Operation modes

AN3223

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Doc ID 17553 Rev 2

The FLASH register value must also be set.

The activation of Flash mode requires the FLASH pin to be high. Flash mode is active only

when the FLASH_ON bit in the COMMAND register is set to 1 and the FLASH pin is high.

This gives the user the possibility to choose between a soft and a hard triggering of the

Flash mode.

Soft triggering is done by writing data to the internal registers only, while the FLASH pin is

microcontroller, which can be used for a different purpose, but this way of triggering is less

accurate than the hard one. The second disadvantage of this solution is that the FLASH

duration can only be set in discrete steps of the internal timer.

Hard triggering of the Flash mode requires the microcontroller to manage the FLASH pin.

The COMMAND and the FLASH registers are loaded with data before the FLASH pin is set

to high. This allows the user to avoid the I²C bus latency. Flash mode then starts as soon as

the FLASH pin is set to high. It takes typically about 0.3 ms to ramp up the LED current to

the adjusted value. When the FLASH pin is kept high long enough, the internal timer

reaches zero and the Flash mode is over. As soon as the FLASH is timed out, the ATN pin is

pulled down for 11 µs to inform the microcontroller that the STATUS register was updated

and that the FLASH is over. If the FLASH pin is set to low before the internal timer reaches

zero, Flash mode is interrupted and can be restarted by setting the FLASH pin to high again.

The internal timer is stopped while the FLASH pin is low. This means that the user can split

the FLASH into several pulses of a total length equal to the FTIM value.

Figure 7 shows the

case for FTIM = 17 (130 ms FLASH time). The cumulative time when the FLASH pin is high

is 150 ms (5 pulses 30 ms long), but the last FLASH pulse is only 10 ms long. The reason is

that the internal FLASH timer reaches zero and the FLASH_ON bit is set to 0.

Table 10.

COMMAND register data for entering Flash mode

CMD_REG

PWR_ON FLASH_ON

TCH_ON

NTC_ON

TCHV_H

CHRG

MONTR

N/A

1

1

0

xxxx

0

MSB

LSB