LT1355 datasheet(35/40 Pages) LINER

35 / 40 page

LTC2358-16

Rev A

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Figure 20. Internal SoftSpan Configuration Register Behavior. Serial LVDS Bus Response to CS

tothedataonSDO.Wheneverpossible,itisrecommended

that rising and falling edges of SCKO be used to capture

DDR serial output data on SDO, as this will yield the best

robustness to delay variations over supply and tempera-

ture. SCKI rising and falling edges also latch SoftSpan

configuration words provided on SDI, which are used to

programtheinternal24-bitSoftSpanconfigurationregister.

See the section Programming the SoftSpan Configuration

Figure 20, the LVDS bus is enabled when CS is low and is

disabled and Hi-Z when CS is high, allowing the bus to be

shared across multiple devices. Due to the high speeds

involved in LVDS signaling, LVDS bus sharing must be

carefullyconsidered.Transmissionlinelimitationsimposed

by the shared bus may limit the maximum achievable bus

clock speed. LVDS inputs are internally terminated with a

100Ω differential resistor when CS is low, while outputs

must be differentially terminated with a 100Ω resistor at

the receiver (FPGA). SCKI must idle in the low state in

LVDS I/O mode, including when transitioning CS.

ThedataonSDOaregroupedinto24-bitpacketsconsisting

of a 16-bit conversion result followed by two zeros, 3-bit

analog channel ID, and 3-bit SoftSpan code, all presented

MSBfirst.AssuggestedinFigures19and20,SDOoutputs

these packets for all analog input channels in a sequential,

circular manner. For example, the first 24-bit packet output

on SDO corresponds to analog input channel 0, followed

by the packets for channels 1 through 7. The data output

on SDO then wraps back to channel 0, and this pattern

repeats indefinitely.

APPLICATIONS INFORMATION

(CMOS)

PD = 0

DON’T CARE

BUSY

(CMOS)

SCKO

(LVDS)

SDO

(LVDS)

SCKI

(LVDS)

SDI

(LVDS)

DON’T CARE

235816 F20

Hi-Z

CHANNEL 0 PACKET

CHANNEL 1 PACKET

CHANNEL 2 PACKET

CHANNEL3PACKET

(PARTIAL)

Hi-Z

NEW SoftSpan CONFIGURATION WORD

(OVERWRITES INTERNAL CONFIG REGISTER)

TWO ALL-ZERO WORDS AND ONE PARTIAL WORD

(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)

tEN

tDIS

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