Philips Semiconductors Product data
PCA9510; PCA9511
Hot swappable I
2
C and
SMBus bus buffer
2003 Dec 18
6
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
CC
is
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
CC
and V
CC
is
generally ignored because a falling edge is only recognized when it
falls below 0.7V
CC
with a slew rate of at least 1.25 V/µs. When a
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
CC
. The first falling pin may have a fast
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/µs, 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.
Propagation Delays
The delay for a rising edge is determined by the combined pull-up
current from the bus resistors and the rise time accelerator current
source and the effective capacitance on the lines. If the pull-up
currents are the same, any difference in rise time is directly
proportional to the difference in capacitance between the two sides.
The t
PLH
may be negative if the output capacitance is less than the
input capacitance and would be positive if the output capacitance is
larger than the input capacitance, when the currents are the same.
The t
PHL
can never be negative because the output does not start to
fall until the input is below 0.7V
CC
, and the output turn on has a non
zero delay, and the output has a limited maximum slew rate, and
even if the input slew rate is slow enough that the output catches up
it will still lag the falling voltage of the input by the offset voltage. The
maximum t
PHL
occurs when the input is driven low with zero delay
and the output is still limited by its turn on delay and the falling edge
slew rate. The output falling edge slew rate is a function of the
internal maximum slew rate which is a function of temperature. V
CC
and process, as well as the load current and the load capacitance.
Rise Time Accelerators
During positive bus transitions a 2 mA current source is switched on
to quickly slew the SDA and SCL lines high once the input level of
0.6 V for the PCA9511 is exceeded. The rising edge rate should be
at least 1.25 V/µs to guarantee turn on of the accelerators. The
PCA9510 doesn’t have any rise time accelerator circuitry.
READY Digital Output
This pin provides a digital flag which is low when either ENABLE is
low or the start-up sequence described earlier in this section has not
been completed. READY goes high when ENABLE is high and
start-up is complete. The pin is driven by an open drain pull-down
capable of sinking 3 mA while holding 0.4 V on the pin. Connect a
resistor of 10 k to V
CC
to provide the pull-up.
ENABLE Low Current Disable
Grounding the ENABLE pin disconnects the backplane side from the
card side, disables the rise-time accelerators, drives READY low,
disables the bus precharge circuitry, and puts the part in a low
current state. When the pin voltage is driven all the way to V
CC
, the
part waits for data transactions on both the backplane and card
sides to be complete before reconnecting the two sides.
