学步园

摘自：http://www.best-microcontroller-projects.com/pic-microcontroller.html

What can youdo with a microcontroller?

Answer: Virtually Any Project
Easily!...

Microcontrollers give you a
fantastic way of creating projects.

APIC microcontroller is a processor with built in memory and RAM and youcan use it to control your projects (or build projects around it). Soit saves you building a circuit that has separateexternal RAM, ROM and peripheral
chips.

What this really means for youis that you have a very powerfuldevice that has many useful built in modules e.g.

Even with just these four modules (note these are just examplemodules - there are more) you can make up manyprojects e.g.:

* Frequencycounter - using the internal timers and reporting throughUART(RS232) or output to LCD.

* Capacitancemeter - analogue comparatoroscillator.

* Event timer- using internal timers.

* Event datalogger -capturing analogue data using an internal ADC andusing the internalEEPROM for storing data (using an external I2C for high data storagecapacity.

* Servocontroller (Control through UART) - using the internal PWMmodule or using a software created PWM.

The PIC Micro is one of the most popularmicrocontrollers and in case you were wondering
the difference betweena microprocessor and a microcontroller is that a microcontroller has aninternal bus with in built memory and peripherals.

In fact the 8 pin (DIL) version of the 12F675has an amazing number of internal peripherals. These are:

• Two timers.
• One 10bit ADC with 4 selectable inputs.
• An internal oscillator (or you can use an externalcrystal).
• An analogue comparator.
• 1024 words of program memory.
• 64 Bytes of RAM.
• 128 Bytes of EEPROM memory.
• External interrupt (as well as interrupts frominternal peripherals).
• External crystal can go up to 20MHz.
• ICSP : PIC standard programming interface.

And all of these work from within an 8 pin DIL package!

Many now have a built in ADC so you can read analogue signal levels soyou don't need to add an external
devices e.g. you can read an
LM35temperature sensordirectly with no interface logic.

Peripherals

At the end is a short summary of the
maindevices used in projects shown on this site.

The best way to start is to learn about the main features of a chip andthen begin to use each peripheral in a project.  I thinklearning by doing is the best way.

Flashmemory

ICSP

I/OPorts

Pindirection

Most PIC microcontroller pins can be set as aninput or and output andthis can be done on the fly e.g. for a dallas 1 wire system a pin canbe written to generate data and read at a later stage. The TRISregister controls the I/O direction and setting a bit in
this registerto zerosets the pin as
outputwhile setting it as one sets thepinas
input.

This allows you to use a pin for multiple operations e.g. the
RealTime clock projectuses RA0, the first pin of PORTA, to output data to a seven segmentdisplay and at a later point in the program read the analogue value asan input.

Current

The PIC I/O ports are high current ports capable ofdirectly driving LEDs (up to 25ma output current) - the
total currentallowed usually ~200mA this is often for the whole chip (or specifiedfor several ports combined together). 

Timer/ Counters

EachPIC microcontroller has up to three timers that you can eitheruseas a timer or a counter (Timer 1 & 2) or a baud clock (Timer2).  

Timer0

Theoriginal timer: Timer 0 was the first timer developed and you can findit in all the earliest devices e.g. 16F84 up to the most current e,g,16F877A.  

It is an 8 bit timer with an 8 bit prescaler that can be driven from aninternal (Fosc/4)orexternal clock.  It generates an interrupt on overflow whenthe count goes from 255 to zero.

Timer 0 always synchronizes the input clock (when using external clock).

Note: Youcan read and write timer 0 but you can not read the prescaler.

Note: The prescaler changes its effect depending on whether it is atimer prescaler or a watch dog prescaler - so the same prescalersetting may prescale by 2 or by 1 depending on its use!

Timer1

This is a 16 bit timer that generatesanoverflow interrupt when it goes from 65535 to zero.   It hasan 8 bitprogrammable prescaler and you can  drive it from the internalclock (Fosc/4) oran external pin. 

To eliminate false triggering it also has an optional inputsynchronizer for external pin input.

This timer can be used in sleep mode and will generate a wakeupinterrupt on overflow.  

Timer 1 is also read by the CCP module to capture an event time.

Note: Usingthis timer in sleep mode will use more current.

Timer2

Thisis an 8 bit timer with an 8 bit prescaler and an 8 bit postscaler. It takes its input onlyfrom the internal oscillator (Fosc/4).

This timer is used for the timebase of a PWM when PWM is active and itcan be software selected by the SSP module as a baud clock.

USART

TheUSART is a useful module and saves having to code up a software versionso it saves valuable program memory.  You can find moreinformation on RS232
hereand how to
makeit work. Look
herefor pin outs.

All you need to interface it to a PC serial port is a MAX232 chip (orequivalent).


Note: An equivalent MAX232 chip is the SP202ECP that has the same pinout asthe MAX232 but lets you use 100nF capacitors - so you don't need thelarge 1uF caps.

