CS5521 datasheet(25/56 Pages) CIRRUS

25 / 56 page

CS5521/22/23/24/28

DS317F8

1.2.4 Serial Port Interface

The CS5521/22/23/24/28’s serial interface consists

of four control lines: CS, SCLK, SDI, SDO.

Figure 10 illustrates the serial sequence necessary

to write to, or read from the serial port’s registers.

CS (Chip Select) is the control line which enables

access to the serial port. If the CS pin is tied low,

the port can function as a three-wire interface.

SDI (Serial Data In) is the data signal used to trans-

fer data to the converters.

SDO (Serial Data Out) is the data signal used to

transfer output data from the converters. The SDO

output will be held at high impedance any time CS

is at logic 1.

SCLK (Serial Clock) is the serial bit clock which

controls the shifting of data to or from the ADC’s

serial port. The CS pin must be held low (logic 0)

before SCLK transitions can be recognized by the

port logic. To accommodate opto-isolators SCLK

is designed with a Schmitt-trigger input to allow an

opto-isolator with slower rise and fall times to di-

rectly drive the pin. Additionally, SDO is capable

of sinking or sourcing up to 5 mA to directly drive

an opto-isolator LED. SDO will have less than a

400 mV loss in the drive voltage when sinking or

sourcing 5 mA.

Command Time

8SCLKs

Data Time 24 SCLKs

Write Cycle

SCLK

SDI

MSB

LSB

Command Time

8SCLKs

SCLK

SDI

Data Time 24 SCLKs

SDO

MSB

LSB

Read Cycle

Command Time

8 SCLKs

8 SCLKs Clear SDO Flag

SDO

SCLK

SDI

Data Time

24 SCLKs

MSB

LSB

* td = XIN/OWR clock cycles for each conversion except the

first conversion which will take XIN/OWR + 7 clock cycles

XIN/OWR

Clock Cycles

t *

Figure 10. Command and Data Word Timing

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