The SPI Function - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 492

negative. The magnitude of a negative temperature can be found by taking the two's comple-
ment of the temperature reading. After this is done, the upper 8 bits become the integer part
of the temperature, whereas the least significant 2 bits become the fractional part of the tem-
perature. A sample of the temperature readings and their corresponding temperature values are
shown in Table 10.3.

Binary
High Byte/Low Byte Hex Temperature

0010 0001/0100 0000 2140 33.25°C

0100 1010/1000 0000 4A80 74.5°C

0001 1010/1100 0000 1AC0 26.75°C

0000 0001/1000 0000 0180 1.5°C

0000 0000/0000 0000 0000 0°C

1111 1111/1000 0000 FF80

2 0.5°C

1111 0010/1100 0000 F2C0

2 13.25°C

1110 0111/0000 0000 E700

2 24°C

1100 1001/0100 0000 C900

2 55°C

Table 10.3 ■ TC72 Temperature output data

10.9.3 Serial Bus Interface
The serial interface consists of chip enable (CE), serial clock (SCK), serial data input (SDI),
and serial data output (SDO). The CE input is used to select TC72 when there are multiple SPI
slaves connected to the microcontroller. TC72 can operate as a SPI slave only.
The SDI input writes data into the TC72's control register, while the SDO output pin reads
the temperature data from the temperature register and the status of the shutdown bit of the
control register. The TC72 can shift data in or out using either the rising or the falling edge of
the SCK input. The CE signal is active high. The SCK idle state is detected when the CE signal
goes high. As shown in Figure 10.14, the clock polarity (CP) of SCK determines whether data is
shifted on the rising or the falling edge. The highest SCK frequency ( f SCK ) is 7.5 MHz.

CP = 0 CP = 1

CE

SCK

Shift
edge

Shift
edge

(b) Data shift on falling edge

(a) Data shift on rising edge

Figure 10.14 ■ Serial clock polarity