PCA9518A_3 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 03 — 3 December 2008 5 of 23
NXP Semiconductors
PCA9518A
Expandable 5-channel I
2
C-bus hub
6. Functional description
The PCA9518A CMOS integrated circuit is a five-way hub repeater, which enables
I
2
C-bus and similar bus systems to be expanded in increments of five with only one
repeater delay and no functional degradation of system performance.
The PCA9518A CMOS integrated circuit contains five multi-directional, open-drain buffers
specifically designed to support the standard low-level contention arbitration of the
I
2
C-bus. Except during arbitration or clock stretching, the PCA9518A acts like a pair of
non-inverting, open-drain buffers, one for SDA and one for SCL.
Refer to Figure 1 “Block diagram of PCA9518A”.
6.1 Enable
The enable pins EN1 through EN4 are active HIGH and have internal pull-up resistors.
Each enable pin ENn controls its associated SDAn and SCLn ports. When LOW, the ENn
pin blocks the inputs from SDAn and SCLn, as well as disabling the output drivers on the
SDAn and SCLn pins. The enable pins should only change state when both the global bus
and the local port are in an idle state to prevent system failures.
The active HIGH enable pins allow the use of open-drain drivers which can be wire-ORed
to create a distributed enable where either centralized control signal (master) or spoke
signal (sub-master) can enable the channel when it is idle.
Unused channels must have pull-up resistors unless their enable pin (ENn) is always
LOW. Port 0 must always have pull-up resistors since it is always present in the bus and
cannot be disabled.
6.2 Expansion
The PCA9518A includes 4 open-drain I/O pins used for expansion. Two expansion pins,
EXPSDA1 and EXPSDA2 are used to communicate the internal state of the serial data
within each hub to the other hubs. The EXPSDA1 pins of all hubs are connected together
to form an open-drain bus. Similarly, all EXPSDA2 pins, EXPSCL1 pins, and all 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 only
contributes 4 ports which can be enabled or disables because the fifth does not have an
enable pin.
Pull-up resistors are required on the EXPxxxn
3
pins even if only one PCA9518A is used.
6.3 I
2
C-bus systems
As with the standard I
2
C-bus system, pull-up resistors are required to provide the logic
HIGH levels on the buffered bus. (Standard open-collector or open-drain configuration of
the I
2
C-bus). The size of these pull-up resistors depends on the system, but each side of
the repeater must have a pull-up resistor. This part is designed to work with
Standard-mode (0 Hz to 100 kHz) and Fast-mode (0 Hz to 400 kHz) I
2
C-bus devices in
addition to SMBus devices. Standard-mode I
2
C-bus devices only specify 3 mA output
drive; this limits the termination current to 3 mA in a generic I
2
C-bus system where
3. ‘xxxn’ is SDA1, SDA2, SCL1 or SCL2. ‘xxx’ is SDA or SCL.