BaudRates

You have to be careful using the baud rates asthey depend onthe main clock in use and normal oscillator values in general do notfit very well with 'real' baud rates.  

CCP 

The Capture/Compare/PWM module has three modes ofoperation:

• Capture- Capture the time of an event.
• Compare- Generate an output when Timer 1 reaches a value.
• PWM- Pulse Width Modulation.

Capture 

Capture mode is used to capture the value of Timer1 when a signal at the CCP pin goes high (or low depending on how theCCP is set up).  The CCP can accurately capture the arrivaltimeof a signal at the CCP pin so it can be used for pulse time measurement.

Compare

Compare mode is used to generate an output whenTimer 1 reaches a value you put into CCPR1.  One special eventtrigger mode lets you start the ADC when the compare mode triggers.

PWM

PWM gives you one Pulse Width Modulation outputwith 10 bit resolution and with no software overhead - once started itoperates all by itself unless you want to change the duty cycle.

It uses Timer 2 to define its operation using Timer 2 period registerto define the frequency of the PWM.


Note: Theduty cycle is not a percentage it is the number of periods of the PWMclock that the output is high!

SSP 

SPI and I2C are shared so you can only use one at a time (or you coulduse the I2C bit banged routines in the
RealTime Clockproject to have both at the same time).

You can find a project that uses I2C
hereand you canfind more information on I2C
here.

Comparatorandcomparator voltage reference 

The comparator is module that has two analoguecomparators which can be set up in one of 8 different ways. Eitherdigital or analogue inputs can be compared to reference voltages.

In one mode an internally generated voltage reference is used as aninput to both comparators and in the same mode multiplexing lets youmonitor up to four different input pins.

You can even send the output of the comparator to a pin so that it isused  independently from the microcontroller e.g. in a circuitwhere you need a comparator you don't need an extra chip!

The analogue level must be between Vdd and Vss as protection diodeswon't allow anything else.  

The module will generate an interrupt if the comparator output changes.

You can use it in sleep mode and the interrupt will wake it up.


The sourceimpedance of the analogue signal must be smaller than 10k.

ADC 

The single 10 bit Analogue to Digital Converter canhave up to 8 inputs for a device multiplexed from input pins.  

The ADC can be used during sleep but you have to use the RC clock mode.One benefit of this is that there will be no digital switching noise soyou will get better conversion accuracy.

The 16F675 can measure 4 analogue input pins!

PSP

The Parallel Slave Port lets you to connectthe PIC microcontroller directly into a microprocessor system.It provides an 8 bit read/write data bus and RD (read) WR(write) and CS (chip select) inputs - all activelow.  

This provides an easy route to adding a PIC microcontroller to an 8bit  system that already exists.

LCD

SpecialFeatures

WDT

If your software goes haywire then this timerresets theprocessor.  To stop the reset the well behaved software mustperiodically issue the CLRWDT instruction to stop a resert.  The WDTruns using its own oscillator.  It runs during sleep andshares Timer 0 prescaler.

POR

Power On Reset startsPIC microcontroller initialization when it detects a rising edge onMCLR.

PWRT

If you enable this then 72ms after a POR the PICmicrocontroller isstarted.

OST

Oscillator Startup Timer delays for 1024 oscillatorcycles after PWRT (if PWRT is enabled) ensuring that the oscillator hasstarted and is stable.  It is automatic and only used forcrystal oscillator modes and is active after POR or wake from sleep.

SLEEP

Sleep mode (or low power consumption mode) isentered by executing the 'SLEEP' command.  The device can wakefrom sleep caused by an external reset, Watch Dog Timer timeout, INTpin RB port change or peripheral interrupt.

Projectdeviceoverview

All three devices are extremely powerful and the maindifference is thatthey have different numbers of pins and memory size.

Note: Thereare differences in using the devices i.e. there are some registers thatare different but in the generally youcan interchange them - this is made easier using a high level language.

The devices used in this site are:

(Note: that all of them have the letter F in - this means it is a Flashre-programmable part - don't go and buy a part with O in as its OTP -programmable only once! - only do that if you are really really sureit's the final design).

PICMicrocontroller Flash Memorysize

You may think that 1k or even 8k is so tiny that itwon't be useful but each PIC microcontroller uses RISC (ReducedInstruction Set Computing) which simply means that it has acleverly arranged instruction set that only has a few instructions. The mid range parts
have 35 instructions.

If you use the high level language as recommended in this site then youwon'tneed to be too aware of the instruction set it just means you can do alot with a small amount of memory.  Most of the projects onthis site although they are fully working projects fit
within2k words!

Note: If youneed more memory you can always move to the 18F series of PICmicrocontrollers.  Another option is to add an I2C serialeprom.

PICmicrocontroller RAM and EEPROMsize

PICmicrocontroller EEROM : Electrically Erasable ROM is usedto storedata that must besaved between power up and power down.  

This area is readableand writable and has a much longer life than the main program storei.e. it has been designed for more frequent use.