STLED325 Functional description
While the interrupt is being read by the MCU and a new GPIO or key data comes in, no new interrupt is generated but the register for GPIO and KEY data is updated so that the MCU does not miss the new KEY and GPIO data.
3.14 Ready
The STLED325 supports cutting-off power to the main board for standby operation for good power management. STBY will be set to high when the READY transitions from high to low.
During a cold boot up or wake up from standby, if the READY pin stays low, the STLED325 will assert the STBY when the guard timer has finished counting down to 0.
When the READY drops to a low, MUTE goes high immediately and soon after (2µs minimum) the STBY is asserted.
In the normal mode of operation, when READY is a high, the STBY is asserted only when the guard timer is enabled and has finished counting down to 0. This is meant to put the system into stand-by as the READY pin was stuck at high and the guard timer register was not cleared before it finished counting down to 0. It is advised to disable the guard timer during normal operation.
3.15 Mute
The MUTE pin will be set to logic high to mute the audio output before power is cut to the host processor. In wake up mode, the MUTE pin will be set to logic low to enable the audio output immediately after the high assertion of the READY pin. In general, MUTE follows
READY pin with an inverted polarity. This pin is used to prevent pop-up sound during powerup and power-down states.
3.16 GPIO
The STLED325 supports 2 additional GPIOs that can be configured as inputs or outputs. As an input, the GPIO can be used to interface to a sensor or a switch or key and as an output, the GPIO can be used to drive individual indicator LEDs.
3.17 Power sense circuits
The STLED325 has a built-in power sense circuit which detects power failures and automatically switches to the battery or super-cap supply when a power failure occurs. The energy needed to sustain the SRAM and clock operations can be supplied by small lithium button supply or a super-cap when a power failure occurs. When operating from the battery or super-cap, all the inputs and outputs are driven to a known state (generally L).
Doc ID 17576 Rev 1 27/62
Functional description
Figure 10.
Power sense circuit
STLED325
For the STLED325 itself, there is the normal operational mode where the supply is from the
5 V V
CC
. When the V
CC
drops below a pre-defined low level, the supply source is switched from the V
CC
to the battery or super-cap supply.
To conserve power and maintain long battery life in this battery supply mode, only the RTC and the clock to the RTC remain operational.
1.
The system will only go into battery mode while:
Vcc < 3.5 V and Vcc < Vbat.
So, it means that the system will only switch to battery mode when Vcc drop below 3.5V and
Battery voltage is higher V
CC
voltage.
2. The system will enter back into Vcc mode from battery mode while:
V
CC
> V bat
It means that the system will switch back to Vcc mode as soon as the V
CC
Vbat.
is higher than
The STLED325 continually monitors Vcc for an out-of-tolerance condition. Should V
CC
fall below the Switchover voltage (V
SO
= 3.5 V), the device goes into a low-power mode. Inputs to the device will not be recognized at this time to prevent any erroneous data or outcome from device. The device also automatically switches over to the battery and powers down into an ultra low current mode of operation to maximize the super-cap or battery duration. As system power returns and V
CC
rises above Vbat, the battery or super-cap is disconnected and the power supply is switched to the external V
CC
. During the battery or super-cap backup mode, the clock registers of RTC are maintained by the attached battery or super-cap.
On power-up, when V
CC
returns to a nominal value, write protection continues for t to timing diagram in later part of spec).
REC
(refer
Upon power-up, the device switches from battery to V
CC
when V
CC
> Vbat. When V above Vbat, it will recognize the inputs.
CC
rises
28/62 Doc ID 17576 Rev 1
