PCA9511A_4

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 04 — 19 August 2009

6 of 24

NXP Semiconductors

PCA9511A

detected, the pull-down will turn off and will not turn back on until a falling edge is

detected.

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

Slave B and then from Slave B to Master. Before the direction change you would observe

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 may 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.

8.4 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

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.

maximum slew rate, and even if the input slew rate is slow enough that the output catches

occurs when the input is driven LOW with zero delay and the output is still limited by its

Fig 4.

System with 3 buffers connected to common node

002aab581

buffer C

buffer B

buffer A

common

node

SLAVE B

SLAVE C

MASTER