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© Koninklijke Philips Electronics N.V. 2005. All rights reserved.

Product data sheet

Rev. 01 — 15 August 2005

7 of 23

Philips Semiconductors

PCA9511A

turn-on delay and the falling edge slew rate. The output falling edge slew rate is a function

well as the load current and the load capacitance.

8.5 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 PCA9511A is exceeded.

The rising edge rate should be at least 1.25 V/

µs to guarantee turn on of the accelerators.

8.6 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

Ω to V

8.7 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

part waits for data transactions on both the backplane and card sides to be complete

before reconnecting the two sides.

8.8 Resistor pull-up value selection

The system pull-up resistors must be strong enough to provide a positive slew rate of

1.25 V/

µs on the SDA and SCL pins, in order to activate the boost pull-up currents during

rising edges. Choose maximum resistor value using the formula:

where R is the pull-up resistor value in

Ω, V

and C is the equivalent bus capacitance in picofarads (pF).

In addition, regardless of the bus capacitance, always choose R

≤ 16 kΩ for V

maximum, R

≤ 24 kΩ for V

voltages on SDAOUT and SCLOUT to connect the backplane to the card, and these

pull-up values are needed to overcome the precharge voltage. See the curves in Figure 5

and Figure 6 for guidance in resistor pull-up selection.

R

800

10

3

×

V

CC min

()

0.6

–

C

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