MCP3302 datasheet(25/40 Pages) MICROCHIP

25 / 40 page

 2002 Microchip Technology Inc.

DS21697B-page 25

MCP3302/04

7.3

Using the MCP3302/04 with

Microcontroller (MCU) SPI Ports

With most microcontroller SPI ports, it is required to

send groups of eight bits. It is also required that the

microcontroller SPI port be configured to clock out data

on the falling edge of clock and latch data in on the ris-

ing edge. Because communication with the MCP3302

and MCP3304 devices may not need multiples of eight

clocks, it will be necessary to provide more clocks than

are required. This is usually done by sending ‘leading

zeros’ before the start bit. For example, Figure 7-4 and

Figure 7-5 show how the MCP3302/04 devices can be

interfaced to a MCU with a hardware SPI port.

Figure 7-4 depicts the operation shown in SPI Mode

0,0, which requires that the SCLK from the MCU idles

in the ‘low’ state, while Figure 7-5 shows the similar

case of SPI Mode 1,1, where the clock idles in the ‘high’

state.

As shown in Figure 7-4, the first byte transmitted to the

A/D Converter contains 6 leading zeros before the start

bit. Arranging the leading zeros this way produces the

13 data bits to fall in positions easily manipulated by the

MCU. The sign bit is clocked out of the A/D Converter

on the falling edge of clock number 11, followed by the

remaining data bits (MSB first). After the second eight

clocks have been sent to the device, the MCU receive

buffer will contain 2 unknown bits (the output is at high

impedance for the first two clocks), the null bit, the sign

bit and the 4 highest order bits of the conversion. After

the third byte has been sent to the device, the receive

conversion results. Easier manipulation of the con-

verted data can be obtained by using this method.

Figure 7-5 shows the same situation in SPI Mode 1,1,

which requires that the clock idles in the high state. As

with mode 0,0, the A/D Converter outputs data on the

falling edge of the clock and the MCU latches data from

the A/D Converter in on the rising edge of the clock.

MCP3302 датащи(PDF) 25 Page - Microchip Technology