PICadilly 12F675 “A Dilly of a Wee PIC” by Ian Burn
Contents:
1. Introduction 2. What is the 12F675 ? 3. Front-end setup 4. Blink! 5. Making Music 6. In Circuit Serial Programming (ICSP) 7. Analog to Digital Converters (ADCs) 8. Sleeping 9. Interrupt on Change 10. Lost Calibration Code 11. Feedback and Acknowledgements
Page 1 Page 1 Page 2 Page 3 - 4 Page 5 - 6 Page 7- 9 Page 10-14 Page 15 Page 16-20 Page 21 Page 22
1. Introduction I have been using the PIC12F675 for the past three years. During that time I have made many mistakes, sometimes more than once. These notes were prepared to assist assist me in future applications using this small but mighty microcontroller. Hopefully you will also find find them useful. Be sure to check the PIC12F629/12F675 datasheet at Microchip for detailed information. (See: www.microchip.com www.microchip.com)) All of my my source code is written for Micro Engineering Labs’ PIC Basic Pro (PBP) Compiler and the chip is programmed using MELab’s EPIC Plus Programmer.
2. What is the 12F675? It in a very versatile 8 pin microcontroller of the Microchip PIC. Some of the features include: • • • • • • • • • • • •
1024 x 14 bit words of programmable memory 64 x 8 bytes of data memory 128 x 8 bytes of EEPROM data memory A built-in RC oscillator operating at 4 MHz (no resonator or crystal required) 5 ports that can be either inputs or outputs 1 port that can be an input or the MCLR (master clear pin) 4 ports can be used as Analog to Digital Converters (ADCs) ADC reference voltage can be set or the supply voltage used Operates from 2.0 to 5.5 VDC with a low current - very low SLEEP current Each port can sink/source 25 mA up to a total of 125 mA Individual programmable weak pull-up resistors for each port Price - about $2 each
The only serious limitations are the number of ports and amount of memory. Ian Burn 1
January 31, 2005
V5.0
12F675 Pin-out Vdd = +2.0 to 5.5 VDC Vss = power ground GP0 to GP5 are the six GPIO ports with all but GPIO.3 (pin 4) being either inputs or outputs. GPIO.3 can be either an input port or MCLR port.
3. Front-End Setup As with most PICs, a number of initial setting must be placed p laced at the beginning of the program. With some PICs, such as the 16F84, one can write a simple program to blink a LED by using u sing all the default settings and no ‘front-end’ ‘front-end’ statements are required. required. Not so with the 12F675. The following lines of code are used in many of my 12F675 projects and are described as follows:
ansel=0 cmcon=7
Sets the ADC inputs as digital when not using the ADC’s. Turns the comparator OFF when using u sing ports digitally
These two statements are critical to digital operation such as the flashing LEDs. Next one would normally configure the GPIO ports as either inputs or outputs. GPIO.3 (pin 4) is the MCRL pin or can be used as an input BUT not as an output. If used as an input, MCLR must be turned OFF in the configuration line below. If it is turned ON, it must have an external pull-up resistor of 4.7k to 10k. You would also likely want a reset button connecting the MCLR pin to ground (Vss) via a 100 ohm resistor.
TRISIO =%000000 is the binary equivalent of TRISIO=0 which sets all ports as outputs The Compiler seems to sense that GPIO.3 must be an input and adjusts for this incorrect code. If all the ports except GPIO.3 are going to be outputs, a preferred code would be: TRISIO=%001000 which is the binary equivalent of TRISIO=8 If all ports except GPIO.0 are inputs, for example, you would use either TRISIO=%111110 or TRISIO=62
Ian Burn
2
January 31, 2005
V5.0
Setting the 12F675 configuration codes (sometimes called ‘fuses’) can be done at the programmer or, for most settings, by way of an @DEVICE statement in the code. For example:
@ Device PIC12F675,WDT_ON,PWRT_ON,PROTECT_OFF,MCLR_ON,BOD_OFF This defines the PIC as a 12F675 The watchdog timer (WDT) is turned ON (usually only OFF if using the @SLEEP command as the WDT keeps waking up the PIC) Power-on reset (PWRT) is turned ON Code protect (PROTECT) is turned OFF MCLR is turned on so an external pull-up resistor is needed Brown out detect (BOD) is turned OFF (this reduces sleep current) • •
• • • •
Other ‘front-end’ statements will be introduced as required.
