MCP3551-ESLASHMS datasheet(19/30 Pages) MICROCHIP

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DS21950B-page 19

MCP3551/3

If 24 bits of data are required from this conversion, they

must be accessed during this communication. You can

terminate data transition by bringing CS high, but the

remaining data will be lost and the converter will go into

Shutdown mode. Once the data has been transmitted

by the converter, the SDO/RDY pin will remain in the

LSB state until the 25th falling edge of SCK. At this

point, SDO/RDY is released from the Data Acquisition

mode and changed to the RDY state.

5.4.2

2-WIRE CONTINUOUS

CONVERSION OPERATION,

(CS TIED PERMANENTLY LOW)

It is possible to use only two wires to communicate with

the MCP3551/3 devices. In this state, the device is

always in Continuous Conversion mode, with internal

conversions continuously occurring. This mode can be

entered by having CS low during power-up or changing

it to a low position after power-up. If CS is low at

power-up, the first conversion of the converter is

initiated approximately 300 µs after the power supply

has stabilized.

5.5

Using The MCP3551/3 with

Microcontroller (MCU) SPI™ Ports

It is required that the microcontroller SPI port be config-

ured to clock out data on the falling edge of clock and

latch data in on the rising edge. Figure 5-6 depicts the

operation shown in SPI mode 1,1 which requires that

the SCK from the MCU idles in the High state, while

Figure 5-7 shows the similar case of SPI Mode 0,0

where the clock idles in the Low state. The waveforms

in the figures are examples of an MCU operating the

SPI port in 8-bit mode, and the MCP3551/3 devices do

not require data in 8-bit groups.

In SPI mode 1,1, data is read using only 24 clocks or

three byte transfers. The data ready bit must be read

by testing the SDO/RDY line prior to a falling edge of

the clock.

In SPI mode 0,0, data is read using 25 clocks or four

byte transfers. Please note that the data ready bit is

included in the transfer as the first bit in this mode.

FIGURE 5-6:

SPI™ Communication – Mode 1,1.

Note:

The RDY state is not latched to CS in this

mode; the RDY flag dynamically updates

on the SDO/RDY pin and remains in this

state until data is clocked out using the

SCK pin.

SCK

SDO/RDY

MCU

Data stored into MCU receive

first byte

Receive

Buffer

Data stored into MCU receive

second byte

Data stored into MCU receive

third byte

DO O

21 20 19 18 17

15 14 13 12 11 10 9

7 6 543

2 1

OL OH 21 20 19 18 17 16

15 14 13 12 11 10 9

RH L

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