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FM24W256-GTR Datasheet(PDF) 3 Page - Ramtron International Corporation |
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FM24W256-GTR Datasheet(HTML) 3 Page - Ramtron International Corporation |
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3 / 13 page  FM24W256 - 256Kb Wide Voltage I2C F-RAM Rev. 1.3 July 2011 Page 3 of 13 Overview The FM24W256 is a serial F-RAM memory. The memory array is logically organized as a 32,768 x 8 bit memory array and is accessed using an industry standard two-wire interface. Functional operation of the F-RAM is similar to serial EEPROMs. The major difference between the FM24W256 and a serial EEPROM with the same pinout relates to its superior write performance. Memory Architecture When accessing the FM24W256, the user addresses 32,768 locations each with 8 data bits. These data bits are shifted serially. The 32,768 addresses are accessed using the two-wire protocol, which includes a slave address (to distinguish other non-memory devices), and a 2-byte address. Only the lower 15 bits are used by the decoder for accessing the memory. The upper most address bit should be set to 0 for compatibility with higher density devices in the future. The access time for memory operation is essentially zero beyond the time needed for the serial protocol. That is, the memory is read or written at the speed of the two-wire bus. Unlike an EEPROM, it is not necessary to poll the device for a ready condition since writes occur at bus speed. That is, by the time a new bus transaction can be shifted into the part, a write operation will be complete. This is explained in more detail in the interface section below. Users expect several obvious system benefits from the FM24W256 due to its fast write cycle and high endurance as compared with EEPROM. However there are less obvious benefits as well. For example in a high noise environment, the fast-write operation is less susceptible to corruption than an EEPROM since it is completed quickly. By contrast, an EEPROM requiring milliseconds to write is vulnerable to noise during much of the cycle. Note that it is the user’s responsibility to ensure that VDD is within datasheet tolerances to prevent incorrect operation. Two-wire Interface The FM24W256 employs a bi-directional two-wire bus protocol using few pins or board space. Figure 2 illustrates a typical system configuration using the FM24W256 in a microcontroller-based system. The industry standard two-wire bus is familiar to many users but is described in this section. By convention, any device that is sending data onto the bus is the transmitter while the target device for this data is the receiver. The device that is controlling the bus is the master. The master is responsible for generating the clock signal for all operations. Any device on the bus that is being controlled is a slave. The FM24W256 always is a slave device. The bus protocol is controlled by transition states in the SDA and SCL signals. There are four conditions including start, stop, data bit, or acknowledge. Figure 3 illustrates the signal conditions that specify the four states. Detailed timing diagrams are in the electrical specifications. Microcontroller SDA SCL FM24W256 A0 A1 A2 SDA SCL FM24W256 A0 A1 A2 VDD Rmin = 1.1 Kohm Rmax = tR/Cbus Figure 2. Typical System Configuration |
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