Several Issues in PIC Microcontroller Development

The PIC series of microcontrollers produced by Microchip Corporation of the United States have been widely used in industrial control, instruments, meters, communications, home appliances, toys and other fields due to their ultra-small size, low power consumption, low cost and wide variety. This article summarizes some of the author's experiences and skills in the development process of PIC microcontrollers for reference by peers.

1 How to further reduce power consumption

2 Note the RBIF bit in INTCON

The interrupt enable bits in INTCON have no effect on the interrupt status bits. When PORT B ​​is configured as input mode, the RB<7:4> pin input is sampled and compared with the old latch value in each read operation cycle. Once different, a high level is generated and RBIF=1 is set. Before opening the RB interrupt, RBIF may have been set to "1", so when opening the RB interrupt, the RBIF bit should be cleared first to avoid being affected by the original value of RBIF. At the same time, it is best to clear the RBIF bit after the interrupt processing is completed.

3 Issues to note when using Mplab-C high-level language to write PIC microcontroller programs

3.1 Pay attention to the writing format when embedding assembly instructions in the program See Example 3.

Example 3

When inlining assembly instructions, each instruction from "#asm" to "endasm" must occupy a line, otherwise an error will occur during compilation.

3.2 The safest representation of addition and multiplication See Example 4.

Example 4

The reason is that Mplab-C compiles c=a*b in 8×8 multiplication mode, returns a single-byte result to c, and the overflow of the result is ignored. It is safest to change the expression "c=a*b;" in the above example to "c=a; c=c*b;" (the same treatment is applied to addition).

3.3 Understanding the register usage of multiplication and division functions

Since the RAM in the PIC microcontroller is only a few dozen bytes, the space is very valuable, and the Mplab-C compiler has the property of not releasing the RAM address, that is, the address used by a variable cannot be allocated to other variables. If the RAM space cannot meet the requirements of too many variables, some variables can only be forced by the user to allocate the same RAM space for alternating use. The multiplication and division functions in Mplab-C need to borrow RAM space to store intermediate results, so if the RAM occupied by the multiplication and division functions overlaps with the address of the user variable, it will lead to unpredictable results. If multiplication and division operations are used in a C program, it is best to first check whether the addresses occupied by the multiplication and division functions conflict with the addresses of other variables through the disassembly code of the program machine code (included in the generated LST file) to avoid the program running away. The Mplab-C manual does not give the specific RAM address occupied by its multiplication and division functions. Example 5 is the address occupation of 0×13, 0×14, 0×19, and 0×1A by the multiplication function.

Example 5

4 Reprogramming the PIC microcontroller chip

For users without hardware emulators, EPROM chips are always used to debug programs. Every time a program is changed, the original content is erased first and then programmed. This process wastes a lot of time and shortens the service life of the chip. If the result of the latter programming is only that the same bit of the corresponding machine code byte changes from "1" to "0" compared to the previous one, data can be written again on the previously programmed chip without erasing the original chip content. In
the process of debugging the program, constant adjustments are often encountered. If the change of constants can ensure that the corresponding bit changes from "1" to "0", programming can continue on the basis of the original chip content. In addition, since the machine code corresponding to the instruction "NOP" is "00", the deletion of instructions during the debugging process is replaced by the "NOP" instruction first, and programming can continue on the original chip content after compilation. In addition,
when programming chips with EPROM, pay special attention to the program confidentiality status bit. The manufacturer has changed the confidentiality status bit of the new generation of EPROM chips from the original EPROM erasable type to the fuse type. Once the program code confidentiality fuse is programmed to "0", the reprogrammable EPROM chip cannot be programmed again. Please pay attention to this when using it to avoid unnecessary waste (Microchip information does not explain this).

References

1 Micorchip PIC16Cxx Data Book
2 MPLAB-C USER ’ S GUIDE