EXERCISE 4: 16F877A Architecture - Lab-Volt 4: 16F877A Architecture ... However Flowcode takes care of the details of PICmicro architecture for you so we will not cover this subject

Microcontroller System Development About PICmicro Chips

Student Manual

FACET by Lab-Volt 43

EXERCISE 4: 16F877A Architecture

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to use the 16F877A PICmicro chip in

various experiments and demonstrations.

EXERCISE DISCUSSION

INTRODUCTION

As you will be using the 16F877A in these tutorials it is important that you understand a little

more about what it does and how to use it. In this section you will see which pins on the

16F877A are available and which connectors on the Multiprogrammer they use. Note that you

should refer to Exercise 2: Inputs and Outputs, Procedure to understand how the connections are

made to the D-type connectors on the Multiprogrammer.

At this point in a traditional programming course you would be introduced in some detail to the

various internal circuit blocks of the PICmicro device. You would need this information to write

code for the PICmicro in C or assembly code. However Flowcode takes care of the details of

PICmicro architecture for you so we will not cover this subject here.

However you will need to have an understanding of the input and output connections of the

PICmicro and the amount of memory available to you. These topics are covered in this section.

PORT A

• Only 6 pins of Port A can be used on a 16F877A device

• RA0-RA5 can be used as digital inputs

About PICmicro Chips Microcontroller System Development

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44 FACET by Lab-Volt

• RA0-RA5 can be used as digital outputs

• AN0-AN4 can be used as analog inputs (=5 analog inputs on port A)

• The pins of port A are mapped on the 16F877A microcontroller as on this drawing

• The pins of port A are connected to the 9-pin D-type connector of port A on the

Multiprogrammer block.

This port is connected to the rest of the PICmicro internals by an 8-bit bus system.


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PORT B

• All 8 pins of Port B can be used on a 16F877A device

• RB0-RB7 can be used as digital inputs

• RB0-RB7 can be used as digital outputs

• An external interrupt pin is possible on RB0

• An external port interrupt is possible on RB4-RB7

• The pins of port B are mapped on the 16F877A microcontroller as on this drawing

• The pins of port B are connected to the 9-pin D-type connector of port B on the




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PORT C

• Full 8 pins of Port C can be used on a 16F877A device

• RC0-RC7 can be used as digital inputs

• RC0-RC7 can be used as digital outputs


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• PWM (Pulse Width Modulation) is possible on RC1 AND RC2

• Port C also contains a USART (Universal Synchronous - Asynchronous Receiver

Transmitter) for serial communication

• The pins of port C are mapped on the 16F877A microcontroller as on this drawing

• The pins of port C are connected to the 9-pin D-type connector of port C on the




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PORT D

• Full 8 pins of port D can be used on a 16F877A device

• RD0-RD7 can be used as digital inputs

• RD0-RD7 can be used as digital outputs

• The pins of port D are mapped on the 16F877A microcontroller as on this drawing

• The pins of port D are connected to the 9-pin D-type connector of port D on the

Multiprogrammer block .



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PORT E

• Only 3 pins of Port E can be used on a 16F877 device

• RE0-RE2 can be used as digital inputs

• RE0-RE2 can be used as digital outputs

• AN5-AN7 can be used as analog inputs (= 3 analog inputs on port E)


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• The pins of port E are mapped on the 16F877A microcontroller as on this drawing

• The pins of port E are connected to the 9-pin D-type connector of port E on the



Flash or EPROM

• EPROM (sometimes referred to as 'Flash') memory is the memory where the program you

write is stored.

• The program you write is 'compiled' by your computer to binary code and then downloaded

into the Flash memory of the PICmicro

• Flash memory is also used in your memory stick or MP3 player. You can read from, and

write to it and it remembers everything, even after a power cut.

• The program lines Flowcode generates tell the PICmicro what to do and when to do it.

• The Flash memory of the 16F877A can store up to 8000 program commands.


