Dual SIM connection
LZT 123 7676 R1A
4
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:
SIGNAL
DESCRIPTION
SIMVCC
3V or 5V Supply
GND System
Ground
SIMCLK
Synchronous Clock Output
SIMRST Reset
Output
SIMDATA
SIM Data Input/Output
SIMPRESENCE
SIM Presence Detection Input
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. For signal conditioning each SIM holder has a supply
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
