STCF04 datasheet(32/43 Pages) STMICROELECTRONICS

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Theory of operation

STCF04

32/43

Doc ID 022927 Rev 3

10.8

External (microprocessor) temporization using the

FLASH_ON bit

Even though it is possible, it is not recommended to use the FLASH_ON bit to start and stop

the flash operation, because of I²C bus latencies: this may result in inaccurate flash timing.

Nevertheless, if this operation mode is chosen, the FLASH pin must be kept HIGH (logic

level or wired to VBAT), leaving the whole flash control to the I²C bus. Also in this operation

mode the time counter times out flash operation and keeps the energy dissipated by the

LED within safe limits in case of software deadlock.

10.9

Single flash using internal temporization

Flash triggering can be obtained either by the FLASH pin (hard-triggering) or by I²C

commands (soft-triggering). The first solution is recommended for an accurate start time,

while the second is less accurate because of the I²C bus time latency. Stop time is defined

by the STCF04 internal temporization and its accuracy is determined by the internal

oscillator. For hard-triggering, it is necessary to set the FLASH_ON bit in advance. For soft-

triggering, the FLASH pin must be kept High (logic level or wired to VBAT) and the flash can

be started by setting the FTIM and the TRIG_EN through I²C (both are located in the CMD

REG). There is a delay time between the moment the flash is triggered and when it appears.

This delay is caused by the time necessary to charge up the output capacitor, which

depends on battery voltage. Once triggered, the flash operation is stopped when the time

counter reaches zero. As soon as the flash is finished, the F_RUN bit is reset, the ATN pin is

pulled down for 11 µs to inform the microprocessor that the STAT_REG has been updated

and the device goes back to Idle mode.

10.10

Light sensor feature

This function works as an optional feature, which is able to optimize the length of the flash

according to the light conditions in the flashed area. It uses an external capacitor CINT,

which is charged by a current coming from the external voltage reference, which is limited by

the light sensor (generally made by a phototransistor). Before the start of the flash

operation, the CINT capacitor is discharged by the internal switch to the zero voltage during

200 µs. During the flash operation the CINT capacitor integrates the charging current

according to the light conditions in the flashed area. When the voltage level on the CINT

capacitor reaches the internal reference voltage, which is set by the light sensor register

(R4, LS_0~2), the STCF04 stops the operation of the flash and discharges the CINT

capacitor through an internal switch.

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