The SPI Function - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 500

10.10.4 Format Data to Be Sent to the MCP4922
According to Figure 10.18, the user needs to represent the value to be converted in 12 bits
and set the most significant 4 bits properly to select the channel, shutdown mode, gain, and
reference voltage buffer mode.
Assume we want to generate a 3-V output from the V OUTA pin, the value to be sent to the
MCP4922 can be calculated as follows:
• Select channel A (set bit 15 to 0)
• Select unbuffered mode (set bit 14 to 0)
• Set gain to 1 (set bit 13 to 1)
• Enable output buffer (set bit 12 to 1)
• 3 V corresponds to the digital value (lowest 12 bits) of 2 12 * 3/5 5 2458 5 0x99A
• The value to be sent to MCP4922 is 0011 1001 1001 1010 5 0x399A

10.10.5 Interfacing the MCP4922 with the HCS12
A typical circuit connection for interfacing the HCS12 wit h t he MCP4922 is shown in
Figure 10.19. Two unused I/O port pins must be used to drive the CS and FS pins.

HCS12 MCP4922

5 V

MOSI

SDI V DD

V REFA

SCK

SCK

SHDN

PK7

CS

LDAC

V OUTA

AVSS

Figure 10.19 ■ Circuit connection between the
HCS12 and MCP4922

Example 10.7
▼ Write a program to generate the waveform from the V OUTA pin shown in Figure 10.20 using
the circuit shown in Figure 10.19 assuming that the E-clock frequency for the HCS12 is 24 MHz.

4.5 V

3.0 V

1.5 V

1 ms 1 ms 1 ms 1 ms 1 ms 1 ms 1 ms 1 ms 1 ms

Figure 10.20 ■ Waveform to be generated