NXP Semiconductors PCA9685, User Manual, Page: 11 / 15

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User manual Rev. 1 — 1 April 2014 11 of 14
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PCA9685 demonstration board OM13332
8.2 Dimming and brightness control for all LEDs and Prescale
The following steps to control dimming/brightness for all LEDs and adjust output
frequency with prescale value:
1. Click the ‘Control Registers’ configuration screen for all LED[0:15] brightness control
and Prescale for LEDs output frequency as shown in Figure 7 .
2. In Figure 7 , uncheck the ‘All LED Full Off’ box (lower bottom), adjust both ‘All LED ON’
and ‘All LED OFF’ bar for all LEDs dimming/brightness effect.
3. Adjust the ‘PRESCALE’ bar to program the LED output frequency (PWM) as shown
below:
– LED output frequency = (25 MHz or EXTCLK)
÷
(4096
×
(pr 1))
– When Prescale = 0x1E (default is 30), the LED output frequency is
(25
×
10
6
)
÷
(4096
×
31) = 197 Hz
– When Prescale = 0x03 (minimum is 3), the LED output frequency is
(25
×
10
6
)
÷
(4096
×
4) = 1526 Hz
– When Prescale = 0xFF (maximum is 255), the LED output frequency is
(25
×
10
6
)
÷
(4096
×
256) = 24 Hz
Remark: The PRESCALE register value can only be set/update when the SLEEP bit
in the MODE1 register is set to logic 1
as shown in Figure 8 . Apply external clock input
to EXTCLK pin, user must set the SLEEP bit in MODE1 register, then write logic 1 to both
the SLEEP and EXTCLK bits in MODE1 register to enable the external clock.
Fig 8. PRESCALE to adjust LED output frequency when sleep enabled