The PCA9618 is a CMOS integrated circuit
that provides Fast-mode Plus (Fm+) I²C-bus or SMBus buffering of either the SCL or SDA line or can be used in any
single bit applications that require buffering. While retaining all the operating modes
and features of the I²C-bus
system, it also permits extension of the I²C-bus by providing bidirectional
buffering, thus enabling buses of 540
pF
at 1 MHz or up to 4000 pF at lower speeds. Using the PCA9618 enables the system designer
to isolate two halves of a bus for capacitance. The DATAA and DATAB pins are overvoltage
tolerant and are high-impedance when the PCA9618 is unpowered.
The PCA9618 can be used to delay the SDA
path by at least the minimum propagation delay,
thereby
providing an effective data hold time for applications that require an SDA hold time
with respect to SCL. The PCA9618 includes an internal glitch filter, so that it will
ignore glitches rather than expand them, this also means that the minimum propagation
delay is on the order of 46 ns and is reasonably stable over temperature and supply
voltage. Although this is not sufficient to provide the SDA delay requirement of the
SMBus hold time specification of 300 ns, many parts that cannot be used with the 0 ns
hold time of the I2C-bus specification only require a few ns of hold time to work
correctly in a system. For these applications, the PCA9618 is an effective solution.
The PCA9618 can also be used as a buffer on
a 1-wire bus similar to how the PCA9617A is used for the I²C-bus or to buffer the master
(microcontroller’s I/O) if the master’s output is not strong enough to drive the 1-wire
bus.
The 2.2 V to 5.5 V bus port B driver has the
static level offset, while the port A driver eliminates the static offset voltage. This
results in a LOW on the port B translating into a nearly 0 V LOW on the port A. This
static offset voltage prevents the PCA9618 from latching into a steady state LOW when
driven LOW.
The static offset design of the port B
PCA9618 I/O driver prevents it from being connected to the static or incremented offset
sides of other bus buffers. Port A of two or more PCA9618s can be connected together,
also, to allow a star topography with port A on the common bus. Port A can be connected
directly to any other buffer with static or incremented offset outputs. Multiple
PCA9618s can be connected in series, port A to port B, with no build-up in offset
voltage with only time of flight delays to consider.
The output pull-down on the port B internal
buffer LOW is set for approximately 0.55 V, while the input threshold of the internal
buffer is set about 90 mV lower (0.45 V). When the port B I/O is driven LOW internally,
the LOW is not recognized as a LOW by the input. This prevents a latching condition from
occurring. The output pull-down on port A drives a hard LOW and the input level is set
at 0.35VCC.
