LED7708 datasheet(39/53 Pages) STMICROELECTRONICS

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LED7708

Recommended operating conditions

Doc ID 022698 Rev 1

39/53

The content of the internal registers, except the read-only bits of DEVCFG0 and DEVCFG1

(i.e. CFP, OVRS, OVFW and EDMI), is not affected when the DEN bit is cleared.

Warning:

In order to prevent damage to the device or to the external

circuitry, some bits of DEVCFG0 and DEVCFG1 registers

(CRS, SDMS and DSYS) are locked when DEN is high.

8.6

Dimming options

The brightness of the LEDs connected to the output channels can be basically controlled in

two ways, depending on the status of the GSME bit of the DEVCFG1 register:

●

Group dimming mode (GSME=0): common dimming performed by using the brightness

value received through the serial interface and stored in the BRT01 register: all the

channels have the same PWM duty cycle;

●

Grayscale dimming mode (GSME=1): independent PWM dimming on each channel

performed by loading the BRT01:BRT16 brightness registers.

In practice, a third way to control the brightness of the LEDs should rely on an intensive use

of the CHSEL register by the host controller. The direct control of the CHSEL register,

combined with either the group dimming or the grayscale dimming, allows the user to

implement particular dimming algorithms.

Once the dimming control mode has been chosen through the GSME bit, the source of the

clock for the PWM generation (grayscale clock) must be selected. The LED7708 can use

either an external clock source or its internal high-frequency oscillator.

8.6.1

Internal dimming oscillator

The LED7708 has a programmable oscillator dedicated to the dimming circuitry.

The output clock is provided at the GSCK pin for the slave devices. If the FOSC pin is forced

high, the oscillator is turned off and the GSCK pin becomes the grayscale clock input.

The oscillator frequency is simply set by connecting a resistor between the FOSC pin and

ground:

Equation 14

Table 20.

FOSC pin setting options

FOSC pin setting

Oscillator

GSCK pin function

High (to LDO3)

OFF

Grayscale clock input

Resistor to GND

Active (4 MHz-25 MHz)

Grayscale clock output

FOSC

OSC

GSCK

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