MCP3919 datasheet(49/88 Pages) MICROCHIP

49 / 88 page

 2014 Microchip Technology Inc.

DS20005347A-page 49

MCP3919

6.8

ADC Channels Latching and

Synchronization

The ADC channel’s data output registers (addresses

0x00 to 0x02) have a double buffer output structure.

The two sets of latches in series are triggered by the

data ready signal and an internal signal indicating the

beginning of a read communication sequence (read

start).

The first set of latches holds each ADC channel data

output register when the data is ready and latches all

active outputs together when DR_LINK = 1. This

behavior is synchronous with the DMCLK clock.

The second set of latches ensures that when reading

starts on an ADC output, the corresponding data is

latched so that no data corruption can occur within a

read. This behavior is synchronous with the SCK clock.

If an ADC read has started, in order to read the follow-

ing ADC output, the current reading needs to be fully

completed (all bits must be read on the SDO pin from

the ADC output data registers).

Since the double output buffer structure is triggered

with two events that depend on two asynchronous

clocks (data ready with DMCLK and read start with

SCK), implement one of the three following methods on

the MCU or processor in order to synchronize the

reading of the channels:

Use the data ready pin pulses as an

interrupt

: once a falling edge occurs on the DR

pin, the data is available for reading on the ADC

output registers after the tDODR timing. If this

timing is not respected, data corruption can

occur.

Use a timer clocked with MCLK as a

synchronization event:

since the data ready is

synchronous with DMCLK, the user can

calculate the position of the data ready

depending on the PHASE, the OSR<2:0> and

the PRE<1:0> settings for each channel. Again,

the tDODR timing needs to be added to this

calculation, to avoid data corruption.

Poll

the

DRSTATUS<2:0>

bits

the

STATUSCOM register:

this method consists of

continuously reading the STATUSCOM register

and waiting for the DRSTATUS bits to be equal

to '0'. When this event happens, the user can

start a new communication to read the desired

ADC data. In this case, no additional timing is

required.

The first method is the preferred one, as it can be used

without adding additional MCU code space but requires

connecting the DR pin to an I/O pin of the MCU. The

last two methods require more MCU code space and

execution time but they allow synchronized reading of

the channels without connecting the DR pin, which

saves one I/O pin on the MCU.

6.9

Securing Read Communications

Through CRC-16 Checksum

Since power/energy metering systems can generate or

receive large EMI/EMC interferences and large

transient spikes, it is helpful to secure SPI

communications as much as possible to maintain data

integrity and desired configurations during the lifetime

of the application.

The communication data on the SDO pin can be

secured through the insertion of a Cyclic Redundancy

Check (CRC) checksum at the end of each continuous

reading

sequence.

The

CRC

checksum

communications can be enabled or disabled through

the EN_CRCCOM bit in the STATUSCOM register. The

CRC message ensures the integrity of the read

sequence bits transmitted on the SDO pin and the CRC

checksum is inserted in between each read sequence

(see Figure 6-9).

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