LTC4332

13

Rev. A

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Figure 12. SS1 and SS2 Transfer. (REM_SS1_POL, REM_SS1_PHA) = (0,0). (REM_SS2_POL, REM_SS2_PHA) = (1,1). WORD_LENGTH = 8

APPLICATIONS INFORMATION

slave select pin is low and the first word is shifted by pin

SCK. It is invalid to assert more than one slave select pin

concurrently.

Remote Mode

The LTC4332 is in SPI master mode when the REMOTE

pin is set high. Pins SCK and MOSI are outputs while pin

MISO is an input. Three slave selects output pins SS1, SS2,

SS3 are available to connect to remote SPI slave devices.

The remote side LTC4332 first asserts the appropriate slave

select pin mirroring the event from the local side and then

toggles pins SCK with MOSI data as write-data becomes

available from the link. Using the same SCK edge, the

LTC4332 captures data on the MISO pin from the remote

slave device, encodes the data, and transmits the created

bit-stream along the second differential pair to the local side.

The remote side SCK period reflects the local side SCK

period offset by a jitter parameter tSCKJ:MASTER. See

Figure 8. Care must be taken to ensure that the maximum

supported frequency of the remote device is not violated.

The SCK duty cycle is as follows. When the remote SPI

mode is set to (0,0) or (1,1), the low time tSCKL:MASTER

applies while SCK high time = 1/fSCK:SLAVE – tSCKL:MASTER

+ tSCKJ:MASTER. When the remote SPI mode is set to (0,1)

or (1,0), the high time tSCKH:MASTER applies while the SCK

low time = 1/fSCK:SLAVE – tSCKH:MASTER + tSCKJ:MASTER.

The remote SPI master device supports SPI modes (0,0),

(0,1), (1,0), and (1,1) for each remote slave select. Each

remote slave select SPI mode can be individually pro-

grammed by the user to match the SPI mode require-

ments of the remote slave device. The SPI mode is pro-

grammed by writing the CONFIG register in the control

interface. SPI mode switching is handled automatically by

the LTC4332. When the user asserts a remote slave select

to start a transfer, the remote side LTC4332 configures

its SPI interface mode to match the mode of the remote

device before asserting the remote slave select signal.

Differential Link Pair

Internally, the LTC4332 utilizes a high performance trans-

ceiver to communicate over the link pair. The A, B, Y,

and Z pins are fault protected to ±60V. In addition, the

transceiver operates over an extended common mode

range of ±25V making it suitable for noisy environments

or systems with ground potential differences. Data is

exchanged between the LTC4332 devices using a cus-

tom packet which has a selectable baud rate based on the

configuration of the SPEED1 and SPEED2 pins. Selectable

baud rates over the cable allow balancing performance

with cable length specific to the application. Both sides

of the link must be set to the same speed configuration

to match the baud rates and allow communication. Each

LOCAL SIDE

READ1

READ2

MISO

SCK

SS1

BUS-HOLD

SS2

READ1

READ2

BUS-HOLD

WRITE2

WRITE3

MOSI

WRITE1

WRITE1

WRITE2

WRITE3

REMOTE SIDE

READ2

READ3

MISO

SCK

SS1

SS2

WRITE2

WRITE3

MOSI

WRITE1

WRITE1

WRITE2

WRITE3

READ1

READ1

READ2

READ3

POL = 1

POL = 0

4332 F12