To know a much more detailed basic of PIC, click on the links below
TAKE A LOOK : INTRODUCTION TO PIC
TAKE A LOOK : BASICS OF MICROCNTROLLERS
Introduction to Peripheral Interface Controllers (PIC)
Peripheral Interface Controllers (PIC) is one of the advanced microcontrollers developed by microchip technologies. These microcontrollers are widely used in modern electronics applications. A PIC controller integrates all type of advanced interfacing ports and memory modules. These controllers are more advanced than normal microcontroller like INTEL 8051. The first PIC chip was announced in 1975 (PIC1650). As like normal microcontroller, the PIC chip also combines a microprocessor unit called CPU and is integrated with various types of memory modules (RAM, ROM, EEPROM ,etc), I/O ports, timers/counters, communication ports, etc.
All PIC microcontroller family uses Harvard architecture. This architecture has the program and data accessed from separate memories so the device has a program memory bus and a data memory bus (more than 8 lines in a normal bus). This improves the bandwidth (data throughput) over traditional von Neumann architecture where program and data are fetched from the same memory (accesses over the same bus). Separating program and data memory further allows instructions to be sized differently than the 8-bit wide data word.
Basic structure of a modern Peripheral interface controller chip is show in the picture below.
The function of CPU in PIC is same as a normal microcontroller CPU. A PIC CPU consists of several sub units such as instruction decoder, ALU, accumulator, control unit, etc. The CPU in PIC normally supports Reduced Instruction Set Computer (RISC) architecture (Reduced Instruction Set Computer (RISC), a type of microprocessor that focuses on rapid and efficient processing of a relatively small set of instructions. RISC design is based on the premise that most of the instructions a computer decodes and executes are simple. As a result, RISC architecture limits the number of instructions that are built into the microcontroller but optimizes each so it can be carried out very rapidly (usually within a single clock cycle.) . These RISC structure gives the following advantages.
• The RISC structure only has 35 simple instructions as compared to others
• The execution time is same for most of the instructions (except very few numbers).
• The execution time required is very less (5 million instructions/second (approximately).
The memory in a PIC chip used to store the data and programs temporary or permanently. As like normal microcontrollers, the PIC chip also has certain amount of RAM, ROM, EEPROM, other flash memory, etc.
ROM memory is used for permanent storage. The ROM memory also called as n program memory. A PI chip has certain amount of ROM memory.
EEPROM memory is another category of ROM memory. The contents in the EEPROM changes during run time and at that time it acts like a RAM memory. But the difference is after the power goes off , the data remains in this ROM chip. This is the one of the special advantages of EEPROM. In the PIC chip the function of EPROM is to store the values created during the runtime.
RAM memory is the one of the complex memory module in a PIC chip. This memory associated with various type of registers (special function registers and general purpose registers) and memory BANK modules (BANK 0, BANK 1, etc.). Once the power goes off, the contents in the RAM will be cleared. As like normal microcontrollers, the RAM memory is used to store temporary data and provide immediate results.
This is a special type of memory where READ, WRITE, and ERASE operations can be done many times. This type of memory was invented by INTEL corporation in 1980. A PIC Chip normally contains a certain amount of flash memory.
Information is stored in a CPU memory location called a register. Registers can be thought of as the CPU’s tiny scratchpad, temporarily storing instructions or data. Registers basically classified into the following.
1) General Purpose Register (GPR)
A general purpose register (or processor register) is a small storage area available on a CPU whose contents can be accessed more quickly than other storage that available on PIC. A general purpose register can store both data addresses simultaneously.
2) Special Function registers (SFR)
These are also a part of RAM memory locations. As compared to GPR, their purpose is predetermined during the manufacturing time and cannot be changed by the user. It is only for special dedicated functions.
Interrupt is the temporary delay in a running program. These delays stop the current execution for a particular interval. This interval/delay is usually called as interrupt. When an interrupt request arrives into a current execution program, then it stops its regular execution. Interrupt can be performed by externally (hardware interrupt) or internally (by using software).
BUS is the communication or data transmission/reception path in a microcontroller unit. In a normal microcontroller chip, two types of buses are normally available.
1) Data bus
Data bus is used for memory addressing. The function of data bus is interfacing all the circuitry components inside the PIC chip.
2) Address bus
Address bus mostly used for memory addressing. The function of address bus is to transmit the address from the CPU to memory locations.
USART or UART
These ports are used for the transmission (TX) and reception (RX) of data. These transmissions possible with help of various digital data transceiver modules like RF, IR, Bluetooth, etc. This is the one of the simplest way to communicate the PIC chip with other devices.
Oscillator unit basically an oscillation/clock generating circuit which is used for providing proper clock pulses to the PIC chip. This clock pulses also helps the timing and counting applications . A PIC chip normally use various types of clock generators. According to the application and the type of PIC used, the oscillators and its frequencies may vary. RC (Resistor-Capacitor), LC (Inductor-Capacitor), RLC (Resistor-Inductor-capacitor), crystal oscillators, etc are the normal oscillators used with A PIC chip.
The entire PIC chip has an area for storing the return addresses. This area or unit called Stack is used in some Peripheral interface controllers. The hardware stack is not accessible by software. But for most of the controllers, it can be easily accessible.
These ports are used for the interfacing various input/output devices and memories. According to the type of PIC, the number of ports may change.
Advanced functioning blocks
These sections include various advanced features of a PIC chip. According to the type of PIC, these features may change. Various advanced features in a peripheral interface controller are power up timer, oscillator start up timer, power on reset, watch dog timer, brown out reset, in circuit debugger, low voltage programming, voltage comparator, CCP modules etc.
Limitations of PIC Architecture
• Peripheral Interface Controller has only one accumulator.
• Small instruction set.
• Register banking switch required to access RAM of other devices.
• Operations and registers are not orthogonal.
• Program memory is not accessible.
Advantages of PIC Controlled System
The PIC controlled system often resides machines that are expected to run continuously for many years without any error and in some cases recover by themselves if an error occurs(with help of supporting firmware).
Many of the PIC based embedded system use a simple pipelined RISC processor for computation and most of them provide on-chip SRAM for data storage to improve the performance.
• Power consumption
A PIC controlled system operates with minimal power consumption without sacrificing performance. Power consumption can be reduced by independently and dynamically controlling multiple power platforms.
Most of the PIC based systems are memory expandable and will help in easily adding more and more memory according to the usage and type of application. In small applications the inbuilt memory can be used.
To know about the basics of PIC 16F877, click on the link below.
TAKE A LOOK : INTRODUCTION TO PIC 16F877
TAKE A LOOK : PIC PROGRAMMING INSTRUCTION SETS