Bootloader implementation of PIC MCU

Reprinted from :http://blog.sina.com.cn/s/blog_77b8b43b0102v9ut.html

lead said  

　　PIC Single chip microcomputer Of BootLoader It belongs to the kind that needs to write its own program , You can write various functions according to your own needs BootLoader The program to . at present , More common BootLoader The program has Microchip The company's C18 Version and famous PICC Compiler manufacturer HI-TECH company （ hereinafter referred to as HI-TECH） Of PICC18 edition . They are similar in that they all occupy Systematic 0x00～0x1FF Program space . Microchip The version of is written in Pure Exchange , and HI-TECH The version of is using C language-written . 

　　 stay PIC Series MCU , Only PIC18 Series and PIC16F87X A series of single chip computers IAP function , To be able to use BootLoader function . I'll just introduce PIC18 Of BootLoader,PIC16F87X Of BootLoader Similar to it . 

1 BootLoader How the program works  

　　BootLoader Is a boot program , Power on the MCU / After reset, run before the user program . After running, judge whether it is necessary to enter the upgrade state . If you don't need to upgrade , Just run Flash Original program in ; If you need to upgrade , First erase the old program , Then receive the user program from the serial port , Write at the same time Flash in . When receiving data , Verification is required , Ensure that the received data is correct , Avoid writing the wrong program into the chip . 

　　PIC18 There is only one single chip microcomputer Flash, Can be in Flash Erase anywhere in （ You can even put BootLoader Self erasure ）; and 51 Single chip computers are generally two Flash, Only from one piece Flash Write another piece on it Flash, Instead of erasing yourself . 

　　BootLoader In principle, a program can be placed anywhere in the whole program space , But for simplicity and convenience 、 It has versatility and minimizes the impact on user programs , Used from 0x00 The beginning of a program space . 

　　BootLoader Programs can obtain data in many ways , Including serial port 、 In parallel 、I2C、SPI、USB etc. ; But from the practical point of view , Using serial port is undoubtedly the most convenient . 

　　 In order to make the user program get the maximum program space , Need BootLoader The program should be as short as possible . ad locum ,Microchip and HI-TECH All will BootLoader The program is designed to be less than 200H Bytes （100H A word ）. Use 0x00～0x1FF Another important reason for this area is , This address space has a special write protection feature . 

　　PIC18 Although the single chip microcomputer has multiple interrupt sources , But only two-level interrupts are supported , There are two interrupt vectors , They are located in 0x08 and 0x18; and BootLoader The program takes up 0x00～0x1FF Space , This means that you need to reposition the interrupt vector , Make the new interrupt vector point to the user's interrupt program , Only in this way can the interrupt program of the user program run normally . 

2 How to use the program  

　　（1） HI-TECH Of BootLoader Program  

　　 stay HI-TECH Of PICC18 compiler Of examples Under the folder , There is one Bootldr A folder , It's inside HI-TECH Of BootLoader Program . This is a complete program , You can compile directly , The compiled HEX It can be downloaded to the chip with a programmer . 

　　（2） Requirements of user system  

　　 If needed BootLoader, The user is required to have one in the system RS232 Serial port can communicate with computer . 

　　（3） The configuration of the program  

　　 Here are the main parameters of the program , If they are not set correctly , Will affect the use of the program . 

　　VERB0： Redundancy mode , There are more tips , But it will take up more program space , It is not recommended that . 
　　BOOT_TIMEOUT： Waiting time out ,0～9s（ No matter how long it is ）, The default is 5s. 
　　BAUD： Baud rate of serial communication , The default is 9 600. 
　　FOSC： Clock frequency of user system , The default is 4 MHz. 
　　NINE： Whether the communication uses the 9 Digit bit , Default not to use . 
　　FILL_BYTE： Filling data used in program space erasure , The default value is 0xFF. 
　　PROG_START： The starting position of the user program , The default is 0x200. 
　　 Other parameters can be ignored , Modify it when necessary , They do not affect communication and downloading . 

