【正文】 her 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 require the standard address bus and data bus wiring. These memories are called serial memories, and they are just the ticket to allow you to store large amounts of information without giving up those precious I/O lines. There are several different styles of serial EEPROM. The focus of this article is going to be SPI based devices. There are also 1, 2, and 3 wire serial EEPROM devices that function in a similar way, but have different interface logic.This article is going to introduce the basics of serial memories, and present the driver software needed to make it operate.Missing the BusThe big advantage to using a Serial EEPROM is that the wiring only requires 4 signal lines from the CPU to operate it. Compare this with a more standard memory, which requires an address bus and a data bus! The amount of glue logic required to make a Serial EEPROM operate is extremely low. For example, check out the following two packages. A traditional memory, such as a 27256, requires a 28pin part to be wired into your circuit. There are 15 address lines, 8 output lines, plus assorted control lines. On the other hand, a serial EEPROM can be purchased using in a 8pin package. There is 1 input line, 1 output line, 1 control line, and 1 clock line required. There are a couple of other control lines available, but are not actually required if you wish to avoid them. As you can imagine, these parts a lot easier to wire in, much smaller and best of all are mighty cheap! An 8kx8 part costs about $ at Dig key.Of course, there are tradeoffs. Since the memory of the serial EEPROM is accessed one bit at a time, there is a speed penalty to be paid. However, these are still extremely fast parts. They can output their data at over 2 million bits per second, which is quite reasonable for many applications. So, for a small amount of software overhead and only 4 I/O pins, you can have yourself some substantial data storage!Would you like to super Size that?Serial EPROM’s are available in a variety of different sizes. They seem to range from 128 bytes all the way up to 32k bytes. Atmen also has a 128k byte device that fits the same form factor. All of these parts are electrically patible. However, some require minor software changes. For example, some allow writes of 32 bytes at a time, where others allow 64 bytes at a time. These minor differences are easily dealt with in software. It turns out that serial EPROM’s are available from several different manufacturers. At least two, Microchip and Atmen, have patible chips. I haven39。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。 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 selecting a part. Luckily, most of the SPI implementations, such as the 68HC11 and 68HC12