hardware design v2.00
Table Of Contents
- Contents
- Version History
- 1 Introduction
- 2 SIM908 Overview
- 3 Package Information
- 4 GSM Application Interface
- 4.1 Power Supply
- 4.2 Power on/down Scenarios
- 4.3 Power Saving Mode
- 4.4 Charging Interface
- 4.5 RTC Backup
- 4.6 Serial Interfaces
- 4.7 Audio Interfaces
- 4.8 SIM Card Interface
- 4.9 LCD Display/SPI Interface
- 4.10 Keypad Interface
- 4.11 ADC
- 4.12 RI Behaviors
- 4.13 Network Status Indication
- 4.14 General Purpose Input/Output (GPIO)
- 4.15 PWM
- 4.16 I2C Bus
- 4.17 GSM Antenna Interface
- 5 GPS Application Interface
- 6 Electrical, Reliability and Radio Characteristics
- 6.1 Absolute Maximum Ratings
- 6.2 Recommended Operating Conditions
- 6.3 Digital Interface Characteristics
- 6.4 SIM Card Interface Characteristics
- 6.5 VDD_EXT Characteristics
- 6.6 SIM_VDD Characteristics
- 6.7 VRTC Characteristics
- 6.8 Current Consumption (VBAT = 3.8V, GPS engine is powered down)
- 6.9 Electro-Static Discharge
- 6.10 Radio Characteristics
- 6.11 Module label information
- Appendix
Smart Machine Smart Decision
SIM908_Hardware Design_V2.00 2012.05.07
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It is very simple to implement charging function, user just needs to connect the charger to the VCHG Pin and
connect the battery to the VBAT Pin.
SIM908 battery charging happens after detecting charger supply and the presence of battery. If there is no charger
supply or no battery present, charging function will not be enabled.
Normally, there are three main states in the whole charging procedure.
z DDLO charge (Pull-up mode) and UVLO charge (Pre-charge mode);
z Fast charge;
DDLO charge and UVLO charge:
DDLO is the state of battery when its voltage is under 1.5V. And UVLO means the battery voltage is less than
3.3V and more than 1.5V. The battery is not suitable for fast charging when its condition is DDLO or UVLO. The
SIM908 provides a small constant current to the battery when the battery is between DDLO and UVLO. In DDLO
charging state, SIM908 gives out 1mA current to the battery. And in UVLO charging state, SIM908 provides
about less than 200mA current to the battery.
DDLO charging terminates when the battery voltage reaches 1.5V. UVLO charging terminates when the battery
voltage is up to 3.3V. Both DDLO and UVLO charge are controlled by the SIM908 hardware only.
Fast charge:
If there is a charger supply and battery present and the battery is not in DDLO and UVLO, SIM908 will enter fast
charge state. Fast charge is controlled by the software to make the current/voltage regulation. The charging
scheme for the Li-Ion battery is constant current (about 550mA) first, followed by constant voltage charging once
4.2V is reached. Charging is stopped when the charging current at constant voltage has decreased down to the
pre-set current.
4.4.4 Operating Modes during Charging
The battery can be charged during various operating mode. That means that charging can be in progress while
SIM908 is in Normal mode (SLEEP, IDLE, TALK, GPRS IDLE or GPRS DATA mode). In this case the voltage
supply should be sufficient. Here Charging in Normal mode is named as Charge mode.
If the charger is connected to the module’s VCHG Pin and the battery is connected to the VBAT Pin while
SIM908 is in POWER DOWN mode, SIM908 will go into the Charge-only mode.
The following table gives the difference between Charge mode and Charge-only mode：
Table 8: Charge operating modes
How to activate mode Features
Charge Mode
Connect charger to module’s VCHG Pin
and connect battery to VBAT Pin of
module while SIM908 is in Normal
operating mode, including:IDLE, TALK
mode; SLEEP mode etc;
z GSM remains operational and registers to GSM
network while charging is in progress;
z The serial interfaces are available in IDLE, TALK
mode, the AT command set can be used fully in this
case;
In SLEEP mode, the serial interfaces are not available.
Once the serial port is connected and there is data in
transferring. SIM908 will exit the SLEEP mode.