PCA9511 PHILIPS [NXP Semiconductors], PCA9511 Datasheet - Page 6

• Manufacturer Part Number: PCA9511
• Description: Hot swappable I2C and SMBus bus buffer
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
OPERATION
Start-up
An under voltage/initialization circuit holds the parts in a
disconnected state which presents high impedance to all SDA and
SCL pins during power-up. A low on the enable pin also forces the
parts into the low current disconnected state when the I
essentially zero. As the power supply is brought up and the enable
is high or the part is powered and the enable is taken from low to
high it enters an initialization state where the internal references are
stabilized and the precharge circuit for PCA9510 (IN only) and
PCA9511 are enabled. At the end of the initialization state the “Stop
Bit And Bus Idle” detect circuit is enabled. With the enable pin high
long enough to complete the initialization state and remaining high
when all the SDA and SCl pins have been high for the bus idle time
or when all pins are high and a stop condition is seen on the SDAIN
and SCLIN pins, SDAIN is connected to SDAOUT and SCLIN is
connected to SCLOUT. The 1 V precharge circuitry is activated
during the initialization and is deactivated when the connection is
made. The precharge circuitry pulls up the SDA and SCL pins to 1 V
through individual 100 k nominal resistors. This precharges the pins
to 1 V to minimize the worst case disturbances that result from
inserting a card into the backplane where the backplane and the
card are at opposite logic levels.
Connect Circuitry
Once the connection circuitry is activated, the behavior of SDAIN
and SDAOUT as well as SCLIN and SCLOUT become identical with
each acting as a bidirectional buffer that isolates the input
capacitance from the output bus capacitance while communicating
the logic levels. A low forced on either SDAIN or SDAOUT will
cause the other pin to be driven to a low by the part. The same is
also true for the SCL pins. Noise between 0.7V
generally ignored because a falling edge is only recognized when it
falls below 0.7V
falling edge is seen on one pin the other pin in the pair turns on a
pull down driver that is referenced to a small voltage above the
falling pin. The driver will pull the pin down at a slew rate determined
by the driver and the load initially, because it does not start until the
first falling pin is below 0.7V
or slow slew rate, if it is faster than the pull down slew rate then the
initial pull down rate will continue. If the first falling pin has a slow
slew rate then the second pin will be pulled down at its initial slew
rate only until it is just above the first pin voltage the they will both
continue down at the slew rate of the first.
Once both sides are low they will remain low until all the external
drivers have stopped driving lows. If both sides are being driven low
to the same value for instance, 10 mV by external drivers, which is
the case for clock stretching and is typically the case for
acknowledge, and one side external driver stops driving that pin will
rise and rise above the nominal offset voltage until the internal driver
catches up and pulls it back down to the offset voltage. This bounce
is worst for low capacitances and low resistances, and may become
excessive. When the last external driver stops driving a low, that pin
will bounce up and settle out out just above the other pin as both
rise together with a slew rate determined by the internal slew rate
control and the RC time constant. As long as the slew rate is at least
1.25 V/ s, when the pin voltage exceeds 0.6 V for the PCA9511, the
rise time accelerators circuits are turned on and the pull down driver
is turned off.
2004 Oct 05
Hot swappable I
CC
with a slew rate of at least 1.25 V/ s. When a
CC
. The first falling pin may have a fast
2
C and SMBus bus buffer
CC
and V
CC
CC
is
is
6
Maximum number of devices in series
Each buffer adds about 0.065 V dynamic level offset at 25 C with
the offset larger at higher temperatures. Maximum offset (V
0.150 V. The LOW level at the signal origination end (master) is
dependent upon the load and the only specification point is the
I
lightly loaded the V
V
about 0.1 V below the threshold of the rising edge accelerator (about
0.6 V). With great care a system with four buffers may work, but as
the V
in firing the rising edge accelerator thus introducing false clock
edges. Generally it is recommended to limit the number of buffers in
series to two.
The PCA9510 (rise time accelerator is permanently disabled) and
the PCA9512 (rise time accelerator can be turned off) are a little
different with the rise time accelerator turned off because the rise
time accelerator will not pull the node up, but the same logic that
turns on the accelerator turns the pull-down off. If the V
will not turn back on until a falling edge is detected; so if the noise is
small enough it may be possible to use more than two PCA9510 or
PCA9512 parts in series but is not recommended.
Consider a system with three buffers connected to a common node
and communication between the Master and Slave B that are
connected at either end of Buffer A and Buffer B in series as shown
in Figure 6. Consider if the V
the V
changing from Master to Slave B and then from Slave B to Master.
Before the direction change you would observe V
Buffer A of 0.3 V and its output, the common node, is 0.4 V. The
output of Buffer B and Buffer C would be 0.5 V, but Slave B is
driving 0.4 V, so the voltage at Slave B is 0.4 V. The output of
Buffer C is 0.5 V. When the Master pull-down turns off, the input of
Buffer A rises and so does its output, the common node, because it
is the only part driving the node. The common node will rise to 0.5 V
before Buffer B’s output turns on, if the pull-up is strong the node will
bounce. If the bounce goes above the threshold for the rising edge
accelerator 0.6 V the accelerators on both Buffer A and Buffer C
will fire contending with the output of Buffer B. The node on the input
of Buffer A will go HIGH as will the input node of Buffer C. After the
common node voltage is stable for a while the rising edge
accelerators will turn off and the common node will return to 0.5 V
because the Buffer B is still on. The voltage at both the Master and
Slave C nodes would then fall to 0.6 V until Slave B turned off. This
would not cause a failure on the data line as long as the return to
0.5 V on the common node ( 0.6 V at the Master and Slave C)
occurred before the data setup time. If this were the SCL line, the
parts on Buffer A and Buffer C would see a false clock rather than a
stretched clock, which would cause a system error.
2
0.6 V and a rising edge is detected, the pull-down will turn off and
OS
C-bus specification of 3 mA will produce V
= 0.1 V, the level after four buffers would be 0.5 V, which is only
OL
OL
MASTER
moves up from 0.1 V, noise or bounces on the line will result
of Slave B (when acknowledging) is 0.4 V with the direction
OL
buffer A
may be 0.1 V. Assuming V
common
OL
Figure 6.
node
PCA9510; PCA9511
at the input of Buffer A is 0.3 V and
buffer B
buffer C
OL
< 0.4 V, although if
IL
SLAVE C
Product data sheet
OL
SLAVE B
SW02353
at the input of
= 0.1 V and
IL
is above
OS
) is