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25AA256XTIST Datasheet(PDF) 6 Page - Microchip Technology |
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25AA256XTIST Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 26 page  25AA256/25LC256 DS21822F-page 6 © 2007 Microchip Technology Inc. 2.0 FUNCTIONAL DESCRIPTION 2.1 Principles of Operation The 25XX256 is a 32,768-byte Serial EEPROM designed to interface directly with the Serial Peripheral Interface (SPI) port of many of today’s popular microcontroller families, including Microchip’s PIC® microcontrollers. It may also interface with microcon- trollers that do not have a built-in SPI port by using discrete I/O lines programmed properly in firmware to match the SPI protocol. The 25XX256 contains an 8-bit instruction register. The device is accessed via the SI pin, with data being clocked in on the rising edge of SCK. The CS pin must be low and the HOLD pin must be high for the entire operation. Table 2-1 contains a list of the possible instruction bytes and format for device operation. All instructions, addresses, and data are transferred MSB first, LSB last. Data (SI) is sampled on the first rising edge of SCK after CS goes low. If the clock line is shared with other peripheral devices on the SPI bus, the user can assert the HOLD input and place the 25XX256 in ‘HOLD’ mode. After releasing the HOLD pin, operation will resume from the point when the HOLD was asserted. 2.2 Read Sequence The device is selected by pulling CS low. The 8-bit READ instruction is transmitted to the 25XX256 followed by the 16-bit address, with the first MSB of the address being a “don’t care” bit. After the correct READ instruction and address are sent, the data stored in the memory at the selected address is shifted out on the SO pin. The data stored in the memory at the next address can be read sequentially by continuing to provide clock pulses. The internal Address Pointer is automatically incremented to the next higher address after each byte of data is shifted out. When the highest address is reached (7FFFh), the address counter rolls over to address 0000h allowing the read cycle to be continued indefinitely. The read operation is terminated by raising the CS pin (Figure 2-1). 2.3 Write Sequence Prior to any attempt to write data to the 25XX256, the write enable latch must be set by issuing the WREN instruction (Figure 2-4). This is done by setting CS low and then clocking out the proper instruction into the 25XX256. After all eight bits of the instruction are transmitted, the CS must be brought high to set the write enable latch. If the write operation is initiated immediately after the WREN instruction without CS being brought high, the data will not be written to the array because the write enable latch will not have been properly set. Once the write enable latch is set, the user may proceed by setting the CS low, issuing a WRITE instruction, followed by the 16-bit address, with the first MSB of the address being a “don’t care” bit, and then the data to be written. Up to 64 bytes of data can be sent to the device before a write cycle is necessary. The only restriction is that all of the bytes must reside in the same page. For the data to be actually written to the array, the CS must be brought high after the Least Significant bit (D0) of the nth data byte has been clocked in. If CS is brought high at any other time, the write operation will not be completed. Refer to Figure 2-2 and Figure 2-3 for more detailed illustrations on the byte write sequence and the page write sequence, respectively. While the write is in progress, the STATUS register may be read to check the status of the WPEN, WIP, WEL, BP1 and BP0 bits (Figure 2-6). A read attempt of a memory array location will not be possible during a write cycle. When the write cycle is completed, the write enable latch is reset. Note: Page write operations are limited to writing bytes within a single physical page, regardless of the number of bytes actually being written. Physical page boundaries start at addresses that are integer multiples of the page buffer size (or ‘page size’) and, end at addresses that are integer multiples of page size – 1. If a Page Write command attempts to write across a physical page boundary, the result is that the data wraps around to the beginning of the current page (overwriting data previously stored there), instead of being written to the next page as might be expected. It is therefore necessary for the application software to prevent page write operations that would attempt to cross a page boundary. |
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