Instructions used in this chapter:

Microcontroller inputs and outputs (I/O)

The microcontroller is a very versatile chip and can be programmed to operate in a number of different configurations. The 16F84 is a 13 I/O device, which means it has 13 Inputs and Outputs. The I/O can be configured in any combination i.e. 1 input 12 outputs, 6 inputs 7 outputs, or 13 outputs depending on your application. These I/O are connected to the outside world through registers called Ports. The 16F84 has two ports, PORTA and PORTB. PORTA is a 5-bit port it has 5 I/O lines and PORTB has 8 I/O.

Timing with the microcontroller

All microcontrollers have timer circuits onboard; some have 4 different timers. The 16F84 has one timer register called TIMER0. These timers run at a speed of ¼ of the clock speed. So if we use a 32,768Hz crystal the internal timer will run at ¼ of 32768Hz i.e. 8192Hz. If we want to turn an LED on for say 1 second we would need to count 8192 of these timing pulses. This is a lot of pulses! Fortunately within the microcontroller there is a register called an OPTION Register, that allows us to slow down these pulses by a factor of 2, 4, 8, 16, 32, 64, 128 or 256. The OPTION Register is discussed in the Instruction Set, Files and Register section in Chapter 19. Setting the prescaler, as it is called to divide by 256 in the OPTION register means that our timing pulses are now 8192/256 = 32Hz, i.e. 32 pulses a second. So to turn our LED on for 1 second we need only to count 32 pulses in TIMER0, or 16 for 0.5 seconds, or 160 for 5 seconds etc.

Programming the microcontroller

In order to program the microcontroller we need to:

Let's consider the first task, writing the program. This can be done on any text editor, such as notepad. I prefer to use an editor supplied by the microcontroller manufacturers, ‘Microchip’. This software is called MPLAB and is available free on www.microchip.com.

As you have seen above we need to configure the I/O and set the Prescaler for the timing. If we do not set them the default conditions are that all PORT bits are inputs. A micro with no outputs is not much use! The default for the Prescaler is that the clock rate is divided by 2.

The program also needs to know what device it is intended for and also what the start address in the memory is.

If this is starting to sound confusing – do not worry, I have written a header program, which sets the all the above conditions for you to use. These conditions can be changed later when you understand more about what you are doing.

The header for the 16F84 sets the 5 bits of PORTA as inputs and the 8 bits of PORTB as outputs. It also sets the prescaler to divide by 256. We will use the 32,768Hz crystal so our timing is 32 pulses per sec. The program instructions will run at ¼ of the 32,768Hz clock, i.e. 8192 instructions per second. The header also includes two timing subroutines for you to use they are DELAY1 – a 1 second delay and DELAYP5 – a half-second delay. A subroutine is a section of code that can be called, when needed, to save writing it again. For the moment do not worry about how the header or the delay subroutines work. We will work through them, in Chapter 6, once we have programmed a couple of applications.

Just one more point, the different ways of entering data.

Entering data

Consider the decimal number 37, this has a Hex value of 25 or a Binary value of 0010 0101. The assembler will accept this as .37 in decimal (note the . is not a decimal point) or as 25H in hex or B’00100101’ in binary.

181 decimal would be entered as .181 in decimal, 0B5H in hex or B’10110101’ in binary. NB. If a hex number starts with a letter it must be prefixed with a 0, i.e. 0B5H not B5H.

NB. The default radix for the assembler MPASM is hex.

Appendix C. illustrates how to change between Decimal, Binary and Hexadecimal numbers.

The PIC Microcontrollers are 8 bit micros. This means that the memory locations, i.e. user files and registers contain 8 bits. So the smallest 8 bit number is of course 0000 0000 which is equal to a decimal number 0 (of course). The largest 8 bit number is 1111 1111 which is equal to a decimal number of 255. To use numbers bigger than 255 we have to combine memory locations. Two memory locations combine to give 16 bits with numbers up to 65,536. Three memory locations combine to give 24 bits allowing numbers up to 16,777,215 and so on. These large numbers are introduced in Chapter 8, Numbers Larger than 255.

