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LTC6820

isoSPI Isolated Communications Interface

Linear Technology 

LTC6820

Operation

Multidrop

Multiple slaves can be connected to a single master by con-

necting them in parallel (multidrop configuration)along one

cable. As shown in Figure 16, the cable should be terminated

only at the beginning (master) and the end. In between, the

additional LTC6820’s and their associated slave devices will

be connected to “stubs” on the cable. These stubs should

be kept short, with as little capacitance as possible, to avoid

degrading the termination along the cable.

The multidrop scheme is only possible if the SPI slaves

have certain characteristics:

n The SPI slaves must be addressable, because they will

all see the same CS signal (as decoded by each slave

LTC6820).

n When not addressed, the slave SDO must remain high.

When a slave is not addressed, its LTC6820 will not trans-

mit data pulses as long as MISO (the SPI device’s SDO)

remains high. This eliminates the possibility for collisions,

as only the addressed slave device will ever be returning

data to the master.

MASTER

SDO

SDI

SCK

CS

LTC6820

MSTR

MOSI IP

MISO

RM

SCK IM

CS

LTC6820

MSTR

IP

MOSI

MISO

IM SCK

CS

1

LTC6820

MSTR

IP

MOSI

MISO

IM SCK

CS

2

LTC6820

MSTR

IP

MOSI

RM

MISO

IM SCK

CS

3

SLAVE 1

1

SDI

SDO

SCK

CS

1

SLAVE 2

2 SDI

SDO

SCK

CS

2

SLAVE 3

3 SDI

SDO

SCK

CS

3

6820 F16

Figure 16. Multidropping Multiple Slaves on a Single Cable

6820f

18

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