LTC2484CDD datasheet(23/40 Pages) LINER

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LTC2484

2484fa

used to shift the conversion result into external circuitry.

EOC can be latched on the first rising edge of SCK and the

last bit of the conversion result on the 32nd rising edge of

SCK. After the 32nd rising edge, SDO goes HIGH

(EOC = 1), SCK stays HIGH and a new conversion starts.

CS remains LOW during the data output state. However,

the data output state may be aborted by pulling CS HIGH

anytime between the first and 32nd rising edge of SCK (see

Figure 9). On the rising edge of CS, the device aborts the

data output state and immediately initiates a new conver-

sion. If the device has not finished loading the last input bit

(SPD) of SDI by the time CS is pulled HIGH, the SDI

information is discarded and the previous configuration is

still kept. This is useful for systems not requiring all 32 bits

of output data, aborting an invalid conversion cycle, or

synchronizing the start of a conversion. If CS is pulled

HIGH while the converter is driving SCK LOW, the internal

pull-up is not available to restore SCK to a logic HIGH state.

This will cause the device to exit the internal serial clock

mode on the next falling edge of CS. This can be avoided

by adding an external 10k pull-up resistor to the SCK pin

or by never pulling CS HIGH when SCK is LOW.

APPLICATIO S I FOR ATIO

When testing EOC, if the conversion is complete (EOC = 0),

the device will exit the low power mode during the EOC

test. In order to allow the device to return to the low power

sleep state, CS must be pulled HIGH before the first rising

edge of SCK. In the internal SCK timing mode, SCK goes

HIGH and the device begins outputting data at time tEOCtest

after the falling edge of CS (if EOC = 0) or tEOCtest after EOC

goes LOW (if CS is LOW during the falling edge of EOC).

The value of tEOCtest is 12µs if the device is using its internal

oscillator. If FO is driven by an external oscillator of

frequency fEOSC, then tEOCtest is 3.6/fEOSC in seconds. If CS

is pulled HIGH before time tEOCtest, the device returns to

the sleep state and the conversion result is held in the

internal static shift register.

If CS remains LOW longer than tEOCtest, the first rising

edge of SCK will occur and the conversion result is serially

shifted out of the SDO pin. The data I/O cycle concludes

after the 32nd rising edge. The input data is shifted in via

the SDI pin on the rising edge of SCK (including the first

rising edge) and the output data is shifted out of the SDO

pin on each falling edge of SCK. The internally generated

serial clock is output to the SCK pin. This signal may be

Figure 8. Internal Serial Clock, Single Cycle Operation

DON’T CARE

FOA

FOB

SPD

SDI

DON’T CARE

SDO

SCK

(INTERNAL)

MSB

SIG

BIT 0

LSB

BIT 5

TEST EOC

BIT 27

BIT 26

BIT 25

BIT 24

BIT 28

BIT 29

BIT 30

EOC

BIT 31

SLEEP

DATA OUTPUT

CONVERSION

2484 F08

<tEOCtest

Hi-Z

TEST EOC

VCC

VREF

IN+

IN–

SCK

SDI

SDO

GND

210

INT/EXT CLOCK

10k

VCC

REFERENCE

VOLTAGE

0.1V TO VCC

ANALOG

INPUT

µF

2.7V TO 5.5V

LTC2484

4-WIRE

SPI INTERFACE

LTC2484CDD Datasheet(PDF) 23 Page - Linear Technology

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