4. Blink! The standard test for successfully programming any PIC is to blink a LED. This circuit uses three AA batteries for a 4.5 volt supply. A 0.1 mfd capacitor is connected between pins 1 and 8 as close as possible to the pins. DON’T omit this capacitor!
+
BA T1
4V5
C1 0.1
The MCLR pin is pulled high via the 10k resistor and can be pulled low to reset the PIC with the button and the 100 ohm resistor. Microchip recommend the 100 ohm resistor on the MCLR pin to Vss.
U1 1
2
R1
3
10k 4
Vdd/+
Vss/Gnd
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
8
R2
7
1k
6
D2
LED
5
PIC12F675
The LED is connected between GPIO.0 and Vss via a 1 k resistor resulting in about 3 mA of LED current. the maximum allowable current is 25 mA.
R3 SW1
Blinking LED with RESET
The LED could be reversed and connected to the Vdd (+4.5V) in which case the LED would come on when GPIO.0 went LOW.
Ian Burn
100
3
January 31, 2005
V5.0
The PIC BasicPro code to blink the LED when using the MCLR RESET is as follows: January 25, 2005 ‘Blinking a LED with a PIC 12F675 ‘The LED will blink 10 times then turn off until the PIC is reset
‘Front-end setup @ Device PIC12F675,WDT_ON,PWRT_ON,PROTECT_OFF,MCLR_ON,BOD_OFF ‘this statement is explained under item 3. above ANSEL=0 ‘Sets the ADC inputs as digital when not using the ADC’s. CMCON=7 ‘Turns the comparator OFF when using ports digitally TRISIO=8 ‘All ports are outputs except MCLR - if a port is not used, set it as an output ‘Define Variables LED VAR GPIO.0 x VAR BYTE
‘sets the GPIO.0 port as LED ‘x is a counter and will not have a value larger than 255
‘Set initial values LED=0
‘ensures that LED is off at the beginning
‘Program FOR x = 1 to 10 LED = 1 PAUSE 500 LED = 0 PAUSE 500 NEXT x END
‘Program will repeat statements between here and ‘next x’ 10 times ‘Turns on LED. Could also use HIGH LED but uses a bit more memory ‘Pause 500 milliseconds ‘Turns off LED ‘Pause 500 milliseconds ‘if x is not = 10, returns to FOR x = 1 to 10 statement ‘ends the program. The reset button will start it at the beginning.
The same result would occur if the MCLR was turned OFF and R1, R3 and SW1 were omitted. GPIO.3 then becomes an input port. I connect the GPIO.3 port to ground or to Vdd if I am not using it as in input, otherwise it will pick up noise and prevents a ‘deep’ SLEEP if using this command. The configuration line would become: @ Device PIC12F675,WDT_ON,PWRT_ON,PROTECT_OFF,MCLR_OFF,BOD_OFF
The LED will only blink the 10 times and cannot be reset except by removing the power. +
Once the power is removed, it may be necessary to discharge C1 to turn off the PIC. To ensure a reset.
