【正文】 ld be appropriate when you want to have extra protection against software bugs, or when you intend for the part to be read only. The other option is to wire it high and leave it that way. 4GNDPower supply GND5SISerial Input pin. In SPI terminology, this would be the Slave In, and should be connected to the MOSI pin of the SPI port. Data being output by the CPU is clocked in on this line.6SCKSerial Clock. This is the SPI serial clock line. Each cycle of this clock causes the data registers to shift both in and out 1 bit. 7HOLDThis interesting pin places the state of the part on 39。HOLD39。. This is useful if you have interrupt driven routines that need to access other parts on the SPI bus. By holding this line low, the part will ignore the SCK line, which in affect causes the part to hold its current state. Your interrupt routine can then use the SPI bus to access a different part, then return to the current operation by raising the HOLD line. If you don39。t plan on doing this, just wire it high.8VCCThe power supply line. Usually connected to +5 volts, though the range of voltages go as low as , depending on the part you are using(* denotes active LOW)The Block DiagramAs always, engineers love to see the block diagram! Here is the block diagram you will find in the documentation for the Microchip 25C640:As you can see, there are a few different parts called out in the block diagram. Of particular interest to us are the EEPROM Array, which is the actual EEPROM memory cells, the Status Register, which is a register that holds configuration and status information, and of course the Page Latches, which are the temporary 39。holding39。 areas for data being written. The other parts are also interesting, but I am going to ignore them for this article. A SchematicConnecting a serial EEPROM is really quite straight forward. I have been offering a board based on the 68HC912B32 Micro controller for some time now. I recently added a spot for a serial EEPROM to the backside of the board. The following snippet of the schematic shows the 25xxx series EEPROM connected to the SPI port for this board. Note that on my 912B32 board, this is an optional device. I left several cut jumpers (these are areas where the trace can be cut, and the wiring done differently) on the board, and they are shown on the schematic. Connecting up a serial EEPROM is pretty straight forward, and it can be added to almost any CPU that supports the SPI bus. Note that I have chosen to leave the WP (Write Protect) and HOLD lines wired high. I don39。t use them in my software, but you are certainly wele to use them. On my board, you merely cut the trace on the jumper, and wire up the appropriate pins to a port for control.Table of mandsThere are a small number of mands that you can issue to the serial EEPROM. The following table is a quick summary.Instruction Instruction CodeDescriptionREAD0000 0011Read data from memory array beginning at selected addressWRITE0000 0010Write data to memory array beginning at selected addressWREN0000 0110Set the write enable latch (enable writing)WRDI0000 0100Clear the write enable latch (disable writing)RDSR0000 0101Read status registerWRSR0000 0001Write status registerEach instruction code has its own argument requirements. Read/Write, for example, is followed by an address. Writing the status register requires an 8bit argument, and so on. A plete set of instructions and their formats are available in the data sheet for your particular part. Most of these SPI parts have extremely similar manuals! You can find a list of data sheets on the Microchip website at which will provide you with a lot of extra detail.Speed issuesMost parts can operate with transfer speeds in excess of 1 megabit per second. Some parts can go up to 8 megabits per second. You need to take this into consideration when you are se