4 Device Operation and Communication
The AT21CS01/11 operates as a client device and utilizes a single‑wire digital serial interface to communicate with a host controller, commonly referred to as the bus host. The host controls all read and write operations to the client devices on the serial bus. The device has two speeds of operation, Standard Speed mode (AT21CS01) and High-Speed mode (AT21CS01 and AT21CS11).
The device utilizes an 8-bit data structure. Data are transferred to and from the device via the single‑wire serial interface using the Serial Input/Output (SI/O) pin. Power to the device is also provided via the SI/O pin, thus only the SI/O pin and the GND pin are required for device operation. Data sent to the device over the single‑wire bus is interpreted by the state of the SI/O pin during specific time intervals or slots. Each time slot is referred to as a bit frame and lasts tBIT in duration. The host initiates all bit frames by driving the SI/O line low. All commands and data information are transferred with the Most Significant bit (MSb) first.
The software sequence sent to the device is an emulation of what would be
sent to an I2C Serial EEPROM with the exception that the
typical 4-bit device type identifier of 1010b in the device address is
replaced by a 4-bit opcode. The device has been architected in this way to allow for rapid
deployment and significant reuse of existing I2C firmware. For more details
about the way the device operates, refer to Device Addressing and I2C Protocol Emulation.
During bus communication, one data bit is transmitted in every bit frame, and after eight bits (one byte) of data have been transferred, the receiving device must respond with either an Acknowledge (ACK) or a No Acknowledge (NACK) response bit during a ninth bit window. 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. If an unavoidable system interrupt is required, refer to the requirements outlined in Communication Interruptions.