Philips Semiconductors

Product data sheet

PCA9551

2004 Oct 01

6

bit

7

6

5

4

3

2

1

0

default

1

1

1

1

1

1

1

1

PSC1 is used to program the period of PWM output.

The period of BLINK1

+

(PSC1

) 1)

38

NOTE:

1. Prescaler calculation is different between the PCA9551 and

other PCA955x LED Blinkers. A divider ratio of 38 instead of 44

is used. This different divider ratio causes the blinking frequency

to be 13% (1 – 38/44) lower when the same 8-bit word is used.

The programmed value of the FREQUENCY PRESCALER must

be adjusted to compensate for this difference in applications

where the PCA9551 is used in conjunction with other PCA955x

LED Blinkers and the observed blinking frequencies need to be

the same.

PWM1 — PWM REGISTER 1

bit

7

6

5

4

3

2

1

0

default

1

0

0

0

0

0

0

0

The PWM1 register determines the duty cycle of BLINK1. The

outputs are LOW (LED off) when the count is less than the value in

PWM1 and HIGH when it is greater. If PWM1 is programmed with

00h, then the PWM1 output is always LOW (LED off).

The duty cycle of BLINK1 is:

256 – PWM1

256

LS0 — LED0–3 SELECTOR

LED 3

LED 2

LED 1

LED 0

bit

7

6

5

4

3

2

1

0

default

0

1

0

1

0

1

0

1

LS1 — LED4–7 SELECTOR

LED 7

LED 6

LED 5

LED 4

bit

7

6

5

4

3

2

1

0

default

0

1

0

1

0

1

0

1

The LSx LED select registers determine the source of the LED data.

00 = Output is set LOW (LED on)

01 = Output is set Hi-Z (LED off – default)

10 = Output blinks at PWM0 rate

11 = Output blinks at PWM1 rate

PINS USED AS GENERAL PURPOSE I/Os

LED pins not used to control LEDs can be used as general purpose

I/Os.

For use as input: Set LEDx to high-impedance (01) and then read

the pin state via the input register.

For use as output: Connect external pull-up resistor to the pin and

size it according to the DC recommended operating characteristics.

LED output pin is HIGH when the output is programmed as

high-impedance, and LOW when the output is programmed LOW

through the “LED selector” register. The output can be pulse-width

controlled when PWM0 or PWM1 are used.

POWER-ON RESET

this point, the reset condition is released and the PCA9551 registers

are initialized to their default states, all the outputs in the off state.

EXTERNAL RESET

A reset can be accomplished by holding the RESET pin LOW for a

be held in their default state until the RESET input is once again

HIGH.

used.