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Enable

Expansion

PCA9518

SCPS132A – JUNE 2006 – REVISED JULY 2007

EN1–EN4 are active-high enable pins and have internal pullup resistors. Each enable pin, ENn, controls its

associated SDAn and SCLn ports. When ENn is low, it isolates its corresponding SDAn and SCLn from the

system by blocking the inputs from SDAn and SCLn and disabling the output drivers on the SDAn and SCLn

pins. It is essential that the ENn change state only when both the global bus and the local port are in an idle

state to prevent system failures. EN1–EN4 also allow the use of open-drain drivers that can be wire-ORed to

create a distributed enable where either centralized control signal (master) or spoke signal (submaster) can

enable the channel when it is idle.

The PCA9518 has four open-drain I/O pins used for expansion. The internal state of the serial data within each

hub is communicated to other hubs through two expansion pins, EXPSDA1 and EXPSDA2. The EXPSDA1 pins

of all hubs are connected together to form an open-drain bus. Similarly, all EXPSDA2 pins, EXPSCL1 pins, and

EXPSCL2 pins are connected together, forming a 4-wire bus between hubs. When it is necessary to be able to

deselect every port, each expansion device contributes only four ports that can be enabled or disabled; the fifth

port does not have an enable pin. Pullup resistors are required on the EXPxxxx pins, even if only one PCA9518

is used.

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