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Code Protection
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In all PIC microcontrollers, one or more code protect bits are included in the con guration fuse register. These bits are used to hinder unauthorized copying or downloading of the hex le of your application once you have completed and released an application. Once the code protection bit is set for a section (or all) of program memory, program memory reads in a typical programmer returns all zeros. In some older devices, program memory data can still be read out, but it is XORed with the adjacent words to allow for verifying the contents of program memory while still making the contents unreadable. In either case, you may nd it preferable to burn the application code into program memory, read it back, and then program the code protect bits before nishing the programming operation. If you are working with EPROM program memory based PIC MCUs, the EPROM cells of the con guration word code protection bits are often covered by an opaque layer of aluminum, as shown in Fig. 4.1. This is to prevent the code protect bits from being
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Figure 4.1 EPROM con guration fuse register cell with aluminum layer preventing selective erasure.
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TABLE 4.2 CODE PROTECTION MODES AND BIT SELECTION FOR THE PIC16F877 BITS CODE PROTECTION OPERATION
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All program memory protected The last 4K (upper half) instructions of program memory are protected The last 256 instructions of program memory are protected No program memory is protected
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selectively erased (normally in the PIC microcontroller, when code protection is disabled, these cells are left unprogrammed) allowing the rest of program memory to be read straight back. The metal layer prevents ultraviolet erasing light from reaching the EPROM cell and effectively prevents it from ever being reprogrammed. For this reason, I recommend that you never enable code protection in EPROMbased PIC microcontrollers unless you are absolutely sure of what you are doing. While some people have reported that a deep erase cycle of several hours to several days will clear code protect bits with the layer of aluminum over them, most have ended up with an interesting (and expensive) piece of abstract art or jewelry. The EEPROM and Flash program memory based PIC MCU code protection is designed so that if it is set, a complete erase of the part is required before it can be reused. This will ensure that all the contents of the chip are cleared before allowing subsequent writes or reading back. There are a lot of options for code protection in many of the PIC microcontrollers. Table 4.2 lists the four ways of specifying code protection in the PIC16F877. This allows you some interesting options and protection for your application. While the PIC microcontroller s code protection hardware is well designed to protect the contents of the PIC microcontroller s program memory, it is not infallible. There are many companies that advertise the capabilities of reading code protected memory (ostensibly for legitimate companies that have lost the source code to a part). The techniques used are somewhat esoteric, but can be accomplished on lab equipment such as scanning electron microscopes, which is available in many chip-making facilities around the world.
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The rst part numbers of General Instrument s PIC (peripheral interface controllers) were programmed using a parallel algorithm: an entire instruction word was presented to the microcontroller and then latched in. This method was reliable and fast but had two major drawbacks. The rst was that the device had to have enough I/O pins to allow a
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PROGRAMMING PIC MICROCONTROLLERS
full instruction word as well as some handshaking, programming voltage, and control bits for the programming operation. This restriction meant that for the early low-end products, there had to be at least 15 I/O pins (which is why when you look at some of the older PIC microcontroller part numbers, like the PIC16F54, there are 18 pins 15 for programming and 2 for power the last pin is the clock input which isn t accessed during the programming operation) and precluded the development of smaller, low pin count products. The second issue was the added complexity of the programmer circuitry. Generally, it is easier and cheaper to create products that transfer data serially than it is to do it in parallel. By having parts that required parallel programming circuits, the PIC microcontrollers would be less attractive to students and hobbyists. All PIC microcontrollers designed after 2000 use the ICSP programming interface (described later in this chapter), but many of the earlier chips in the low-end and PIC17 families use parallel programming algorithms, which are described in the following sections.
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