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RAM

• RAM is the memory where the 'variables' (values in your program that alter as your program

runs) you declare are stored.

• Data from inputs, outputs, analog inputs, calculations etc. is typically stored in variables

inside Flowcode.

• This memory is of the RAM-type. It's erased every time the power gets cut or a reset occurs.

• The RAM of the 16F877A can store up to 368 bytes of data.

EEPROM

• EEPROM is the memory where the variables can be permanently stored.

• This memory is of the PROM-type. It is preserved every time the power gets cut or a reset

occurs.

• The EEPROM of the 16F877A can store up to 256 bytes of data.

Memory in the 16F877A

ALU

• The ALU Arithmetic Logic Unit is the heart of the PICmicro.

• Everything passes through this unit.

• The program in the Flash memory tells the ALU what to do.

• The ALU can send data to, and fetch data from all the separate blocks and Ports in the

PICmicro by the 8-bit wide data-bus.

• The ALU needs 4 external oscillator clock pulses to execute one whole instruction.

• How an ALU works is actually very complicated. Fortunately Flowcode programmers do not

need to know more than this.


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TIMER 1

• This timer interrupt is used to provide the PICmicro with exact timing information.

• It is clocked by the system clock or by an external clock on RC0.

• This system clock runs exactly 4 times slower than the external oscillator clock.

• Either the external clock or the system clock can be divided by 1, 2, 4 or 8 by configuring the

Prescaler of TMR1 in Flowcode.

• This divided clock triggers the TMR1 to increment the TMR1 register.

• This TMR1 register is an 8-bit register and will have an overflow when it reaches 256.

• On the exact moment when this overflow occurs, TMR1 generates an interrupt and the

TMR1 register is set back to 0.

• This TMR1 Interrupt will stop the main program immediately and start up the TMR1 Macro.

• After the TMR1 Macro is finished, the main program continues on from where it had left off.

Example:

External clock oscillator = XTAL : 19,660,800Hz

System Clock = /4 : 4,915,200 Hz

Set prescaler to 8 = /8 : 614,400 Hz

Overflow when TMR1 = 256 = /256 : 2400 Hz

Conclusion: In this situation, TMR1 will interrupt the main program and execute the TMR1

Macro 2400 times per second.

TIMER 0

• This timer interrupt is used to provide the PICmicro with exact timing information.

• It is clocked by the system clock or by an external clock on RA4.

• This system clock runs exactly 4 times slower than the external oscillator clock.

• Either the external clock or the system clock can be divided by 1, 2, 4 or 8, 16, 32, 64, 128,

or by 256 by configuring the Prescaler of TMR0 in Flowcode.

• This divided clock triggers TMR0 to increment the TMR0 register.

• This TMR0 register is an 8-bit register and will have an overflow when it reaches 256.

• On the exact moment when this overflow occurs, TMR0 generates an interrupt and the

TMR0 register is set back to 0.

• This TMR0 Interrupt will stop the main program immediately and start up the TMR0 Macro.

• After the TMR0 Macro is finished, the main program continues on from where it had left off.

Example:

External clock oscillator = XTAL : 19.660.800Hz

System Clock = /4 : 4.915.200 Hz

Set prescaler to 256 = /256 : 19200 Hz

Overflow when TMR0 = 256 = /256 : 75 Hz

Conclusion: In this situation, TMR0 will interrupt the main program and execute the TMR0

macro 75 times per second.


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RBO External Interrupt

• A logic level change on RB0 can be configured to generate an interrupt.

• It can be configured in Flowcode to react to a rising or to a falling edge on RB0.

• When it is set to react to a rising edge and a rising edge occurs at RB0 then:

This will immediately stop the main program

The RB0 related macro is executed

After this RBO macro is executed, the main program continues on from where it had left

off.

This will happen every time a rising edge is detected at pin RB0.

PORTB External Interrupt

• A logic level change on either RB4 or RB5 or RB6 or RB7 can be configured to generate one

and the same single interrupt.

