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and there is no request to load in a new one, execution jumps to StartVector, and the page and bank bits are reset. Then the four original instructions are allowed to execute. Note that there is a nal goto 0x0004 | ($ & 0x1800) instruction that executes if in the rst four instructions execution does not branch to some other location. As indicated earlier, as this is being written, there are no USB-equipped mid-range PIC microcontroller part numbers that can support a bootloader through the USB. If you cannot use the UART port, then there are a number of other interfaces that can be used, including
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I2C, with the PIC microcontroller as a slave device SPI, again with the PIC microcontroller as a slave device The parallel slave port (PSP) A proprietary bit banging synchronous protocol
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Of these four options, the one that probably would be most reasonable to use with a PC is to the PSP connected to the parallel (printer) port. When the printer port is con gured for a basic dot matrix printer (such as the EPSON FX80), the data is strobed out continuously with just polling for a busy pin, which could be used to synchronize the transfer of the application le to the PIC microcontroller (waiting for the PIC to program the data into its program memory). The need for synchronizing the write of program memory with the new data coming in is a very important point when implementing a bootloader in a PIC microcontroller. You probably will nd that the program memory write will be slower than the data rate that you would like to use, and even more important, when program memory is being written, the PIC microcontroller s processor stops executing, meaning that any incoming data cannot be processed, leading to the possibility of overwrites of the incoming data buffer. You always should make sure that you have implemented some kind of handshaking protocol between your bootloader-equipped system and the host application downloader.
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SAMPLE BOOTLOADER CODE
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Implementing a generic bootloader in the PIC microcontroller was surprisingly dif cult, although I am very pleased with the nal functionality of the result. A big part of the issue was designing the bootloader to work within the con nes of the PIC16F877A microcontroller and with its Flash program memory burning algorithms; I was expecting that I would be able to write the code in 256 instructions, but it turned out that I needed about twice that to implement the function. The resulting application does work well and provides a method of downloading an application over a single 9,600-bps serial link using a basic Terminal emulator. The development hardware for the bootloader that I used was a PIC16F877A running at 4 MHz with the serial port interface wired to an MAX232 and a button placed at RC1. The circuit was the same base circuit I used to develop the BASIC87 application (but using a PIC16F877A instead of a PIC16F877). The difference between the two devices made quite a bit of difference in the code because the PIC16F877A requires four instructions to be loaded before programming can commence.
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To interface to this bootloader, I used my old standby Hyperterminal by Hilgreave (www.hilgreave.com) and wired the application to my PC via a serial port. Communication takes place at 9,600 bps and does not use any hardware handshaking, but the XON/OFF protocol is enabled. To send the updated application .hex le, the user simply clicks on File Transfer followed by Send Text File. This interface is very simple and very reliable, and when you look at the various RS-232-connected applications used in this book, you will see that I tend to use this interface almost exclusively. The XON/XOFF protocol is used for handshaking and stopping the PC from sending code to the bootloader application if it is currently burning program memory. While at 9,600 bps the timing is such that the data cannot come in faster than the bootloader program could burn the program memory, as noted earlier, the processor is stopped when programming is taking place, so I felt that it was best to indicate to the PC to stop sending data until the programming operation had completed. Before starting to develop the application, I created the pseudocode shown below and converted it into assembler (bl87a.asm, which can be found in the bl87a folder).
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main(){ if ((0 == RC1) || (0xFF == TestFlag)) { UARTInit(); ProgramIns(TestFlag, 0x3FFF); do { printf(XOFF); printf( Ready\n ); ProgramGood = 1; printf(XON); // // // // Button Pressed No Program Load Hex File Mark as Can t Use
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