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X9521V20I-B Datasheet(PDF) 3 Page - Intersil Corporation |
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X9521V20I-B Datasheet(HTML) 3 Page - Intersil Corporation |
3 / 24 page 3 FN8207.1 January 3, 2006 PRINCIPLES OF OPERATION SERIAL INTERFACE Serial Interface Conventions The device supports a bidirectional bus oriented protocol. The protocol defines any device that sends data onto the bus as a transmitter, and the receiving device as the receiver. The device controlling the transfer is called the master and the device being controlled is called the slave. The master always initiates data transfers, and provides the clock for both transmit and receive opera- tions. Therefore, the X9521 operates as a slave in all applications. Serial Clock and Data Data states on the SDA line can change only while SCL is LOW. SDA state changes while SCL is HIGH are reserved for indicating START and STOP conditions. See Figure 1. On power-up of the X9521, the SDA pin is in the input mode. Serial Start Condition All commands are preceded by the START condition, which is a HIGH to LOW transition of SDA while SCL is HIGH. The device continuously monitors the SDA and SCL lines for the START condition and does not respond to any command until this condition has been met. See Figure 2. Serial Stop Condition All communications must be terminated by a STOP condition, which is a LOW to HIGH transition of SDA while SCL is HIGH. The STOP condition is also used to place the device into the Standby power mode after a read sequence. A STOP condition can only be issued after the transmitting device has released the bus. See Figure 2. Serial Acknowledge An ACKNOWLEDGE (ACK) is a software convention used to indicate a successful data transfer. The transmit- ting device, either master or slave, will release the bus after transmitting eight bits. During the ninth clock cycle, the receiver will pull the SDA line LOW to ACKNOWL- EDGE that it received the eight bits of data. Refer to Figure 3. The device will respond with an ACKNOWLEDGE after recognition of a START condition if the correct Device Identifier bits are contained in the Slave Address Byte. If a write operation is selected, the device will respond with an ACKNOWLEDGE after the receipt of each subse- quent eight bit word. In the read mode, the device will transmit eight bits of data, release the SDA line, then monitor the line for an ACKNOWLEDGE. If an ACKNOWLEDGE is detected and no STOP condition is generated by the master, the device will continue to transmit data. The device will ter- minate further data transmissions if an ACKNOWLEDGE is not detected. The master must then issue a STOP condition to place the device into a known state. SCL SDA Data Stable Data Change Data Stable Figure 1. Valid Data Changes on the SDA Bus SCL SDA Start Stop Figure 2. Valid Start and Stop Conditions X9521 |
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