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AT24C256-10CC-2.7 Datasheet(PDF) 7 Page - ATMEL Corporation |
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AT24C256-10CC-2.7 Datasheet(HTML) 7 Page - ATMEL Corporation |
7 / 15 page AT24C128/256 7 Device Addressing The 128K/256K EEPROM requires an 8-bit device address word following a start condition to enable the chip for a read or write operation (refer to Figure 1). The device address word consists of a mandatory one, zero sequence for the first five most significant bits as shown. This is common to all 2-wire EEPROM devices. The 128K/256K uses the two device address bits A1, A0 to allow as many as four devices on the same bus. These bits must compare to their corresponding hardwired input pins. The A1 and A0 pins use an internal proprietary circuit that biases them to a logic low condition if the pins are allowed to float. The eighth bit of the device address is the read/write opera- tion select bit. A read operation is initiated if this bit is high and a write operation is initiated if this bit is low. Upon a compare of the device address, the EEPROM will output a zero. If a compare is not made, the device will return to a standby state. DATA SECURITY: The AT24C128/256 has a hardware data protection scheme that allows the user to write protect the whole memory when the WP pin is at VCC. Write Operations BYTE WRITE: A write operation requires two 8-bit data word addresses following the device address word and acknowledgment. Upon receipt of this address, the EEPROM will again respond with a zero and then clock in the first 8-bit data word. Following receipt of the 8-bit data word, the EEPROM will output a zero. The addressing device, such as a microcontroller, then must terminate the write sequence with a stop condition. At this time the EEPROM enters an internally-timed write cycle, t WR, to the nonvolatile memory. All inputs are disabled during this write cycle and the EEPROM will not respond until the write is complete (refer to Figure 2). PAGE WRITE: The 128K/256K EEPROM is capable of 64- byte page writes. A page write is initiated the same way as a byte write, but the microcontroller does not send a stop condition after the first data word is clocked in. Instead, after the EEPROM acknowledges receipt of the first data word, the microcon- troller can transmit up to 63 more data words. The EEPROM will respond with a zero after each data word received. The microcontroller must terminate the page write sequence with a stop condition (refer to Figure 3). The data word address lower 6 bits are internally incre- mented following the receipt of each data word. The higher data word address bits are not incremented, retaining the memory page row location. When the word address, inter- nally generated, reaches the page boundary, the following byte is placed at the beginning of the same page. If more than 64 data words are transmitted to the EEPROM, the data word address will “roll over” and previous data will be overwritten. The address “roll over” during write is from the last byte of the current page to the first byte of the same page. ACKNOWLEDGE POLLING: Once the internally-timed write cycle has started and the EEPROM inputs are dis- abled, acknowledge polling can be initiated. This involves sending a start condition followed by the device address word. The read/write bit is representative of the operation desired. Only if the internal write cycle has completed will the EEPROM respond with a zero, allowing the read or write sequence to continue. Read Operations Read operations are initiated the same way as write opera- tions with the exception that the read/write select bit in the device address word is set to one. There are three read operations: current address read, random address read and sequential read. CURRENT ADDRESS READ: The internal data word address counter maintains the last address accessed dur- ing the last read or write operation, incremented by one. This address stays valid between operations as long as the chip power is maintained. The address “roll over” during read is from the last byte of the last memory page, to the first byte of the first page. Once the device address with the read/write select bit set to one is clocked in and acknowledged by the EEPROM, the current address data word is serially clocked out. The microcontroller does not respond with an input zero but does generate a following stop condition (refer to Figure 4). RANDOM READ: A random read requires a “dummy” byte write sequence to load in the data word address. Once the device address word and data word address are clocked in and acknowledged by the EEPROM, the microcontroller must generate another start condition. The microcontroller now initiates a current address read by sending a device address with the read/write select bit high. The EEPROM acknowledges the device address and serially clocks out the data word. The microcontroller does not respond with a zero but does generate a following stop condition (refer to Figure 5). SEQUENTIAL READ: Sequential reads are initiated by either a current address read or a random address read. After the microcontroller receives a data word, it responds with an acknowledge. As long as the EEPROM receives an acknowledge, it will continue to increment the data word address and serially clock out sequential data words. When the memory address limit is reached, the data word address will “roll over” and the sequential read will con- tinue. The sequential read operation is terminated when the microcontroller does not respond with a zero but does generate a following stop condition (refer to Figure 6). |
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