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1
Introduction
The SIM card provides the identity of a particular subscriber to the GSM
operator network. The network operator provides a set of services to the
subscriber based on his/her identity.
As a result of the varying costs, services, and cell coverage of different
GSM operators it may be beneficial to have a choice of GSM operator
network at the time of making or receiving a call. This can be achieved in
a system which actively selects which one of two SIM cards to activate
before attaching to a network.
This application note provides the design of one such "Dual SIM" system
when used in conjunction with a GR47 GSM module from Sony Ericsson.
2
Design Overview
The SIM connection signals from the GR47 GSM module consist of:
SIMDATA
SIMPRESENCE
Each of the output signals and the supply connections can be connected
directly to both SIM holders. With a little signal conditioning these signals
have enough drive to fan out to two SIM holders without need for signal
buffering.
decoupling capacitor associated with it.
The SIMDATA data input/output signal is the key to successful SIM
access. When active, the SIMDATA signal is pulled-up by the GR47. The
signal is bidirectional and is pulled-down by whichever side is sending
data. In the case of the disabled SIM holder in our dual SIM solution the
unused SIMDATA must be pulled-up to SIMVCC. This is a standard idle
condition for a SIM.
The method we have used to redirect the SIMDATA signal to whichever
SIM has been actively selected is by a dual SPST analogue switch.
The dual SPST analogue switch is part number NLAS325 from "ON
Semiconductor™". This part has a wide operating voltage range and very
low quiescent supply current. In addition it has two independent switch
enable controls, ideally suited to this application as one is active high, the
LZT 123 7676 R1A
Dual SIM connection
SIGNAL
SIMVCC
GND
SIMCLK
Synchronous Clock Output
SIMRST
SIM Presence Detection Input
For signal conditioning each SIM holder has a supply
DESCRIPTION
3V or 5V Supply
System Ground
Reset Output
SIM Data Input/Output
4
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