BAT2
4V5
C1 0.1 U2 1
Vdd/+
Vss/Gnd
8
R2
Blinking LED without RESET
2
3
4
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
1k
D1
7
6
LED
5
PIC12F675
Ian Burn
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January 31, 2005
V5.0
5. Making Music I have had a lot of fun making music with PICs. This is a case where the 12F675 is a bit limiting because of its memory size but one or two tunes are possible. One application was to blink the ‘nose’ of a reindeer that played the tune, Rudolph the Red Nosed Reindeer. Others have included ‘ musical clocks’ and door chimes. Although the PIC cannot get the exact audio frequency for musical notes, it can come quite close. The key command used is:
SOUND pin, [tone #, tone duration, tone #, tone duration, …..] pin is the port used to put out the sound [I usually define the port such as: MPIN VAR GPIO.1] tone # is a value between 0 and 127 with 0 being silence (127 is about 10,000 Hz) tone duration is the length of the tone in 12 millisecond increments The following tables can be used to equate tone # values to approximate musical notes:
Tone # 1 6 13 20 26 31 37 42 47 51 56 60 64 67 71 74 77 80 83 85
Note Ds2 E2 F2 Fs2 G2 Gs2 A2 As2 B2 C3 Cs3 D3 Ds3 E3 F3 Fs3 G3 Gs3 A3 As3
Tone # 88 90 92 94 96 98 99 101 102 104 105 107 108 109 110 111 112 113 114 115
Note B3 C4 Middle C Cs4 D4 Ds4 E4 F4 Fs4 G4 Gs4 A4 As4 B4 C5 Cs5 D5 Ds5 E5 F5 Fs5 missing G5
For example: E2 is the musical note E in the second octave. Ds3 is the musical note D sharp in the third octave. To convert this information into a musical tune takes a bit of patience.
Ian Burn
5
January 31, 2005
V5.0
I did find one problem when using the 12F675 for music. When using the 12F675’s internal oscillator (may not be significant) and operating at lower voltages (3 to 4.5) distortion of the higher notes can occur if the coupling capacitor is 10 mfd. This may also be a function of the speaker used. By dropping the cap to 3.3 mfd the problem was eliminated 3 AA BATTERIES and it worked down to 3V. In the Sound Example circuit the 12F675 is normally in SLEEP mode and draws less than 1 microamp. SW1 operates the RESET on the MCLR pin and ‘wakes up’ the PIC, starting the music.
4V5
C2 0.1
U1 1
2
The @SLEEP command is used in this program and is explained later on in Section 8’
R1 10k
3
Vdd/+
Vss/Gnd
GP5
GP0/AN0
GP4/AN3
GP1/AN1
8
7
C1 4
GP3/MCLR
GP2/AN2
5
3.3 mfd
PIC12F675
This short ‘tune’ is “Shave and a Haircut”. I have programmed a number of full length songs but it does a fair bit of time.
SPEAKER
6
SW1
R2
8 Ohms
100
A Sound Example 'A Sound Example - January 27, 2005 ‘The tune, “Shave and a Haircut’ is played and then the 12F675 goes to sleep toconserve the batteries ‘sleep current is less than I microamp ‘MCLR reset is used to wake up the 12F675 ‘Front-end Seup @ Device PIC12F675,WDT_OFF,PWRT_OFF,PROTECT_OFF,MCLR_ON,BOD_OFF ‘sets configurations ADCON0=0 'no ADC used ANSEL=0 'no ADC input - IMPORTANT if using ports in digital mode CMCON=7 ' turns off comparator function - IMPORTANT if using ports in digital mode TRISIO= 8 ‘ Sets GPIO.3 as input, all others as outputs. Equivalent to TRISIO =%001000 ‘Define variables MPIN VAR GPIO.2 y VAR BYTE
'music pin set as GPIO.2 ‘adjustable time delay between notes
‘Program START: y=4 ‘set delay between notes = 4 x 12 mSec = 28 mSec Sound MPIN, [109,40,0,y,102,20,0,y,102,20,0,y,105,40,0,y,102,20,0,y,0,40,108,40,0,y,109,40,0,y] @ SLEEP ‘assembler ‘sleep forever’ command Pause 100 ‘give PIC time to wakeup GOTO START End
‘music
‘171 words
Ian Burn
6
January 31, 2005
V5.0
HINT: If memory space becomes critical, you can save a few words by using LED=0 and LED=1 each use 1 word of memory instead of: Low LED and High LED each use 3 words of memory
HINT: Constantly moving a PIC between a test circuit breadboard and the chip programmer can lead to pin damage. One way to minimize this damage is to mount the PIC in a machine pin socket and move the complete unit back and forth. I know, you do that already!!
