MCP3302 datasheet(28/46 Pages) MICROCHIP

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MCP3302/04

DS21697E-page 28

6.3

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

rising 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 6-4 and Figure 6-5 show how the

MCP3302/04 devices can be interfaced to a MCU with

a hardware SPI port. Figure 6-4 depicts the operation

shown in SPI Mode 0,0, which requires that the SCLK

from the MCU idles in the ‘low’ state, while Figure 6-5

shows the similar case of SPI Mode 1,1, where the

clock idles in the ‘high’ state.

As shown in Figure 6-4, 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

converted data can be obtained by using this method.

Figure 6-5 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.

FIGURE 6-4:

SPI Communication with the MCP3302/04 using 8-bit segments

(Mode 0,0: SCLK idles low).

1213

1415

SCLK

DIN

X = Don’t Care Bits

DOUT

NULL

BIT

B11 B10 B9

B6 B5

B3 B2

B1 B0

HI-Z

MCU latches data from A/D Converter

Data is clocked out of

A/D Converter on falling edges

on rising edges of SCLK

Don’t Care

SGL/

DIFF

Start

0000

XXX

B11 B10 B9

???

SGL/

DIFF

Start

Bit

(Null)

MCU Transmitted Data

(Aligned with falling

edge of clock)

MCU Received Data

(Aligned with rising

edge of clock)

Data stored into MCU receive

bits

Data stored into MCU receive

second 8 bits

Data stored into MCU receive

8 bits

? = Unknown Bits

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