The Header for the 16F84, HEADER84.ASM

The listing below shows the header for the 16F84 microcontroller. I suggest you start all of your programs, for this chip, with this header, or a modified version of it. A full explanation of this header file is given in Chapter 6.

; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1means input).

; and PORTB as an OUTPUT (NB 0 means output).

; The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.

;1second and 0.5 second delays are included in the subroutine section.

;*********************************************************

; EQUATES SECTION

;*********************************************************

;******************************************************************

; Configuration Bits

;*********************************************************

;SUBROUTINE SECTION.

;*********************************************************

;CONFIGURATION SECTION

;*********************************************************

;Program starts now.

NB. In the program any text on a line following the semicolon (;) is ignored by the assembler software. Program comments can then be placed there.

The section is saved as HEADER84.ASM you can use it to start all of your 16F84 programs. HEADER84 is the name of our program and ASM is its extension.

Program example

The best way to begin to understand how to use a microcontroller is to start with a simple example and then build on this.

Let us consider a program to flash an LED ON and OFF at 0.5 second intervals. The LED is connected to PortB bit 0 as shown in Figure 2.1.

Figure 2.1 Circuit diagram of the microcontroller flasher

Notice from Figure 2.1 how few components the microcontroller needs – 2 × 68pF capacitors, a 32.768kHz crystal for the oscillator and a 0.1μF capacitor for decoupling the power supply. Other oscillator and crystal configurations are possible – see Microchip's data sheets for other combinations. I have chosen the 32kHz crystal because it enables times of seconds to be produced easily.

The program for this circuit can be written on any text editor, such as Notepad or on Microchip's editor MPLAB.

Open HEADER84.ASM or start a new file and type the program in, saving as HEADER84.ASM If using Notepad saveas type “All Files” to avoid Notepad adding the extension .TXT

Once you have HEADER84.ASM saved on disk and loaded onto the screen alter it by including your program as shown below:-

; HEADER84.ASM for 16F84. This sets PORTA as an INPUT (NB 1means input).

; and PORTB as an OUTPUT (NB 0 means output).

;The OPTION Register is set to /256 to give timing pulses of 1/32 of a second.

;1second and 0.5 second delays are included in the subroutine section.

;*******************************************************

; EQUATES SECTION

;*********************************************************

;******************************************************************

; Configuration Bits

;*****************************************************

;SUBROUTINE SECTION.

;*********************************************************

;CONFIGURATION SECTION.

;*********************************************************

;Program starts now.

Saving and assembling the code

The program is then saved as FLASHER.ASM. The next task is to assemble this text into the HEX code that the microcontroller understands.

Open MPLAB the screen shown below in Figure 2.2 will open up.

Figure 2.2 MPLAB initial screen

Open the file FLASHER.ASM using the FILE menu as shown in Figure 2.3.

Figure 2.3 Opening FLASHER.ASM

From the CONFIGURE Menu, Select Device then choose the micro 16F84 in this example, as indicated in Figure 2.4.

Figure 2.4 CONFIGURE –; select device

Next choose CONFIGURE – Configuration Bits as shown in Figure 2.5 and set as indicated.

Figure 2.5 Configuration bits setting

Our configuration bits setting, select the LP Oscillator, turn the Watchdog Timer Off, turn the Power Up Timer on and turn Code Protect off.

Notice the value of this configuration is 3FF0 in hex. This configuration setting can be written into the header program so there is no need to here. The code is __CONFIG H’3FF0’

The choice of configuration bit settings for the 16F84 are:

Then we have to convert our text, FLASHER.ASM into a machine code file FLASHER.HEX to do this choose PROJECT – Quickbuild Flasher.ASM as shown in Figure 2.6.

Figure 2.6 Compiling FLASHER.ASM to FLASHER.HEX

If the program has compiled without any errors then MPLAB will return with a message Build Succeeded as indicated in Figure 2.7. There may be some warnings and messages but do not worry about them, the compiler has seen something it wasn't expecting.