6. In Circuit Serial Programming (ICSP) I have experimented with this method of programming and have found some things that work and some that don’t. The following is based on experience with the PIC12F675 and MeLabs’ EPIC Plus Programmer although I expect most points will be directly transferable to other PICs and other programmers.
Advantages of ICSP The advantage of ICSP in circuit development is that you do not have to continually transfer the PIC to the programmer every time you make a software change.
What is ICSP? ICSP, as I have been using it, is the normal high voltage programming usually done with the PIC mounted in the programmer only in this case it is left in the circuit with the other components attached. Five lines run from the programmer to the PIC and associated circuit. These are: +5VDC (Vdd) Ground (Vss) Data line Clock line Vpp/MCLR programming line Ideally one would have a jack mounted on the circuit board or breadboard that would accept the 5 lines via a plug and cable connected to the programmer. This allows the programming lines to be easily removed while the program is running.
Design requirements The Microchip data sheet indicates that the external circuitry connected to the CLOCK , DATA and MCLR ports must be done via a resistor so as not to load down the programming voltages. Similarly the capacitance on these ports must be limited for the same reason. Ian Burn
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January 31, 2005
V5.0
I have found that R1 and R2 can be as low as 470 ohms thus allowing the driving of LEDs from the CLOCK and DATA ports.
To normal circuit
PIC
Connections from Programmer
If a PIC is used that is set up for an internal MCLR resistor (MCLR OFF), R3 can be omitted unless it is used as an input port.
Vdd (+5V)
Vdd (+5V)
Vss (gnd)
Vss (gnd)
Programmer current limitations
Clock
Clock
Data
Data
Vpp
MCLR/Vpp
Normally the supply voltage to the breadboard or end product circuit board is removed during the ICSP process.
R1
R2
R3 10k
Vdd
To normal circuit
Since the +5VDC from the programmer is feeding the PIC and since this lead on Basic ICSP Connections the PIC is connected to other components, the programmer will have to be able to supply enough current to operate the overall circuit. There are ways around this if it is a problem.
Testing ICSP - Things that work and things that don’t. I used the internal RC oscillator and the internal and turned the MCLR OFF. Diodes D1 and D2 allowed me to have both the programmer and the circuit activated at the same time even though I was using 4.5 volts to operate the circuit and 5 volts from the programmer. C2
D2
+4.5 VDC
0.1
U1 1
D4 DIODE D5
D3
LED
Vss/Gnd
8
D1 LED 2
LED
Vdd/+
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
7
R2 470
D6 LED 3
4
R3 470
6
C1 3.3 mfd
5
DIODE R1 470
PIC12F675
SPEAKER
R4 470
8 Ohms
GND
Vdd
VPP
Vcc
Data
PROGRAMMER
Clock
2k
Testing ICSP with the 12F675 Testing ISCP with the 12F675
Ian Burn
8
January 31, 2005
V5.0
I was able to blink LEDs D1, D4 and D5 and have a tone at the speaker. I was even able to leave the programmer connected and send in new programming data to the 12F675 with the circuit under power. But … as soon as LED D6 was added I ran into trouble if the programmer was connected and I tried to run the circuit. Within the EPIC Programmer there is an effective resistance of 2K between the CLOCK and DATA lines such that the outputs from the PIC at these two ports were effectively connected together causing the LEDs to interfere with each other. This problem is obviously eliminated by disconnecting the programmer lines when the circuit is operating. I also could NOT get the PIC to sleep properly with the programmer connected. I next tried to use the ‘external’ MCLR resistor (MCLR=ON) and found that I could not run the circuit with the programmer Vpp/MCLR line left connected.
The best practice is to disconnect the programmer once the programming has been completed. Again, a convenient plug/jack would be helpful.
