VPOR is critical to the power-on reset. VPOR is the voltage level at which the reset condition is released and all

the registers and the I2C/SMBus state machine are initialized to their default states. The value of VPOR differs

based on the VCC being lowered to or from 0. Figure 8-9 and Table 8-1 provide more details on this specification.

V

CC

V

POR

V

PORF

Time

POR

Time

Figure 8-9. VPOR

8.4 Layout

8.4.1 Layout Guidelines

For printed circuit board (PCB) layout of PCF8574A, common PCB layout practices should be followed but

additional concerns related to high-speed data transfer such as matched impedances and differential pairs are

not a concern for I2C signal speeds.

In all PCB layouts, it is a best practice to avoid right angles in signal traces, to fan out signal traces away from

each other upon leaving the vicinity of an integrated circuit (IC), and to use thicker trace widths to carry higher

amounts of current that commonly pass through power and ground traces. By-pass and de-coupling capacitors

are commonly used to control the voltage on the VCC pin, using a larger capacitor to provide additional power

in the event of a short power supply glitch and a smaller capacitor to filter out high-frequency ripple. These

capacitors should be placed as close to the PCF8574A device as possible. These best practices are shown in

Figure 8-10.

For the layout example provided in Figure 8-10, it would be possible to fabricate a PCB with only 2 layers

by using the top layer for signal routing and the bottom layer as a split plane for power (VCC) and ground

(GND). However, a 4 layer board is preferable for boards with higher density signal routing. On a 4 layer PCB,

it is common to route signals on the top and bottom layer, dedicate one internal layer to a ground plane, and

dedicate the other internal layer to a power plane. In a board layout using planes or split planes for power and

ground, vias are placed directly next to the surface mount component pad which needs to attach to VCC or GND

and the via is connected electrically to the internal layer or the other side of the board. Vias are also used when

a signal trace needs to be routed to the opposite side of the board, but this technique is not demonstrated in

Figure 8-10.

PCF8574A

SCPS069H – JULY 2001 – REVISED SEPTEMBER 2024

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