PCA9551D datasheet(5/21 Pages) PHILIPS

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Philips Semiconductors

Product data sheet

PCA9551

8-bit I2C LED driver with programmable blink rates

2004 Oct 01

DEVICE ADDRESSING

Following a START condition the bus master must output the

address of the slave it is accessing. The address of the PCA9551 is

shown in Figure 4. To conserve power, no internal pull-up resistors

are incorporated on the hardware selectable address pins and they

must be pulled HIGH or LOW.

SLAVE ADDRESS

su01420

FIXED

HARDWARE SELECTABLE

R/W

Figure 4. Slave address

The last bit of the address byte defines the operation to be

performed. When set to logic 1 a read is selected while a logic 0

selects a write operation.

CONTROL REGISTER

Following the successful acknowledgement of the slave address,

the bus master will send a byte to the PCA9551 which will be stored

in the Control Register.

SW01034

AUTO-INCREMENT FLAG

RESET STATE: 00h

Figure 5. Control register

The lowest 3 bits are used as a pointer to determine which register

will be accessed.

If the auto-increment flag is set, the three low order bits of the

Control Register are automatically incremented after a read or write.

This allows the user to program the registers sequentially. The

contents of these bits will rollover to ‘000’ after the last register is

accessed.

When auto-increment flag is set (AI = 1) and a read sequence is

initiated, the sequence must start by reading a register different from

‘0’ (B2 B1 B0

0 0 0 0)

Only the 3 least significant bits are affected by the AI flag.

Unused bits must be programmed with zeroes.

CONTROL REGISTER DEFINITION

NAME

TYPE

FUNCTION

INPUT

READ

INPUT

PSC0

READ/

WRITE

FREQUENCY

PRESCALER 0

PWM0

READ/

WRITE

PWM

PSC1

READ/

WRITE

FREQUENCY

PRESCALER 1

PWM1

READ/

WRITE

PWM

LS0

READ/

WRITE

LED0–LED3

SELECTOR

LS1

READ/

WRITE

LED4–LED7

SELECTOR

INPUT — INPUT REGISTER

LED

bit

default

The INPUT register reflects the state of the device pins. Writes to

this register will be acknowledged but will have no effect.

NOTE: The default value “X” is determined by the externally applied

logic level, normally ‘1’ when used for directly driving LED with

pull-up to VDD.

PSC0 — FREQUENCY PRESCALER 01

bit

default

PSC0 is used to program the period of the PWM output.

The period of BLINK0

(PSC0

) 1)

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.

PWM0 — PWM REGISTER 0

bit

default

The PWM0 register determines the duty cycle of BLINK0. The

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

PWM0 and HIGH when it is greater. If PWM0 is programmed with

00h, then the PWM0 output is always LOW.

The duty cycle of BLINK0 is:

256 – PWM0

256

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