PCA9551 Datasheet, PDF (6/21 Pages) NXP Semiconductors – 8-bit I2C LED driver with programmable blink rates

Philips Semiconductors
8-bit I²C LED driver with programmable blink rates
Product data sheet
PCA9551
PSC1 — FREQUENCY PRESCALER 1¹
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
When power is applied to V_DD, an internal Power-On Reset holds
the PCA9551 in a reset condition until V_DD has reached V_POR. At
this point, the reset condition is released and the PCA9551 registers
are initialized to their default states, all the outputs in the off state.
Thereafter, V_DD must be lowered below 0.2 V to reset the device.
EXTERNAL RESET
A reset can be accomplished by holding the RESET pin LOW for a
minimum of t_W. The PCA9551 registers and I²C state machine will
be held in their default state until the RESET input is once again
HIGH.
This input requires a pull-up resistor to V_DD if no active connection is
used.
2004 Oct 01
6