Datasheet

© 2012 Microchip Technology Inc. DS22286A-page 43
MCP3911
6.0 SERIAL INTERFACE
DESCRIPTION
6.1 Overview
The MCP3911 device is compatible with SPI Modes 0,0
and 1,1. Data is clocked out of the MCP3911 on the
falling edge of SCK and data is clocked into the
MCP3911 on the rising edge of SCK. In these modes,
SCK can Idle either high or low.
Each SPI communication starts with a CS
falling edge
and stops with the CS
rising edge. Each SPI
communication is independent. When CS
is high, SDO
is in high-impedance, transitions on SCK, and SDI
have no effect. Additional controls: RESET, DR and
MDAT0/1 are also provided on separate pins for
advanced communication.
The MCP3911 interface has a simple command
structure. The first byte transmitted is always the
CONTROL byte and is followed by data bytes that are
8-bits wide. Both ADCs are continuously converting
data by default and can be reset or shut down through
a CONFIG register setting.
Since each ADC data is either 16 or 24 bits (depending
on the WIDTH bits), the internal registers can be
grouped together with various configurations (through
the READ bits) in order to allow easy data retrieval within
only one communication. For device reads, the internal
address counter can be automatically incremented in
order to loop through groups of data within the register
map. The SDO will then output the data located at the
ADDRESS (A<4:0>) defined in the control byte and then
ADDRESS + 1 depending on the READ<1:0> bits,
which select the groups of registers. These groups are
defined in Section 7.1 “CHANNEL REGISTERS - ADC
Channel Data Output Registers” (Register Map).
The Data Ready pin (DR
) can be used as an interrupt
for an MCU and outputs pulses when a new ADC
channel data is available. The RESET
pin acts like a
Hard Reset and can reset the part to its default power-
up configuration. The MDAT0/1 pins give the modulator
outputs (see Section 5.4 “Modulator Output Block”).
6.2 Control Byte
The control byte of the MCP3911 contains two device
Address bits, A<6:5>, 5 register Address bits, A<4:0>,
and a Read/Write bit (R/W). The first byte transmitted
to the MCP3911 is always the control byte.
The MCP3911 interface is device addressable (through
A<6:5>) so that multiple MCP3911 chips can be pres-
ent on the same SPI bus with no data bus contention.
This functionality enables three-phase power metering
systems, containing three MCP3911 chips, controlled
by a single SPI bus (single CS
, SCK, SDI and SDO
pins).
FIGURE 6-1: Control Byte.
The default device address bits are ‘00’. Contact the
Microchip factory for additional device address bits. For
more information, please see the Product Identification
System section.
A read on undefined addresses will give an all zeros
output on the first, and all subsequent transmitted
bytes. A write on an undefined address will have no
effect, and also will not increment the address counter.
The register map is defined in Table 7-1.
6.3 Reading from the Device
The first data byte read is the one defined by the
address given in the CONTROL byte. After this first
byte is transmitted, if the CS
pin is maintained low, the
communication continues and the address of the next
transmitted byte is determined by the status of the
READ bits in the STATUSCOM register. Multiple
looping configurations can be defined through the
READ<1:0> bits for the address increment (see
Section 6.6 “SPI MODE 0,0 – Clock Idle Low, Read/
Write Examples”).
6.4 Writing to the Device
The first data byte written is the one defined by the
address given in the control byte. Two write mode con-
figurations for the address increment can be defined
through the WRITE bit in the STATUSCOM register.
When WRITE = 1, the write communication automati-
cally increments the address for subsequent bytes. The
address of the next transmitted byte within the same
communication (CS
stays logic low) is the next address
defined on the register map. At the end of the register
map, the address loops to the beginning of the writable
part of the register map (address 0x06). Writing a non-
writable register has no effect. When WRITE = 0, the
address is not incremented on the subsequent writes.
The SDO pin stays in high-impedance during a write
communication.
A6
A5
A4 A3
A2
A1
A0
R/W
Read/
Write Bit
Register
Device
Address Bits
Address
Bits