The PICmicro MCU, on the other hand, is a Microcontroller and has all the CPU, memory, oscillator, watchdog and I/O incorporated within the same chip. This saves space, design time and external peripheral timing and compatibility problems, but in some circumstances can limit the design to a set memory size and I/O capabilities.
The PIC family of microcontrollers offers a wide range of I/O, memory and pecial functions to meet most requirements of the development engineer. You will find many general books on library shelves exploring the design of microcontrollers, microprocessors and computers, so the subject will not be expanded or duplicated here other than to explain the basic differences.
Why use the PIC
Code Efficiency
The PIC is an 8 bit Microcontroller based on the Harvard architecture – which means there are separate internal busses for memory and data. The throughput rate is therefore increased due to simultaneous access to both data and program memory. Conventional microcontrollers tend to have one internal bus handling both data and program. This slows operation down by at least a factor of 2 when compared to the PICmicro MCU.
Safety
All the instructions fit into a 12 or 14 bit program memory word.
There is no likelihood of the software jumping onto the DATA section of aprogram and trying to execute DATA as instructions. This can occur in a non Harvard architecture microcontroller using 8-bit busses.
Instruction
Set There are 33 instructions you have to learn in order to write software for the 16C5x family and 14 bits wide for the 16Cxx family.
Each instruction, with the exception of CALL, GOTO or bit testing instructions (BTFSS, INCFSZ), executes in one cycle.
Speed
The PIC has an internal divide by 4 connected between the oscillator and the internal clock bus. This makes instruction time easy to calculate, especially if you use a 4 MHz crystal. Each instruction cycle then works out at 1 uS. The PIC is a very fast micro to work with e.g. a 20MHz crystal steps through a program at 5 million instructions per second! – almost twice the speed of a 386SX 33!
Static
Operation The PIC is a fully static microprocessor; in other words, if you stop the clock, all the register contends are maintained. In practice you would not actually do this, you would place the PIC into a Sleep mode – this stops the clock and sets up various flags within the PIC to allow you to know what state it was in before the Sleep. In Sleep, the PIC takes only its standby current which can be less the 1uA.
Drive Capability
The PIC has a high output drive capability and can directly drive LEDs and triacs etc. Any I/O pin can sink 25mA or 100mA for the whole
device. Options A range of speed, temperature, package, I/O lines, timer functions, serial comms, A/D and memory sizes is available from the PIC family to suit virtually all your requirements.
pic function block diagram
Versatility
The PIC is a versatile micro and in volume is a low cost solution to
replace even a few logic gates; especially where space is at a premium.
Security
The PICmicro MCU has a code protection facility which is one of the
best in the industry. Once the protection bit has been programmed, the
contents of the program memory cannot be read out in a way that the
program code can be reconstructed.
Development
The PIC is available in windowed form for development and OTP (one time programmable) for production. The tools for development are readily available and are very affordable even for the home enthusiast.
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