May 2006, Issue 190

Mobile Phone Book
M16C/62P-Based Data Backup System
Renesas M16C 2005 Honorable Mention

WHAT’S A SIM?

A smart card chip is typically found embedded in a card no bigger than a credit card. But because of the small form factor required for a mobile phone, the ISO developed the SIM-size standard for smart cards.

The SIM smart card’s interface to the outside world is simple in comparison to other technologies (e.g., the multimedia card (MMC), secure digital (SD) card, and CompactFlash (CF) card). The SIM micromodule has room for eight contacts, two of which are reserved for things like the USB protocol and a contact-less interface. The interface comprises eight contact points (see Figure 1).

The VCC contact provides the card’s power supply. All SIM smart cards typically run between 3 and 5 V because mobile phones have low power requirements.

The RST contact provides the card with the RESET signal. It’s low-level activated. The CLK contact provides the card with the clock signal. Its range is between 1 and 5 MHz with a duty cycle between 40% and 60%.

The GND contact is the ground signal of the power supply. The VPP contact isn’t used anymore. In the past, it provided the programming power required to write and erase a smart card’s internal nonvolatile memory.

The I/O contact is used as either an input (Reception mode) or output (Transmission mode) using a half-duplex asynchronous serial protocol. The two RFU contacts are reserved for future use.

The procedure for managing the SIM card interface is simple. When the contacts are activated, the interface device should perform several operations in sequence. First, it holds RST at a low level. It then applies a stable power to the VCC contact, puts the I/O contact in Reception mode, and disconnects (or holds) the VPP contact stable at an idle level. Lastly, the interface provides a suitable and stable clock to the CLK contact. The SIM card is then ready to be reset.

After maintaining the RST signal at a low state for at least 40,000 clock cycles, it should be raised to a high state. The answer to reset (ATR) begins between 400 and 40,000 clock cycles after the rising edge of the signal on RST. If the ATR doesn’t begin within 40,000 clock cycles with the RST signal in a high state, RST should be returned to a low state and the contacts should be deactivated (if the card is mute or has been removed from the interface socket).

Subsequent exchanges of information between the SIM card and the interface device can be accomplished by driving the I/O line while the RST signal is held high. On the other hand, the interface device can reset the SIM card at any time.