PCA9521 Просмотр технического описания (PDF) - NXP Semiconductors.

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PCA9521

Fast dual bidirectional bus buffer

NXP Semiconductors. 

NXP Semiconductors

PCA9521

Fast dual bidirectional bus buffer

7. Functional description

Refer to Figure 1 “Block diagram”.

7.1 VCC, GND — supply pins

The power supply voltage for the PCA9521 may be any voltage in the range 2.7 V to

5.5 V. The threshold level below which the output will begin to match the input is 33 % of

VCC. Hence, the operating voltage should be chosen with the required bus voltage,

switching threshold, and noise margins, in mind.

7.2 SA1, SA2, SB1, SB2 — buffer inputs/outputs

The two buffers (SA and SB) are identical and symmetrical. The buffers can be driven

from either direction, with the same response. When port 1 of the buffer is being driven

LOW (< 0.3VCC) by another device on the bus, port 2 will be driven LOW by the IC to

provide the buffered output.

The ‘input’ side is determined by the lowest externally driven signal. Therefore if port 1 is

externally pulled to VSx1 = 250 mV, and port 2 is externally pulled to VSx2 = 500 mV, the

buffer will pull port 2 down further such that it becomes VSx2 = VSx1 + Voffset. Should port 2

subsequently become lower than port 1 by the amount of the offset voltage

(VSx2 + Voffset < VSx1) by means of an external device pulling it LOW, control of the

buffering operation will switch, and port 2 will become the ‘input’. The voltage at port 1 will

then become VSx1 = VSx2 + Voffset. When both ports are being held almost equal (less than

an offset voltage) the external devices are effectively in control.

7.3 EN — enable; activate buffer operations

The enable input, EN, is used to disable the buffer, for the purpose of isolating sections of

the bus. The IC should only be disabled when the bus is idle. This prevents truncation of

commands which may confuse other devices on the bus.

Enable may also be used to progressively activate sections of the bus during system

start-up. Bus sections slow to respond on power-up can be kept isolated from the main

system to avoid interference and collisions.

The EN pin may be pulled up higher than the VCC of the buffer, further enhancing the

capability of the PCA9521 in a level shifting role. For example, a microprocessor could

drive EN, SA1 and SB1 at 5 V, while the buffer VCC, SA2 and SB2 ports are at 3.3 V.

Similarly, the threshold level of the EN pin allows a 1.8 V device to disable a PCA9521

with a VCC of 3.3 V.

The EN pin includes an internal 2 A pull-down current, which will act to disable the

device should the pin be left floating.

PCA9521

Product data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 2 — 19 March 2013

© NXP B.V. 2013. All rights reserved.

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