　　（4） Compilation of program  

　　 There are two ways to BootLoader The program compiles ： 

　　① Command line mode . When using the command line , The typical usage is ： 
　　picc18 -8f452 bootldr.c -o -zg -noerrata 
　　 The last parameter above -noerrata yes PIC18Fxx2 Required by series single chip microcomputer , For other models , You don't have to join ; And the parameters -18f452 It is the model of the designated single chip microcomputer , If it's another model , Change it to the actual model . 


　　② Use MPLAB IDE. Use MPLAB IDE when , First, create a new project file in the normal way , Select the project file directory , Then join HI-TECH Of Samples\Bootldr Files under directory bootldr.c and bootldr.h. When establishing engineering documents , choice MCU The model of is the actual model , choice C Compiler is HI-TECH Of PICC18 compiler . 

　　（5） Configuration of user program  

　　 Use HI-TECH Version of BootLoader, The modification of user program is very few and simple , Just modify the offset of the user program , There is no need to modify any program code or make any special configuration . because BootLoader The program needs to occupy 0～1FFH Space , So the user program needs to start from 200H Start running at , That is a Set the offset of the program to 200H. The method is in MPLAB IDE in , Select from the menu Project→Build Options...→Project, stay PICC-18 Linker Under the page mark Specify offset for ROM（ROM Code offset ） Enter the offset in 200（ Note that the input here is already hexadecimal , There's no need to switch ）, Pictured 1 Shown . 

　　　　 
　　　　　　　　　　　　　　　　　　　　　 chart 1 

　　（6） Program configuration word  

　　 For each PIC Single chip microcomputer Chip , You need to set the correct program configuration word before it can run normally . Although in BootLoader The configuration word can be modified in , But it's not safe , It's not convenient . In general , After the program configuration word is set, it does not need to be modified , therefore Configuration word is burning BootLoader The program is written into the single chip microcomputer chip together , In the future, only use BootLoader To upgrade ( burning ) Program , This will not damage the configuration word of the chip . 

　　（7） Debug user program  

　　 At ordinary times , When writing and debugging user programs , It's still the same as the normal way , SCM does not contain BootLoader Program , Also do not set the offset （ Or set to 0）, For ease of use ICD2 Wait for the simulator to simulate the program . After the program is debugged , Then modify the offset of the program to 200H, And recompile the program , Generate end users HEX Code . Only in this way can the compiled program be used BootLoader Download . 

　　（8） Download user program  
　　　　 
　　 Use HI-TECH Of BootLoader Program , When downloading user program to MCU , You don't need a special download program , Just use Windows The self-contained HyperTerminal program is ok . The communication parameters of the super terminal need to be set to and BootLoader The procedure is the same , Including baud rate 、 check 、 Data bits 、 Stop bit, etc . 

　　 First, connect the serial port cable , Then start the HyperTerminal program , Then reset the MCU （ SCM should have written BootLoader Program ）. At this time , A countdown counter will be displayed in the window of the HyperTerminal , The initial value of the counter is the above BOOT_TIMEOUT Parameters . The counter refreshes every second . When the counter is 0 when , Will run the previous program . If any data is input from the HyperTerminal during this period （ Just press any key , Or the single chip microcomputer receives any data from the serial port ）, The counter will stop counting , Enter upgrade state . At this time BootLoader The program will first erase the old program space , Then a colon will appear on the screen “：”, Prompt to wait for the user program to download . At this time, you can select transfer from the menu of the HyperTerminal → Send a text file , Choose the compiled HEX File can . If the download is successful , A small bracket will be displayed in the window of the HyperTerminal “)”, Prompt that the download is complete , At the same time, the user program will automatically start running . Use BootLoader When the download , Because the speed of serial port is relatively slow （ Compared with programmer ）, So you need to wait for a while . The specific time is related to the size of the user program . If there is an error in the download , The single chip microcomputer will reset automatically , Enter countdown state , Repeat the process above . Using HyperTerminal is relatively simple , No special download software is required ; But lack of interaction , No process instructions , If there are errors in the download process, you can't stop . 

