Datasheet

Product data sheet Rev. 08 — 31 July 2008 5 of 26

NXP Semiconductors

PCA9551

8-bit I

C-bus LED driver with programmable blink rates

If the Auto-Increment (AI) 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 the 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 ≠ 000).

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

programmed with zeroes.

6.2.1 Control register deﬁnition

6.3 Register descriptions

6.3.1 INPUT - Input register

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

acknowledged but will have no effect.

Remark: The default value ‘X’ is determined by the externally applied logic level (normally

logic 1) when used for directly driving LED with pull-up to V

6.3.2 PSC0 - Frequency Prescaler 0

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

The period of BLINK0 = (PSC0 + 1) / 38.

Remark: 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 Frequency Prescaler 0 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.

Table 3. Register summary

B2 B1 B0 Symbol Access Description

0 0 0 INPUT read only input register

0 0 1 PSC0 read/write frequency prescaler 0

0 1 0 PWM0 read/write PWM register 0

0 1 1 PSC1 read/write frequency prescaler 1

1 0 0 PWM1 read/write PWM register 1

1 0 1 LS0 read/write LED0 to LED3 selector

1 1 0 LS1 read/write LED4 to LED7 selector

Table 4. INPUT - Input register description

Bit 7 6 5 4 3 2 1 0

Symbol LED7 LED6 LED5 LED4 LED3 LED2 LED1 LED0

Default XXXXXXXX