Ian Burn
9
January 31, 2005
V5.0
7. Analog to Digital Converters (ADCs) The 12F675 has four ADC inputs, AN0 - AN3 (check the pin numbers on page 2. ) The output from the ADC converter will be a number between 0 and 1023 representing the lower and upper voltage limits. The input voltage must not exceed the Vdd-Vss range. The upper voltage limit can be set at Vdd or to a Vref+ voltage less than Vdd. In the example below, the Upper limit of the input is set at Vdd and would give a value of 1023. If Vref+ was used and was set at 1.5 VDC, then 1.5 V would equal a value of 1023 and 0 V = a zero ADC value. When Vref+ is used, it is applied to pin6 (see pin out on Page 3) All ADC inputs must use a common upper limit of either Vdd or Vref+. They cannot be set individually on the 12F675.
BAT1
4V5
C1
U1
0.1
The 12F675 does not offer a Vref- option. 1
To use one or more of the four ADC inputs, a number of parameters must be set.
Vdd/+
2
R1
Since it is a 10 bit ADC use:
3
Vss/Gnd
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
8
7
6
10k 4
5
DEFINE ADC_BITS 10 PIC12F675
Use the internal clock and set:
DEFINE ADC_CLOCK3 ADC Input at AN3, GPIO.4 (Input range Vss to Vdd)
Sample timing is set by:
DEFINE ADC_SAMPLEUS 50 BAT1
For detailed explanations of these statements, please refer to the Microchip PIC12F629/675 Datasheet.
Vref+
1
2
Assuming that the Comparator function is not being used, it must be turned off:
R1
3
4V5
C1
U1
0.1
Vdd/+
Vss/Gnd
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
8
7
6
10k 4
CMCON=7
Vref+
5
PIC12F675
ADC Input at AN3, GPIO.4 (Input range Vss to Vref+) It is next necessary to set the ADCON0, ANSEL and TRISIO registers Ian Burn
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January 31, 2005
V5.0
From Microchip’s PIC12F629/675 Datasheet - Page 41
ADCON selects the output format, the reference voltage and which ADC inputs you wish to use. In the example that follows we want the following: Right justified output therefore BIT 7 = 1 Reference voltage at Vdd therefore BIT 6 = 0 Bit 5 and 4 are not used and set at 0 One ADC input, GPIO.4 is used (AN3) therefore BIT 3 = 1 BIT 2 = 1 The ADC input will start right away and always be on therefore BIT 1 = 1 BIT 0 = 1 This results in an ADCON0=10001111 for the 8 bits but this must be expressed as a decimal number which is 143, therefore
ADCON0=143
Ian Burn
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January 31, 2005
V5.0
From Microchip’s PIC12F629/675 Datasheet - Page 42
The ANSEL register is used to set the timing and to set the ADC pin. In the example that follows, timing was set at Fosc/2 (see Microchip Datasheet for details) therefore: BIT 7 = 0 Unimplemented BIT 6 = 0 BIT 5 = 0 BIT 4 = 0 We are selecting GPIO.4 or AN3 for the ADC input line, therefore: BIT 3 = 1 BIT 2 = 0 BIT 1 = 0 BIT 0 = 0 ANSEL = 00001000
ANSEL=8
Ian Burn
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January 31, 2005
V5.0
TRISIO
Setting the input/output pins
In our example it is necessary to set all pins as outputs except for GPIO.4 and the MCLR or GPIO.3. GPIO.4 will be the ADC, input pin
TRISIO=%011000
or TRISIO=24
The previous ADC circuits are fine except it would be useful to have some form of output. The following example will use a variable voltage input into AN3 (GPIO.4) with the range being between ground and the supply voltage, Vss and Vdd. This will provide ADC values from 0 to 1023. The outputs will be a LED that turns on if ADC is above 500 and sound ranging from “note values” of 1 to 120, corresponding to ADC values of 0 to 1023. The MCLR will be turned OFF, making GPIO.3 an input. To prevent noise entering this un-used port, it will be connected to Vss.