• It can not be configured to react to a rising or to a falling edge. It is triggered by both rising

and falling edges.

• When it is configured in Flowcode and a level change occurs on either of these 4 input pins

of Port B:

This will immediately stop the main program

The PORTB related macro is executed

After this PORTB macro is executed, the main program continues on from where it had

left off.

This will happen every time a level change is detected on one of the 4 MSB's of PORTB.

A/D Conversion

• This 16F877A PICmicro Microcontroller has 8 pins that have an extra A/D function.

• This PICmicro controller has only one single 10-bit A/D converter.

• This implies that these 8 analog inputs can't all be read at the same time.

• A built in analog switch is the answer to this problem.

• In Flowcode you can select which of the 8 analog inputs you want to sample.

• After this 'sample' instruction, the analog switch is set to the correct input and this analog

input is converted to a 10-bit binary value.

• In Flowcode, you can select to only use the 8 MSB's of this 10-bit value by using the

'ReadAsByte' instruction, or you can select to use the full 10 bits by selecting the 'ReadAsInt'

instruction. The 10 bits will fill up the 10 LSB's of the selected 16-bit integer variable.

• After this, you can select another analog input that needs to be read.

Busses:

• A PICmicro is a typical Harvard-type Microcontroller.

• This means that there is a separate bus for Instructions and one for Data.

• The data bus is 8-bit wide and connects every block and port together.

• The instruction bus is 14 bit wide and transports 14 bit long instructions from the program

memory to the ALU.


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EXERCISE QUESTIONS

The various ports are connected to the rest of the PICmicro internals by a(n) _______ system.

a. 32-bit bus system

b. 16-bit bus system

c. 8-bit bus system

A Harvard-type microcontroller uses

a. the same bus for both instructions and data.

b. one bus for instructions and a separate bus for data.

c. an ACLU data-bus.


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EXERCISE PROCEDURE

Sensor Block

• The male D-type connector fits into the female connector of one of the ports of an upstream

block.

• This block is developed for learning how to 'read' analog inputs from analog sensors and

inputs from specific digital sensors.

• The PICmicro can 'read' analog voltages between VDD and VSS.

• The Analog input is always converted by the PICmicro to a 10-bit Binary Value (0 -1023).

• There are 2 analog sensors on the block itself to experiment with:

An LDR (Light dependent resistor) on PIN 1

A Variable Resistor on PIN 2

• External Analog Sensors can be connected to analog and their output is routed to the

PICmicro on PIN 4.

• External Digital Sensors can be connected to digital, and their output is sent to the PICmicro

on PIN 3. Some digital sensors are active and require a stimulus from the PICmicro. For

these a signal from the PICmicro to the Digital Sensor is available on PIN 5.


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LED Block

• The male D-type connector fits into the Female connector of one of the ports of an upstream

block.

• LED D0 and its series resistor R0 will be connected to pin 1 of the Male D-type connector.

• The series resistor is here to control the current that will flow from PICmicro to the LED.

A LED usually has an Imax of 20mA

The PICmicro has an Imax of 25mA (sink or source)

The LED has a Forward Voltage drop of 1.7V

In this case the 560 Ohm resistor will limit the current at:

(5V-1.7V)/560 Ohm= 5.9mA

This is well under the current limits of both PICmicro and LED


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LCD Block


block.

• An LCD actually has its own microcontroller on it that takes care of all of the LCD functions.

• All we need to do is send data or instructions to the LCD.

• Data is sent over the 4 data pins.

• Instructions like 'shift left' are also sent over the same 4 data pins.

• The RS pin 'tells' the LCD whether you are sending Data or Instructions.

• The enable pin is active low and allows you to select several LCD's that are on the same data

and RS lines.

• Flowcode 3 takes care of all of the driving issues for you.

• BE AWARE of the fact that the timing of the data that is sent to the LCD is quite critical. It

is best to use an LCD in combination with a crystal in the PICmicro.