3 BootLoader Program improvement  

　　（1） HI-TECH Of BootLoader Defects in the program  

　　 although HI-TECH Version of BootLoader The program is already very convenient , Have all the necessary basic elements ; But there are some defects in the program , Even a major hidden danger , It is not suitable for direct use in practical projects . Here are the main problems  

　　①BootLoader It is marked by receiving any data from the serial port BootLoader State of . Get into BootLoader Post state ,BootLoader The first thing a program does is erase the space of the previous program . If in actual use , The single chip microcomputer is reset for some unexpected reason , And run after reset BootLoader When there is any data on the serial port （ For example, the interference signal or the system is in the state of serial port communication ）, It will cause the loss of user programs .

　　② BootLoader The watchdog is not used in the program . If the upgrade fails or the program crashes during the upgrade , Will not be able to return to the original upgrade state . This is not a big problem for the direct serial connection , But using remote upgrade is a fatal problem . 
　　③ If data error occurs during writing user program , It will reset , At this time, the user program has been partially written . If it runs after reset BootLoader When the program does not receive a signal , Will start the user program . In this way, incomplete user programs may cause running failures and unpredictable results . If the watchdog is opened in the user program , At this time, the watchdog may crash without being started , This is a serious problem in remote upgrade . 
　　 
　　 ④ BootLoader Program It is allowed to write EEPROM And chip configuration word . Although this increases flexibility , But it's not safe . If the configuration word is accidentally set incorrectly , After downloading, the whole chip will not work properly . At this time, you need to use the programmer to modify the configuration word . 

　　 Only try to overcome the defects mentioned above , Only then BootLoader The program is applied to the actual system . 

　　（2） Yes HI-TECH Of BootLoader Program improvement  

　　 In response to the above questions , Yes HI-TECH Of BootLoader Some modifications have been made , Removed some rarely used functions and unsafe functions , At the same time, the entry BootLoader Condition for judging the state . In addition to using enhanced serial port data recognition , The method of level detection is also added , Used to determine whether to enter BootLoader state . 

　　 ① For the problem in the first item above , Modify it to recognize a specific string before entering BootLoader Upgrade status of . The content and length of a specific string can be defined by the user （ The length cannot exceed 12 byte . In general ,12 The identification string of bytes should be long enough ）. You can use any data （ Include 0）. 

　　 ② Added specific pin level judgment , stay BootLoader After program running , Judge whether the voltage on a particular pin is the predetermined voltage , This determines whether it is necessary to enter BootLoader Upgrade status . Pin and predetermined voltage （ high / low ） You can set it up freely . This way is safer , But it is not suitable for remote upgrade . 

　　 ③ Added watchdog option , Can be set to use / Don't use a watchdog . Generally, the overflow time of the watchdog is set at 0.5～2s More appropriate . 

　　 ④ Added programming response . One line is successfully received at each HEX After the data , Send a response byte , Used for error detection during programming .PC The download program of the client can judge whether there is an error in the download process according to this byte . 

　　 ⑤ For the above 3(1) in ③ The problem of , It can be solved in this way . Will be compiled HEX The file is slightly modified manually , take 0x200～0x220（ Suppose the user program is from 0x200 At the beginning ） The data of the address segment moves from the beginning of the file to the end of the file . such 0x200 The code at will be written into the MCU at the end . If a failure occurs during writing , Even if BootLoader Started the user program , It will also reset because there is no code in this area , Instead of running a partially written user program . But this method needs to be right HEX Only with a certain understanding of the structure of the document . A simple judgment HEX The method of data address is ,HEX The first character on each line of the file is a colon “:”, The... After the colon 3、4、5、6 These four numbers represent the address of this line of data , It is expressed in hexadecimal . Such as ：“：100200...” It means the address is 0x200. Under normal circumstances , The compiled HEX The file data is arranged in the order of addresses from low to high . 

