【正文】 his into consideration when you are selecting a part. Luckily, most of the SPI implementations, such as the 68HC11 and 68HC12 SPI ports, allow you to adjust the clock speed for the SPI port with a clock scalar. Thus, if you are connecting a 25LC640, for example, to a 16mhz 68HC12, you will need to select an SPI presale that slows the SPI bus down to 2mhz. This is a mon oversight, and a good source of bugs!Cycle lifeSerial EPROM’s are typically rated to endure 1 million write operations per byte. That’s pretty decent. However, you still need to be careful here that you are not constantly writing data to the part. EEPROM is not a substitute for general purpose RAM. If you write to these parts in a tight loop, it would only take an hour or so to exceed the 1million write operations. Your software needs to be written to take that into account. SoftwareAs you can see, wiring up an SPI based EEPROM to the SPI port is relatively simple. Nothing es for free, however, and you will find the software mands needed to work with the SPI memory is a little more plex than just doing memory writes. The basic operation of the SPI based EPROM’s is to lower the CS line, send a mand, such as WRITE, followed by an address and the data, then raise the CS line. In a WRITE operation, raising the CS line causes the EEPROM to actually store the data. That is an important point you don39。. 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。Void sermem_StartWrite (into address)。當(dāng)然，你可以給它們中的大多數(shù)掛上外部存儲(chǔ)器，但是，那要做許多工作，而且通常要消耗其他有價(jià)值的資源，僅僅是那些輸入/出引腳使芯片變得有用。當(dāng)然，存在一些折中。它們（EEPROM）也有幾種封裝型號(hào)，包括一個(gè)8引腳雙列直插封裝以及SOIC。CPU輸出的數(shù)據(jù)在此線上隨時(shí)鐘輸出。最近我在板子的背面為串行EEPROM添加了一塊地方。當(dāng)你選擇了一個(gè)器件后需要將此考慮進(jìn)去。微芯片25C640使用32字節(jié)的頁(yè)。其中一些具有不同的頁(yè)容量和數(shù)據(jù)//容量Void sermem_Initialize ()。串行EEPROM在它往陣列里保存數(shù)據(jù)時(shí)需要一個(gè)爆發(fā)時(shí)間。狀態(tài)轉(zhuǎn)換之所以重要是因?yàn)镃S線在微控制器和串行EEPROM之間被用作主要的握手線。}以上我展示了sermem_StopWrite()函數(shù)的執(zhí)行。，是使用該規(guī)則的一個(gè)測(cè)試程序。當(dāng)你完成時(shí)，sermem_StopRead()會(huì)被調(diào)用。在一個(gè)WRITE操作中，升高CS線引起EEPROM存儲(chǔ)數(shù)據(jù)。寫入狀態(tài)寄存器需要8bit的格式，等等。對(duì)于我們尤其有趣的是EEPROM陣列（它實(shí)際上是EEPROM存儲(chǔ)單元），狀態(tài)寄存器（保存結(jié)構(gòu)和狀態(tài)信息），當(dāng)然還有頁(yè)鎖存（它是暫時(shí)保存當(dāng)前寫入數(shù)據(jù)的區(qū)域）。這適合于當(dāng)你想對(duì)軟件錯(cuò)誤有額外的保護(hù)，或者你打算僅僅從器件讀出。這些較小的差別很容易在軟件中解決。另一方面，一個(gè)有8引腳封裝的串行電擦除可編程存儲(chǔ)器能夠買到。}Above I show my implementation of the sermem_StopWrite () function. This functions job is to cause the serial EEPROM to mit a previously transferred page to EEPROM. The function razes the CS line, and then issues a RDSR instruction and loops waiting for the WIP bit to go to zero. Note that the spi_deselect () and spi_select () instructions needed to be there. The state transition is important because the CS line is used as the primary handshake between the micro controller that the serial EEPROM. It signals the start and stop of operations. Void sermem_StartRead (into address)。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 packa