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Multiprogrammer Block

• Circuit Description:

5 Ports A-B-C-D-E

Full 8-bit functionality on PORT B-C-D

6-bit functionality on PORT A

3-bit functionality on PORT E

Pins 1-8 are one-on-one connections with the pins of the corresponding port (e.g.

Pin1 is connected to PB0, Pin 2 to PB1 etc.)

Pin 9 of the D-type connector is always GND. This implies that ground will not have

to be connected externally to the downstream boards.

Power supply:

standard is from FACET Base Unit.

an external power supply can be used and should be at least 12V and be able to

deliver 300mAusing this connector.


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The screw terminals provide the downstream blocks in the pegboard area with the correct

power. In all of the cases in this course, this will be 5V. You can provide downstream

blocks with a stable 5V by connecting them to any of the 5V screw terminals. GND has

to be connected in the same way.

The Reset switch is connected to the PICmicro and if pressed, it will reset the PICmicro.

The program in the PICmicro will start all over again every time you press the reset

switch.


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Clocking

Every 4 clock-pulses, the PICmicro executes one single instruction. This implies that

the external clock source that provides the PICmicro with these pulses actually

determines the speed of the execution of the program.

A single instruction is NOT the same as one Flowcode symbol. A Flowcode program

is compiled into C and then into Assembly. One line of Assembly code is called 'a

single instruction.'

Some Flowcode symbols are compiled into only a few assembly lines while others

could easily be compiled into 50 or more assembly lines. You will have to look at the

compiled assembly code to know the exact timing.

XTAL-MODE

For very fast operation: up to 20MHz

In this course we will use a 19,660,800Hz Crystal.

19,660,800 / 1024 = 19200

(19,660,800 / 1024) / 256 = 75

Settings on the Multiprogrammer Block:

Put Toggle switch in XTAL position.

Settings in Flowcode 3:

CHIP...CONFIGURE Select 'XTAL'

RC-MODE

For lower Frequencies - RC is not as stable as Crystal mode

RC SLOW : 10Hz to 25Hz (Can be Changed by RV1)

RC FAST : 600kHz to 1200kHz (Can be Changed by RV1)

Settings on the Multiprogrammer Block:

Put Toggle switch in RC position.

Another Toggle switch is used to select SLOW or FAST.

Settings in Flowcode 3:

CHIP...CONFIGURE Select 'RC'


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SWITCH Block


block.

• Switch pressed:

Small current flow through 390R-SW and 4K7.

Voltage over 4K7 is +/- 4.7V is a clear '1' for the input pin of the PICmicro

• Switch not pressed:

No current flow

No voltage over 4K7 resistor

This is a clear '0' for the input of the PICmicro


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• BEWARE:


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7-Segment Display

1. The male D-type connectors fit into the Female connectors of 2 of the ports of an upstream

block.

2. A 7-segment display is actually no more than:

7 LEDs with long shaped segments, placed in an '8' shape.

1 LED to indicate a decimal point.

3. 7-segment displays can be:

Common Cathode: where the Cathode of all of the LEDs is common and the LEDs are lit

by applying a positive voltage to the anode of the specific segment.

Common Anode: where the Anode of all of the LEDs is common and the LEDs are lit by

applying a negative voltage to the cathode of the specific segment.

The Displays used on this block are of the Common Anode type.


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4. The tricky bit is that we do not have enough pins to drive all these LEDs (8 x 4 = 32 separate

LEDs) by our controller. We will solve this by:

Using the speed of the PICmicro.

By driving pin 1-2-3 or 4 of the anodes connector we determine which of the four 7-

segment displays is active to receive data.

Data is sent to the active display by the 8 data pins on the cathodes connector.

LEDs will emit light for a short time after they have been turned off.

So the trick is that you alternately activate each of the four 7 segment displays in turn, by

making the appropriate anode high, and then send the data to each display by making the

appropriate cathodes low.

