MCP3002-ISLASHMS datasheet(17/34 Pages) MICROCHIP

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DS21294D-page 17

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

configuration. 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

transmitted 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 out-

put 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

provided 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 DIN 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 “Using the MCP3002 with Microcon-

troller (MCU) SPI Ports”

for more details on using the

MCP3002 devices with hardware SPI ports.

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

conversion period at any time during the transmission.

Faster conversion rates can be obtained by using this

technique 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:

CONFIGURING BITS FOR

THE MCP3002

CONFIG BITS

CHANNEL

SELECTION

GND

SGL/

DIFF

ODD/

SIGN

Single-Ended

Mode

—

Pseudo-

Differential

Mode

IN+

IN-

—

IN-

IN+

—

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