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A tutorial on the 12F675 PIC microcontroller which shows you how to program and use it with a series of projects starting out with a simple LED flasher and progressing on to more advanced projects.
Although the 12F675 is an older device it is still a useful one and has many peripehrals built into it including the standard 10bit ADC. In fact it has two timers and analogue comparator and the ADC which can read analogue values from 4 pins. With its 1k of programming memory you can make this device do many different tasks.
PIC 12F675 Tutorial Index |
Features |
Programming |
ICSP Connection |
Power Supply |
Oscillator Calibration |
Tip for storing the calibration value |
Oscillator Modes |
Tutorial 1 : Flash LED (Simple port output) |
Tutorial 2 : Key reading and debounce (Simple port input) |
Tutorial 3 : PIC Serial Transmit (Soft Serial Transmit) |
Tutorial 4 : LM35 Temperature sensor to serial port(ADC) |
Tutorial 5 : LM35 EEPROM Temperature data logger(EEPROM) |
Tutorial 6 : Servo Motor driver using Timer 0 interrupt(Timer 0) |
Tutorial 7 : Servo controller T0 & T1 interrupts (Soft Serial Rx, T1) |
The 12F675 microcontroller is packaged in an 8 pin chip and even though it
is tiny it is packed with peripherals. It even has a 10bit ADC built in (this
is the same ADC that you can find on the 16F877A and 16F88 used elsewhere on
this site). So learning about this peripheral is also useful for these other
parts.
The 12F675 has 1024 words of program memory, 64 Bytes of RAM and 128 Bytes of
EEPROM, an internal oscillator, timers an ADC and a comparator.
Note:The 12F629 is identical except that it
does not have the ADC.
[Source microchip datasheet]
The following bubble diagram shows the major peripherals and features of the 12F675 in a visual format:
>br> Note: you can compare this chip
(using bubble diagrams) to some others used on this site by clicking here.
You can program the microcontroller using an ICSP programmer (you can use it
for any PIC chip). ICSP connections are shown in the diagram below.
To use it you will need software running on the PC : ICPROG. This lets you flash the hex file
generated by the compiler into the 12F675
You can find a programmer circuit here and information on using ICPROG here.
Other views:
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If you don't have a bench power supply then you should use the following standard circuit.
All you will need is a wall power supply block with dc output (greater than 8V
and no more than 35V) or a 9V battery to plug into CN1.
Note: It is best to use the 5V power supply
circuit as it not only correctly regulates the dc voltage but it protects your
PIC chip. The input voltage can go up to 35V without damaging the 7805.
You would not want to use that high voltage for very long if using
reasonable current as the 7805 would have to get rid of the excess power as
heat. Say you used 100mA dropping 35V to 5V gives P=VxI = 30 *0.1 = 3W - a
huge power output - the 7805 would get very hot and go into thermal
shutdown!
Before Programming it with your hex file make a note of the oscillator
calibration value which is factory set by Microchip.
Note:The calibration value is located at the
last memory address 0x3FF
This value calibrates the 4MHz oscillator to 1%. If overwritten you have to
re-calculate it yourself. Click here
for m ore detailed information (in a further tutorial) and then come back
here.
If you use ICPROG then it warns you that you are about to overwrite the
oscillator calibration value and asks if you use the value from the hex file -
you should answer No to keep the original value.
Note: Each oscillator calibration value will
be different so you have to note down each value for each chip and not muddle
them up! If you loose it you can recalculate it but you will need a frequency counter.
This is a tip I have seen on the web for storing your calibration value on
the device itself - it's so good I thought I would include it here.
All you do is think of the pins of the 8 pin device as a binary number and mark
those pins with the value you read out using the programmer (in read mode)
All you need is the last hex number as the 1st is always 34.
So lets say you read your device and get 348C. Just use the 8C part.
As with the
16F88 the 12F675 microcontroller has eight oscillator modes but unlike the
16F88 the internal oscillator is fixed at 4Mhz.
You can use an external oscillator either a resistor capacitor pair, an
external clock signal or a crystal (or resonator). You can even operate the
crystal to 20Mhz if you need extra performance.
Note: Only use the external modes if
absolutely necessary as you loose the use of pins (loosing 2 out of 6 I/O pins
is a lot to loose).
The first
program is a flashing LED - it always is! The reason is that there is the
least hardware to go wrong so it gives a good test of your system setup.
This project also uses the 12F675's internal oscillator and you don't need a
crystal so there is even less to go wrong!
Use the solderless breadboard to
construct the following circuit:
Note: Double check your connections on the
breadboard.
Note: the plus sign on the 10u electrolytic
capacitor which must connect to the positive input voltage and have a voltage
rating stamped on it of greater than 35V (or greater than your maximum dc power
block output). The LED must be connected with the flat side to ground.
The following diagram shows the above Plugblock circuit in schematic form. It is exactly the same circuit but lets you view the circuit in an easier way and shows the layout of the circuit from the point of view of the circuit block functions rather than how you have to place the components (using the Plugblock).
Note: The LED current limiter resistor (1k) is not the ideal one it just lets
you see the led (you don't need the maximum current to see the light from the
LED) - to use the LED at higher output replace it with 220R. This gives lots
more current so it is brighter I=(5-2)/220=13mA (most leds let you use 20mA but
you would have to check the forward diode drop, here assumed as 2V, to get the
exact resistor. It's only an LED I always assume 2V as the slight variations
for different colored LEDs won't make much difference.
Using ICPROG
(you can
find a description of how to use it here)
set the
device to 12F675 and hit the read button. Remember to note down the contents
of address 0x3FF.
The next thing to do is
to flash the LED to prove that the system you have is working as reading back
data is not very interesting.
You can use the hex
file directly to program the 12F675 then it will flash the led on and off or
you can re-compile the files using the free compiler from Mikroelectronika. You
can find a very brief compiler tutorial here.
Some of the C source code is :
////////////////////////////////////////////////////////////////////// void init_ports(void) { TRISIO = 0; // set as output } ////////////////////////////////////////////////////////////////////// // Start here void main() { init_ports(); while(1) { // infinite loop GPIO = (1<<4); delay_ms(200); GPIO = 0; delay_ms(200); } } |
First of all the
init_ports() routine sets up the direction of pins in the GPIO port - in common
with all the other PIC Micros you can change the port direction at any time
using a TRIS keyword (which is just another register location). Setting a
bit in the TRISIO register to zero sets the pin direction to an output. Here
all bits are zero so all GPIO bits are set as outputs.
As you can see main() is a very simple easily readable program the only
slightly non obvious part is the (1<<4) statement.
This just takes the value 1 and bit shifts it left four times so the number 4
is the same as bit position 4. Bits in a byte are labeled 7 to 0 from left
to right and (1<<0)=1, (1<<1)=2, (1<<2)=4, (1<<3)=8
etc. so this gives an easy way of setting an individual bit in a byte that is
also easy to read. If you wanted to set bit 5 you could write GPIO = 32; (or
0x20 in hex) but GPIO = (1<<5) is much easier to read.
Note: The C programming course has more on
PORT control techniques.
Try changing the
delay time (in both delay_ms statements) to a smaller or larger value and
re-compile and re-flash the chip to see the effect.
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