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FM24C256-SE Datenblatt(PDF) 5 Page - Ramtron International Corporation |
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FM24C256-SE Datenblatt(HTML) 5 Page - Ramtron International Corporation |
5 / 12 page FM24C256 Rev 3.1 May 2005 Page 5 of 12 Addressing Overview After the FM24C256 (as receiver) acknowledges the device address, the master can place the memory address on the bus for a write operation. The address requires two bytes. The first is the MSB (upper byte). Since the device uses only 15 address bits, the value of the upper bits is a “don’t care”. Following the MSB is the LSB (lower byte) with the remaining eight address bits. The address value is latched internally. Each access causes the latched address value to be incremented automatically. The current address is the value that is held in the latch, either a newly written value or the address following the last access. The current address will be held as long as power remains or until a new value is written. Reads always use the current address. A random read address can be loaded by beginning a write operation as explained below. After transmission of each data byte, just prior to the acknowledge, the FM24C256 increments the internal address latch. This allows the next sequential byte to be accessed with no additional addressing externally. After the last address (7FFFh) is reached, the address latch will roll over to 0000h. There is no limit to the number of bytes that can be accessed with a single read or write operation. Data Transfer After the address information has been transmitted, data transfer between the bus master and the FM24C256 can begin. For a read operation the FM24C256 will place 8 data bits on the bus then wait for an Acknowledge from the master. If the Acknowledge occurs, the FM24C256 will transfer the next sequential byte. If the Acknowledge is not sent, the FM24C256 will end the read operation. For a write operation, the FM24C256 will accept 8 data bits from the master then send an acknowledge. All data transfer occurs MSB (most significant bit) first. Memory Operation The FM24C256 is designed to operate in a manner very similar to other 2-wire interface memory products. The major differences result from the higher performance write capability of FRAM technology. These improvements result in some differences between the FM24C256 and a similar configuration EEPROM during writes. The complete operation for both writes and reads is explained below. Write Operation All writes begin with a device address, then a memory address. The bus master indicates a write operation by setting the LSB of the device address to a 0. After addressing, the bus master sends each byte of data to the memory and the memory generates an acknowledge condition. Any number of sequential bytes may be written. If the end of the address range is reached internally, the address counter will wrap from 7FFFh to 0000h. Unlike other nonvolatile memory technologies, there is essentially no write delay with FRAM. Since the read and write access times of the underlying memory are the same, the user experiences no delay on the bus. The entire memory cycle occurs in less time than a single bus clock. Therefore, any operation including a read or write can occur immediately following a write. Acknowledge polling, a technique used with EEPROMs to determine if a write has completed is unnecessary and will always return a ready condition. Internally, an actual memory write occurs after the 8 th data bit is transferred. It will be complete before the Acknowledge is sent. Therefore, if the user desires to abort a write without altering the memory contents, this should be done using a Start or Stop condition prior to the 8 th data bit. The FM24C256 uses no page buffering. The memory array can be write protected using the WP pin. Pulling the WP pin high will write-protect all addresses. The FM24C256 will not acknowledge data bytes that are written when WP is active. In addition, the address counter will not increment if writes are attempted to these addresses. Setting WP low will deactivate this feature. WP is internally pulled down. The state of WP should remain stable from the Start command until the address is complete. Figure 5 and 6 below illustrate both a single-byte and multiple-write. |
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