Datasheet

PCA9551_8 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 08 — 31 July 2008 6 of 26
NXP Semiconductors
PCA9551
8-bit I
2
C-bus LED driver with programmable blink rates
6.3.3 PWM0 - Pulse Width Modulation 0
The PWM0 register determines the duty cycle of BLINK0. The outputs are LOW (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 = (256 − PWM0) / 256.
6.3.4 PSC1 - Frequency Prescaler 1
PSC1 is used to program the period of the PWM output.
The period of BLINK1 = (PSC1 + 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 1 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.
6.3.5 PWM1 - Pulse Width Modulation 1
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 = (256 − PWM1) / 256.
Table 5. PSC0 - Frequency Prescaler 0 register description
Bit 7 6 5 4 3 2 1 0
Symbol PSC0[7] PSC0[6] PSC0[5] PSC0[4] PSC0[3] PSC0[2] PSC0[1] PSC0[0]
Default 11111111
Table 6. PWM0 - Pulse Width Modulation 0 register description
Bit 7 6 5 4 3 2 1 0
Symbol PWM0
[7]
PWM0
[6]
PWM0
[5]
PWM0
[4]
PWM0
[3]
PWM0
[2]
PWM0
[1]
PWM0
[0]
Default 10000000
Table 7. PSC1 - Frequency Prescaler 1 register description
Bit 7 6 5 4 3 2 1 0
Symbol PSC1[7] PSC1[6] PSC1[5] PSC1[4] PSC1[3] PSC1[2] PSC1[1] PSC1[0]
Default 11111111
Table 8. PWM1 - Pulse Width Modulation 1 register description
Bit 7 6 5 4 3 2 1 0
Symbol PWM1
[7]
PWM1
[6]
PWM1
[5]
PWM1
[4]
PWM1
[3]
PWM1
[2]
PWM1
[1]
PWM1
[0]
Default 10000000