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24AA64T-I/MC Datasheet(PDF) 7 Page - Microchip Technology |
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24AA64T-I/MC Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 40 page  2010 Microchip Technology Inc. DS21189R-page 7 24AA64/24LC64/24FC64 5.0 DEVICE ADDRESSING A control byte is the first byte received following the Start condition from the master device (Figure 5-1). The control byte consists of a four-bit control code. For the 24XX64, this is set as ‘1010’ binary for read and write operations. The next three bits of the control byte are the Chip Select bits (A2, A1, A0). The Chip Select bits allow the use of up to eight 24XX64 devices on the same bus and are used to select which device is accessed. The Chip Select bits in the control byte must correspond to the logic levels on the corresponding A2, A1 and A0 pins for the device to respond. These bits are, in effect, the three Most Significant bits of the word address. For the SOT-23 and Chip Scale packages, the address pins are not available. During device addressing, the A2, A1 and A0 Chip Select bits (Figure 5-2) should be set to ‘0’. The last bit of the control byte defines the operation to be performed. When set to a ‘1’, a read operation is selected. When set to a ‘0’, a write operation is selected. The next two bytes received define the address of the first data byte (Figure 5-2). Because only A12...A0 are used, the upper-three address bits are “don’t care” bits. The upper-address bits are transferred first, followed by the Less Significant bits. Following the Start condition, the 24XX64 monitors the SDA bus, checking the device-type identifier being transmitted. Upon receiving a ‘1010’ code and appro- priate device-select bits, the slave device outputs an Acknowledge signal on the SDA line. Depending on the state of the R/W bit, the 24XX64 will select a read or write operation. FIGURE 5-1: CONTROL BYTE FORMAT 5.1 Contiguous Addressing Across Multiple Devices The Chip Select bits A2, A1 and A0 can be used to expand the contiguous address space for up to 512K bits by adding up to eight 24XX64 devices on the same bus. In this case, software can use A0 of the control byte as address bit A13; A1 as address bit A14; and A2 as address bit A15. It is not possible to sequentially read across device boundaries. The SOT-23 and Chip Scale packages do not support multiple device addressing on the same bus. FIGURE 5-2: ADDRESS SEQUENCE BIT ASSIGNMENTS 10 1 0 A2 A1 A0 SACK R/W Control Code Chip Select Bits Slave Address Acknowledge Bit Start Bit Read/Write Bit 1 010 A 2 A 1 A 0 R/W xx x A 11 A 10 A 9 A 7 A 0 A 8 •• • • •• A 12 Control Byte Address High Byte Address Low Byte Control Code Chip Select bits x = “don’t care” bit |
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