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FM25C041U Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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FM25C041U Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 11 page 7 www.fairchildsemi.com FM25C041U Rev. B Functional Description (Continued) SPI communication As mentioned before, serial communication with the EEPROM is enabled when the /CS pin is held low and the /HOLD pin is held high. Input data (Instruction Opcodes, Array addresses and Data) on the SI pin is latched in on the falling edges of SCK clock signal, starting from the first falling edge after the /CS pin goes low. During the time the SI data is input into the EEPROM, the SO pin remains in high impedance state. If the intended instruction is of read nature (Array read and Status register read), then data from the EEPROM is driven out actively on the SO pin from every rising edge of the SCK after the last input data (SI) is latched in. During the time the SO data is output from the EEPROM, the data on the SI pin is ignored. Figure 2 illustrates the above. Refer Figure 1 for timing information. HOLD function An active communication with the EEPROM can be temporarily suspended by bringing the /HOLD pin low when a EEPROM is selected (/CS pin should be low) and a serial sequence with the EEPROM is currently underway. To suspend the communication, /HOLD pin must be driven low while SCK is high, otherwise the Hold function will not be invoked until the next SCK low to high transition. The EEPROM must remain selected during this se- quence. Transitions on the SCK and SI pins are ignored during the time the part is suspended and the SO pin will be in high impedance state. Releasing the /HOLD pin back to high state will allow the operation to resume from the point it was suspended. /HOLD pin must be driven high while the SCK pin is high, otherwise serial communication will not resume until the next SCK low to high transition. Asserting a low on the /HOLD pin at any time will tri-state the SO pin. Figure 3 illustrates Hold timing. System Configuration When multiple SPI peripherals (for e.g. EEPROMs) are present on the bus, the SI, SO and the SCK signals can be tied together. Figure 4 illustrates a typical system configuration with respect to /CS, SCK, SI and SO pins. FIGURE 4. System Configuration SI SO SCK /CS DATA OUT (MOSI) DATA IN (MISO) SERIAL CLOCK (SPICK) SS0 SS1 SS2 SS3 SI SO SCK /CS SI SO SCK /CS SI SO SCK /CS SELECTION CHIP SPI MASTER MCU FM25Cxxx SPI Modes 1 and 2 FM25C041U supports both MODE 1 and MODE 2 of operations. The difference between MODE 1 and MODE 2 is determined by the state of the SCK clock signal when a SPI cycle starts (when / CS is driven low) as well as when the SPI cycle ends (when /CS is driven high). Under MODE 2 of operation, the SCK signal is held low both at the start and at the end of a SPI cycle. Under Mode 1 of operation, the SCK signal is held high both at the start and at the end of a SPI cycle. However in both of these two modes, the input data (SI) is sampled (latched in) at the falling edge of the SCK clock signal and the output data (SO) is driven after the rising edge of the SCK clock signal. See Figure 1 and Figure 2. READ SEQUENCE (READ) Reading the memory via the serial SPI link requires the following sequence. The /CS pin is pulled low to select the EEPROM. The READ opcode is transmitted on the SI pin followed by the byte address (A7–A0) to be read. After this is done, data on the SI pin becomes don’t care. The data (D7–D0) at the address specified is then shifted out on the SO pin. If only one byte is to be read, the /CS pin can be pulled back to the high level. It is possible to continue the READ sequence as the byte address is automatically incremented and data will continue to be shifted out as clock pulses are continuously applied. When the end of memory array is reached (last byte location), the address counter rolls over to the start of memory array (first byte location) allowing the entire memory to be read in one continuous READ cycle. See Figure 5. FIGURE 5. Read Sequence /CS SI SO Read Opcode Byte Addr Data (1) Data (2) Data (n) READ STATUS REGISTER (RDSR): The Read Status Register (RDSR) instruction provides read access to the status register. As mentioned before, of the 8bits of data, only the LSB 4bits are valid and they indicate Block Protec- tion information (BP1 and BP0), Write Enable status (WEN) and Busy/Ready status (/RDY) of the EEPROM. MSB 4bits of are invalid (Don’t cares) Following is the format of RDSR data: TABLE 3. Status Register Format Bit Bit Bit Bit Bit Bit Bit Bit 7 6 54 32 1 0 X X X X BP1 BP0 WEN RDY Bit3 (BP1) and Bit2 (BP0) together indicate Block write protection previously set on the EEPROM. Refer Table 2. Bit1 (WEN) indicates the Write enable status of the EEPROM. This bit is a read-only bit and is read by executing RDSR instruction. If this bit is “1” then the EEPROM is write enabled. If this bit is “0” then the EEPROM is write disabled. Bit0 (/RDY) indicates the Busy/Ready status of the EEPROM. This bit is a read-only bit and is read by executing RDSR instruction. If this bit is “1” then the EEPROM is busy doing a program cycle. If this bit is “0” then the EEPROM is ready. Note that if a RDSR instruction is executed when an internal programming cycle is in progress, only the /RDY bit is valid. All other bits are don’t cares. |
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