5. If you only need a single display, then this can be activated permanently by putting a two-

post connector in the single location above the displays. This way only the cathode connector

is used to drive the left display and no multiplexing is necessary.

Keypad Block


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6. This Keypad input block provides you with 12 input buttons on an 8-bit port.

• This is done by alternately making the column lines high and then scanning the 4 row input

lines.

• If one of the row inputs reads as high, you know which button was pushed.

• This is called a 'multiplexed input'.

• Flowcode provides the programmer with a hardware macro that takes care of this all.

• The keypad is best connected to port B because this port has a Port interrupt on the lines

RB4-RB7, exactly the same lines as the row connections.

Prototype Board Block Diagram

Which pins are connected to each other?

7. The male D-type connectors fit into the Female connectors of an upstream board or block.

8. This board lets you build and test your own PICmicro circuits over and over again.

9. BE AWARE OF:

the current limit per PIN of the PICmicro is 25mA. Stay well below this limit!


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Connecting input devices on pins that are configured as outputs (see switch block)

The total current limit of the PICmicro (all of the pins) is 200mA.

10. If you want a more permanent circuit built, or bad contacts in the prototype-board worry you,

use the small Patch board and solder the circuit you want

11. To make connections easier you can use a special set of jumper leads which fit neatly into the

turned pin connectors and into the proto board holes.

Example.

Patch Board Block Diagram


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12. The male D-type connectors fit into the Female connectors of an upstream board or block.

13. This board lets you build and test your own PICmicro circuits.

14. BE AWARE OF:

the current limit per PIN of the PICmicro is 25mA. Stay well below this limit!

Connecting input devices on pins that are configured as outputs (see switch block)

The total current limit of the PICmicro (all of the pins) is 200mA.

15. If you want to test a less permanent circuit, you may prefer the Prototype Board.

An example of how the Patch board is used:

These two photographs show how you can use the Patch board. On the bottom side of the patch

board you can use 'wiring pencil wire' to speed up the prototyping process. This thin wire has a

layer of polyurethane insulating coating that burns away with a soldering iron to reveal the

copper wire beneath. To use it you just apply the soldering iron for a second and then solder it to

the appropriate component pin.


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REVIEW QUESTIONS

1. The Sensor Block has two ___________ sensors to experiment with - a Light Dependent

resistor (LDR) and a Variable Resistor.

a. digital

b. analog

c. digital to analog

2. Every time you press the reset switch on the PICmicro it will start the program at the

previous step.

a. True

b. False

EXERCISE CONCLUSION

• Each pin on the PICmicro chip has specific characteristics, availability for digital or analog

data and connection to the entire system.

• The PICmicro chip uses EPROM, RAM, and EEProm memory.

• The ALU (Arithmetic Logic Unit) is at the heart of the PICmicro - everything passes through

it.

• The timer interrupt provides the PICmicro with exact timing and PORT B enables external

interrupt of activities.

• This model of the PICmicro chip also has an A/D Converter.

REVIEW QUESTIONS

1. The pins at each port on the PICmicro chip have the same capabilities.

a. True

b. False

2. The program you write for the PICmicro is stored in the EPROM or Flash memory of the

chip.

a. True

b. False

3. The ALU can:

a. send data.

b. fetch data.

c. send or fetch data.

d. compile the program.

4. The PICmicro controller has a 10-bit A/D converter that permits:

a. reading of one input at a time.

b. reading of multiple inputs at one time.

c. reading of 16-bit information only.

d. The A/D converter is not capable of reading anything.


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5. The PICmicro is a(n):

a. Columbia-type controller.

b. Intel-type controller.

c. HP-type controller.

d. Harvard-type controller.

CMS AVAILABLE

FAULTS AVAILABLE

Documents

EXERCISE 4: 16F877A Architecture - Lab-Volt 4: 16F877A Architecture ... However Flowcode takes care of the details of PICmicro architecture for you so we will not cover this subject