MCP3918 datasheet(55/88 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005287A-page 55

MCP3918

7.0

2-WIRE SERIAL INTERFACE

DESCRIPTION

7.1

Overview

The 2-Wire Interface mode is designed for applications

that require galvanic isolation. It allows a minimum

number of digital isolator channels, specifically one

bi-directional or two unidirectional channels, to be

connected to the MCP3918 when interfacing through

an isolation barrier. This functionality reduces the total

system cost in an isolated application system, like a

polyphase

shunt-based

energy

meter.

recommended to use the MCP3918 with the 2-Wire

mode for digitally isolated applications and with the SPI

mode for other applications where galvanic isolation is

not required.

The principle of this 2-Wire interface is simple: it has a

serial clock input pin (SCK/MCLK) and a serial data

output pin (SDO), and it automatically sends output

data in packets (frames) at a DRCLK data rate (every

time new data is available on the ADC output). It has no

serial input pin to diminish the number of isolated

channels. At the same time, the serial clock pin SCK

also becomes the master clock (MCLK) input pin of the

device, and the part becomes fully synchronous with

SCK = MCLK. The system then becomes fully

synchronous and can be driven by only one master

clock for multiple phases, which ensures proper

synchronization and constant phase angle between

phases, which is important for an energy metering

application.

The SDO pin becomes the only output of the device

and is fully synchronous with the serial/master clock.

The SDO pin is never in high-impedance in this mode,

and is by default at logic low when not transmitting

data. The SDO pin idles logic low in this mode because

most of the digital isolator devices consume less

current in a logic low state than in a logic high state.

This effectively reduces the total power consumption of

a system with digital isolation devices.

When the part has entered 2-Wire mode, the logic pins

RESET, SDI, CS, OSC1 and DR become logic input

pins for the configuration of the device (respectively

OSR0/OSR1/BOOST/GAIN0/GAIN1).

These

pins

need to have well-defined logic states for low-power

applications. These pins define the only settings that

can be modified in 2-Wire mode.

The MDAT0 pin is always disabled and kept in a

high-impedance state during the 2-Wire Interface

mode. This pin can be grounded for applications using

exclusively the 2-Wire Interface mode so that the

EMI/EMC susceptibility of the part is improved.

FIGURE 7-1:

MCP3918 2-Wire Interface Typical Application Schematic.

SDO

GAIN1

SCK/MCLK

OSR0

BOOST

CH0+

CH0-

MCP3918

ANALOG

DIGITAL

SINC

PGA

Modulator

MOD<3:0>

Isolator

2-wire

Interface

OSR1

To SPI Ports

Of an MCU

GAIN0

Logic inputs

connected

to DVDD or

DGND

Main MCU/

CPU Board

Isolation

Barrier

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