically not latched and, thus,
the switch will turn-off when the temperature falls below
the temperature set point. Note that it is necessary to
have hysteresis so the switch does not “chatter” when
crossing the temperature setpoint.
DS00929A-page 4
 2004 Microchip Technology Inc.