Specifications

Host/Module Interfaces
Rev 2.0 Apr.10 Proprietary and Confidential 61
USIM operation
Note: For interface design
requirements, refer to:
(2G) 3GPP TS 51.010-1,
section 27.17, or
(3G) ETSI TS 102 230
V5.5.0, section 5.2.
When designing the remote USIM interface, you must make sure that the USIM
signal integrity is not compromised.
Some design recommendations include:
• Total impedance of the VCC and GND connections to the USIM, measured at
the module connector, should be less than 1  to minimize voltage drop
(includes any trace impedance and lumped element components—inductors,
filters, etc.).
Note: The MC8xxx is designed for use with either a 1.8 V or 3 V USIM.
• Position the USIM connector no more than 10 cm from the module. If a longer
distance is required because of the design of the host device, a shielded wire
assembly is recommended—connect one end as close as possible to the
USIM connector and the other end as close as possible to the module
connector. The shielded assembly may help shield the USIM interface from
system noise.
• Reduce crosstalk on the XIM_data line to reduce the risk of failures during
GCF approval testing.
• Avoid routing the XIM_CLK and XIM_DATA lines in parallel over distances
greater than 2 cm—cross-coupling of these lines can cause failures.
• Keep USIM signals as short as possible, and keep very low capacitance
traces on the XIM_DATA and XIM_CLK signals to minimize signal rise time—
signal rise time must be <1 µs. High capacitance increases signal rise time,
potentially causing your device to fail certification tests.
• Add external pull-up resistors (15 k–30 k), if required, between the
SIM_IO and SIM_VCC lines to optimize the signal rise time.
• 3GPP has stringent requirements for I/O rise time (<1 µs), signal level limits,
and noise immunity—consider this carefully when developing your PCB
layout.
• VCC line should be decoupled close to the USIM socket.
• USIM is specified to run up to 5 MHz (USIM clock rate). Take note of this
speed in the placement and routing of the USIM signals and connectors.
• You must decide if, and how much, additional ESD protection and series
resistors are suitable for your product. The MC8xxx already includes
additional ESD protection. Adding more protection (additional circuits) than is
necessary could decrease signal rise time, increase load impedance, and
cause USIM certification failure.
• Putting an optional decoupling capacitor at XIM_VCC near the USIM socket
is recommended—the longer the trace length (impedance) from the socket to
the module, the greater the capacitance requirement to meet compliance
tests.
• Putting an optional series capacitor and resistor termination (to ground) at
XIM_CLK at the USIM socket to reduce EMI and increase signal integrity is
recommended if the trace length between the USIM socket and module is
long—47 pF and 50  resistor are recommended.