Figure 2.7 Build Succeeded

Incidently, I always have line numbers on my code to find my way around, especially in larger programs. Line numbers can be turned on and off with the path: EDIT – PROPERTIES.

Suppose that you have a syntax error in your code. The message Build Failed will appear as shown in Figure 2.8. You then have to correct the errors. MPLAB has indicated the error in the message box. If you ‘double click’ on the error message then MPLAB will indicate, with an arrow, where the error is in your code. Correct the errors and compile (Quickbuild) again to produce an error free build.

Figure 2.8 Build failed

The error I have written into my code occurs in line 61, with the message, ‘symbol not previously defined (PORT)’. I should have written PORTB the compiler does not understand ‘PORT’.

After successfully building the program, the HEX code is ready to be programmed into the Microcontroller.

You can view your compilation using VIEW – PROGRAM MEMORY as shown in Figure 2.9.

Figure 2.9 Program memory

The FLASHER.HEX file is now ready to be programmed into the chip.

PICSTART PLUS programmer

If you do not have a programmer I would recommend Arizona Microchip's own PICSTART PLUS. When Arizona bring out a new microcontroller as they do regularly, the driver software is updated and can be downloaded free off the internet from MICROCHIP.COM.

Once installed on your PC it is opened from MPLAB i.e.

Switch on the PICSTART Plus Programmer.

Select, Programmer – Select Programmer – PICSTART Plus, shown in Figure 2.10.

Figure 2.10 Selecting the PICSTART plus programmer

Select Enable Programmer from the Programmer box, Figure 2.10.

The final stage is to program your code onto the chip. To do this click the programming icon shown in Figure 2.11 or via the menu on Programmer – Program.

Figure 2.11 Programming icon

After a short while the message success will appear on the screen.

You will be greeted with the success statement for a few seconds only, if you miss it check the program statistics for Pass 1 Fail 0 Total 1, which will be continually updated.

The code has been successfully blown into your chip and is ready for use.

If this process fails – check the chip is inserted correctly in the socket, if it is then try another chip.

So we are now able to use the microcontroller to switch an LED on and off – Fantastic!

But use your imagination. There are 35 instructions in your micro vocabulary. The PIC Microcontroller range at the moment includes devices with 64k bytes of EPROM-program memory, 3938 bytes of RAM-data memory, 1024 bytes of EEPROM, 72 Input and Output pins, 11 interrupts, 15 channel A/D converter, 20MHz. clock, real time clock, 4 counter/timers, 55 word instruction set. See Appendix A for a detailed list. If the 64k of EPROM or 3938 bytes of RAM is not enough your system can be expanded using extra EPROM and RAM. In the end the only real limits will be your imagination.

Programming flowchart

Problem: flashing two LEDs

There has been a lot to do and think about to get this first program into the microcontroller and make it work in a circuit. But just so that you are sure what you are doing – Write a program that will flash two LEDs on and off alternately. Put LED0 on B0 and LED1 on B1. NB you can use the file FLASHER.ASM it only needs two extra lines adding! Then save it as FLASHER2.ASM

The circuit layout is shown in Figure 2.12.

Figure 2.12 Circuit to flash 2 LEDs

Try not to look at the solution below before you have attempted it.

Solution to the problem, flashing two LEDs

The header is the same as in FLASHER.ASM. just include in the section, program starts now, the following lines:

;Program starts now.

Did you manage to do this? If not have a look at my solution and see what the lines are doing. Now try flashing 4 LEDs on and off, with 2 on and two off alternately. You might like to have them on for 1 second and off for half a second. Can you see how to use the 1-second delay in place of the half-second delay.

The different combinations of switching any 8 LEDs on PORTB should be relatively easy once you have mastered these steps.

Perhaps the most difficult step in understanding any new technology is getting started. The next chapter will introduce a few more projects similar to Flasher.ASM to help you progress.