MCP3002-ISLASHMS datasheet(13/28 Pages) MICROCHIP

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DS21294C-page 13

MCP3002

5.0

SERIAL COMMUNICATIONS

5.1

Overview

Communication with the MCP3002 is done using a

standard SPI-compatible serial interface. Initiating

communication with the device is done by bringing the

CS line low. See Figure 5-1. If the device was powered

up with the CS pin low, it must be brought high and

back low to initiate communication. The first clock

received with CS low and DIN high will constitute a start

bit. The SGL/DIFF bit and the ODD/SIGN bit follow the

start bit and are used to select the input channel config-

uration. The SGL/DIFF is used to select single ended

or psuedo-differential mode. The ODD/SIGN bit selects

which channel is used in single ended mode, and is

used to determine polarity in psuedo-differential mode.

Following the ODD/SIGN bit, the MSBF bit is transmit-

ted to and is used to enable the LSB first format for the

device. If the MSBF bit is high, then the data will come

from the device in MSB first format and any further

clocks with CS low, will cause the device to output

zeros. If the MSBF bit is low, then the device will output

the converted word LSB first after the word has been

transmitted in the MSB first format. Table 5-1 shows the

configuration bits for the MCP3002. The device will

begin to sample the analog input on the second rising

edge of the clock, after the start bit has been received.

The sample period will end on the falling edge of the

third clock following the start bit.

On the falling edge of the clock for the MSBF bit, the

device will output a low null bit. The next sequential 10

clocks will output the result of the conversion with MSB

first as shown in Figure 5-1. Data is always output from

the device on the falling edge of the clock. If all 10 data

bits have been transmitted and the device continues to

receive clocks while the CS is held low (and the MSBF

bit is high), the device will output the conversion result

LSB first as shown in Figure 5-2. If more clocks are pro-

vided to the device while CS is still low (after the LSB

first data has been transmitted), the device will clock

out zeros indefinitely.

If necessary, it is possible to bring CS low and clock in

leading zeros on the D

IN line before the start bit. This is

often done when dealing with microcontroller-based

SPI ports that must send 8 bits at a time. Refer to

Section 6.1 for more details on using the MCP3002

devices with hardware SPI ports.

If it is desired, the CS can be raised to end the conver-

sion period at any time during the transmission. Faster

conversion rates can be obtained by using this tech-

nique if not all the bits are captured before starting a

new cycle. Some system designers use this method by

capturing only the highest order 8 bits and ‘throwing

away’ the lower 2 bits.

TABLE 5-1:

Configuration Bits for the MCP3002.

CONFIG

BITS

CHANNEL

SELECTION

GND

SGL/

DIFF

ODD/

SIGN

SINGLE

ENDED MODE

PSEUDO-

DIFFERENTIAL

MODE

IN+

IN-

IN+

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