4.2 I2C Interface

This interface is designed to be compatible at the protocol level with the Microchip AT24C16 Serial EEPROM operating at 400 kHz.

Tip: There are some differences between the two devices (for example, the ECC204-TFLXAUTH and AT24C16 have different default I2C addresses); therefore, it is recommended that designers read the respective data sheets carefully.

The SDA pin is normally pulled high with an external pull-up resistor because the ECC204-TFLXAUTH only includes an open-drain driver on its output pin. The host system may use either an open-drain or a totem pole driver. In the latter case, it must be tri-stated when the ECC204-TFLXAUTH is driving results on the bus. The SCL pin is an input and must be driven both high and low at all times by an external device or pulled high by an external resistor.

The serial interface is comprised of two signal lines: Serial Clock (SCL) and Serial Data (SDA). The SCL pin is used to receive the clock signal from the host, while the bidirectional SDA pin is used to receive command and data information from the host as well as to send data back to the host. Data are always latched into the ECC204-TFLXAUTH on the rising edge of SCL and always output from the device on the falling edge of SCL. Both SCL and SDA pins incorporate integrated glitch suppression filters and Schmitt Triggers to minimize the effects of input spikes and bus noise.

All command and data information is transferred with the MSb first. During bus communication, one data bit is transmitted every clock cycle and after eight bits (one byte) of data are transferred, the receiving device must respond with either an ACK or a NACK response bit during a ninth clock cycle (ACK/NACK clock cycle) generated by the host. Therefore, nine clock cycles are required for every one byte of data transferred. There are no unused clock cycles during any read or write operation, so there must not be any interruptions or breaks in the data stream during each data byte transfer and ACK or NACK clock cycle.

During data transfers, data on the SDA pin must only change while SCL is low, and the data must remain stable while SCL is high. If data on the SDA pin change while SCL is high, either a Start or a Stop condition will occur. Start and Stop conditions are used to initiate and end all serial bus communication between the host and the client devices. The number of data bytes transferred between a Start and a Stop condition is not limited and is determined by the host. For the serial bus to be idle, both the SCL and SDA pins must be in the logic high state at the same time.