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FM25V40 Fiches technique(PDF) 4 Page - Cypress Semiconductor |
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FM25V40 Fiches technique(HTML) 4 Page - Cypress Semiconductor |
4 / 23 page PRELIMINARY FM25V40 Document Number: 001-87288 Rev. *A Page 4 of 23 Overview The FM25V40 is a serial F-RAM memory. The memory array is logically organized as 524,288× 8 bits and is accessed using an industry-standard serial peripheral interface (SPI) bus. The functional operation of the F-RAM is similar to serial flash and serial EEPROMs. The major difference between the FM25V40 and a serial flash or EEPROM with the same pinout is the F-RAM's superior write performance, high endurance, and low power consumption. Memory Architecture When accessing the FM25V40, the user addresses 512K locations of eight data bits each. These eight data bits are shifted in or out serially. The addresses are accessed using the SPI protocol, which includes a chip select (to permit multiple devices on the bus), an opcode, and a three-byte address. The upper 5 bits of the address range are 'don't care' values. The complete address of 19 bits specifies each byte address uniquely. Most functions of the FM25V40 are either controlled by the SPI interface or handled by on-board circuitry. The access time for the 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 SPI bus. Unlike a serial flash or EEPROM, it is not necessary to poll the device for a ready condition because writes occur at bus speed. By the time a new bus transaction can be shifted into the device, a write operation is complete. This is explained in more detail in the interface section. Serial Peripheral Interface – SPI Bus The FM25V40 is a SPI slave device and operates at speeds up to 40 MHz. This high-speed serial bus provides high-performance serial communication to a SPI master. Many common microcontrollers have hardware SPI ports allowing a direct interface. It is quite simple to emulate the port using ordinary port pins for microcontrollers that do not. The FM25V40 operates in SPI Mode 0 and 3. SPI Overview The SPI is a four-pin interface with Chip Select (CS), Serial Input (SI), Serial Output (SO), and Serial Clock (SCK) pins. The SPI is a synchronous serial interface, which uses clock and data pins for memory access and supports multiple devices on the data bus. A device on the SPI bus is activated using the CS pin. The relationship between chip select, clock, and data is dictated by the SPI mode. This device supports SPI modes 0 and 3. In both of these modes, data is clocked into the F-RAM on the rising edge of SCK starting from the first rising edge after CS goes active. The SPI protocol is controlled by opcodes. These opcodes specify the commands from the bus master to the slave device. After CS is activated, the first byte transferred from the bus master is the opcode. Following the opcode, any addresses and data are then transferred. The CS must go inactive after an operation is complete and before a new opcode can be issued. The commonly used terms in the SPI protocol are as follows: SPI Master The SPI master device controls the operations on a SPI bus. An SPI bus may have only one master with one or more slave devices. All the slaves share the same SPI bus lines and the master may select any of the slave devices using the CS pin. All of the operations must be initiated by the master activating a slave device by pulling the CS pin of the slave LOW. The master also generates the SCK and all the data transmission on SI and SO lines are synchronized with this clock. SPI Slave The SPI slave device is activated by the master through the Chip Select line. A slave device gets the SCK as an input from the SPI master and all the communication is synchronized with this clock. An SPI slave never initiates a communication on the SPI bus and acts only on the instruction from the master. The FM25V40 operates as an SPI slave and may share the SPI bus with other SPI slave devices. Chip Select (CS) To select any slave device, the master needs to pull down the corresponding CS pin. Any instruction can be issued to a slave device only while the CS pin is LOW. When the device is not selected, data through the SI pin is ignored and the serial output pin (SO) remains in a high-impedance state. Note A new instruction must begin with the falling edge of CS. Therefore, only one opcode can be issued for each active Chip Select cycle. Serial Clock (SCK) The Serial Clock is generated by the SPI master and the communication is synchronized with this clock after CS goes LOW. The FM25V40 enables SPI modes 0 and 3 for data communication. In both of these modes, the inputs are latched by the slave device on the rising edge of SCK and outputs are issued on the falling edge. Therefore, the first rising edge of SCK signifies the arrival of the first bit (MSB) of a SPI instruction on the SI pin. Further, all data inputs and outputs are synchronized with SCK. Data Transmission (SI/SO) The SPI data bus consists of two lines, SI and SO, for serial data communication. SI is also referred to as Master Out Slave In (MOSI) and SO is referred to as Master In Slave Out (MISO). The master issues instructions to the slave through the SI pin, while the slave responds through the SO pin. Multiple slave devices may share the SI and SO lines as described earlier. The FM25V40 has two separate pins for SI and SO, which can be connected with the master as shown in Figure 3. |
Numéro de pièce similaire - FM25V40 |
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Description similaire - FM25V40 |
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