　　 The improved program adds some parameters , They are ： 
　　BOOT_SIGNAL—— Use the MCU pin level trigger mode to enter BootLoader; 
　　BOOT_SIGNAL_PORT—— Define the pin used to detect the level trigger starting mode ; 
　　BOOT_SIGNAL_LEVEL—— Define the detection level 1= High level trigger ,0= Low level trigger ; 
　　BOOT_TIME_DELAY—— Use timeout mode to enter BootLoader, This parameter and the above BOOT_SIGNAL Not at the same time  Use ; 
　　USE_EXTEND_HEX—— Whether to receive extended HEX Code , Not using it can save code , It is not recommended that ; 
　　USEWDT—— Whether in BootLoader Use a watchdog in , It is recommended to use ; 
　　CONFIRM_TIME—— Number of bytes synchronized online , Use multiple bytes for synchronization in timeout mode ; 
　　CONFIRM_DATA—— User definable online data ; 
　　USE_ECHOBACK—— Whether to respond when programming , Use can increase the security of downloading , It is recommended to use .

　　 Used BOOT_SIGNAL After the way , The parts related to timeout mode no longer work . At this time, it can be defined to use any pin to determine whether it needs to enter BootLoader. In the use of BOOT_TIME_DELAY( Timeout mode ) when , Added some related content , Such as CONFIRM_DATA, You can use any specific string to confirm whether you need to enter BootLoader state , Added BootLoader The security of the program , Avoid being disturbed and entering the program upgrade state by mistake . 

　　（3） Other suggestions and methods for improvement  

　　 ① HI-TECH Of BootLoader The program receives standard HEX file . This is not safe enough in many cases , It is not conducive to the encryption of the program , Easy to disassemble and crack . It can be done to HEX Encrypt , Become data that cannot be viewed directly . 

　　 ② Using HyperTerminal to download programs is slow , meanwhile , If there is an error in the download , The HyperTerminal program cannot be found and stopped in time , Instead, you can't stop until you send the file . At this time BootLoader The program will repeatedly enter BootLoader state , There is certain damage to the single chip microcomputer . It is best to write a special download program on the computer side by yourself , It can not only improve the download speed , It can also improve the success rate of downloading . 

　　 ③ Use RS422/485 The way . Sometimes , It's not that RS232 A serial port , It is RS422/RS485 A serial port . They are actually similar , There are only some differences in the interface mode .RS422/RS485 Need to control sending , So in BootLoader Add a sending control to the program . 

　　 ④ stay BootLoader in , Setting the baud rate very high is not very useful , It does not improve the overall download speed , And can only speed up the speed of data transmission . Because the whole download is divided into data communication （ The data transfer ） and Flash write in / erase （ Data waiting ） Two parts . Program code Flash The erasing and writing speed of space is relatively slow （ The typical value is 3～4 ms）, It's easy to lose data if it's too fast , Cause download failure . Use 9600 bps The baud rate of , transmission 1 The data of bytes is about 1 ms, Receive a buffer 8 Bytes need about 8 ms, Greater than the write delay , So there is no need to delay ; When the communication rate exceeds 9600 bps when , receive 8 The time of byte buffer may be less than the write time , Need to delay in communication . The actual test results are ： Use 9600 bps Than using 14 400 bps Time slow 50%, Use 57 600 bps Than 9600 bps Double the speed , While using 115 200 bps Time and 57 600 bps There is hardly any difference . If you use HyperTerminal to download , There is no need to set a high baud rate . Because when using the most commonly used three wire communication （ No control signal ）, The super terminal adopts a more conservative way to send data , Itself is relatively slow . 
　 
Conclusion  

　　 A good BootLoader The program should have good maintainability and can handle exceptions correctly , It will not cause damage and collapse of the system due to accidents . 

　　 Can be in http://shaoziyang.logchina.com/blog/article_156363.936124.html Download the improved BootLoader Program （ This program has been used in practical projects for a long time , Very stable ）, And a replacement HyperTerminal PIC18 Dedicated download program ( The download speed of this program is much faster than that of HyperTerminal , You can define baud rate and online string by yourself , have BootLoader Zone code protection function and download error detection function ). 

　　　　　　　　　　　　　　　　　 reference  

1 Microchip.PIC18F1220/1320/2320/6620/6621 Data manual  
2 Microchip Application instructions AN851：A Flash Bootloader for PIC16 and PIC18 Devices 
3 HI-TECH Of PICC18（ver8.35pl2） The software BootLoader Program code and program description