3 AA BATTERIES
4V5
C2
U1 1
Vdd/+
0.1 Vss/Gnd
8
D1 2
R1
GP5
GP0/AN0
7
R2 470
3
GP4/AN3
GP1/AN1
6
10k
LED
SPEAKER
C1 4
GP3/MCLR
GP2/AN2
5
3.3 mfd
PIC12F675
8 Ohms
ADC Example with Audio Frequency Output Proportional to Input Voltage
Ian Burn
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January 31, 2005
V5.0
'ADC Example #1 'A variable voltage is applied to AN3, GPIO.4 The range will be Vss to Vdd 'The LED will come on if the ADC value equals or exceeds 500 'Sound output, a 120 mSec beep, will have a frequency proportional to the input voltage. 'A #120 tone is used at the top end as the higher audio frequencies sometimes distort 'depending upon the speaker. A 0.001 mfd cap across the speaker may allow use of up to #127 'Front-end setup @ Device PIC12F675,WDT_ON,PWRT_ON,PROTECT_OFF,MCLR_OFF,BOD_OFF 'sets configuration ADCON0=143 'set AN3 ON ie GPIO.4 ANSEL=8 'sets Fosc/2 and GPIO.4 [AN3] as ADC input CMCON=7 'turns off comparator function TRISIO=%011000 'sets GPIO.4 as in input and all others as outputs could also use TRISIO=24 ‘DEFINE OSCCAL_1K 1 DEFINE ADC_BITS 10 DEFINE ADC_CLOCK 3 DEFINE ADC_SAMPLEUS 50
' Set OSCCAL for 1K device - calibrates the internal oscillator - not really needed here 'ADC settings 'ADC settings 'ADC settings
‘Define variables RDG VAR WORD MPIN VAR GPIO.2 LED VAR GPIO.0 TONE VAR WORD
'ADC reading from AN3 'Music pin output to speaker 'LED output 'tone value for the sound command
'Initial settings LED=0 'Program START: LED=0 ADCIN 3, RDG Pause 100 IF RDG =>500 Then LED=1 TONE = 1+(rdg/10)*119/102
Sound MPIN,[TONE,20] GoTo START End
‘reset LED to off at the beginning of each program cycle 'get voltage reading from AN3 and place ADC value in 'RDG' ‘allow time for ADC calculation 'turn on LED if ADC value is equal to or larger than 500 'converts ADC value to a 'TONE' value of 1 to 120 'necessary to divide by 10 to avoid 65535 word limit 'ADC of 0 = tone of 1, ADC of 1023 = tone of 120 'provides a tone proportional to input voltage. Duration =20x12=120 mSec
'277 words
HINT: ADC circuits frequently have a voltage divider across the battery, constantly drawing current. Depending upon how often ADC readings are needed, you can conserve battery life by supplying the ADC reference voltage from an unused port. Make the port ‘high’ just prior to taking an ADC reading then return it to ‘low’ when finished. You will still be using the Vdd as reference, not the Vref+ option. For example a thermistor used in a divider to monitor a deepfreeze temperature may require a ‘reading’ every 10 minutes - no need to be drawing current all the time.
Ian Burn
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January 31, 2005
V5.0
8. Sleeping Many of my 12F675 circuits operate on 3 AAA or AA batteries. If the circuits operate continuously there is often an opportunity to have the PIC SLEEP during a major part of its operating cycle. The typical sleep current for the 12F675 is less than 1 microamp. I am not sure how much less as my meters don’t go low enough, but the current will be close to the shelf life of the battery! The two SLEEP commands that I use are:
@SLEEP
and
SLEEP period
@SLEEP is an assembler sleep instruction which puts the PIC to sleep until it is awakened by: the watchdog timer because you forgot to turn it off!! an interrupt (eg interrupt on change) a reset of the MCLR if it is used A Pause 100 after the @SLEEP command allows time for the PIC to wakeup • • •
SLEEP period is a PBP Compiler statement where ‘period’ is the number of seconds between 1 and 65535 - a range of from 1 second to 18.2 hours. Longer periods have to be accomplished by using loops. (eg for x= 1 to 10; SLEEP 64800; next x ….= 18 hours x 10 =180 hours). This command uses the watchdog timer which must be turned ON, unlike the @SLEEP command. Many things can influence the sleep current, including: Brown out detection - turn off to minimize sleep current ie BOD_OFF in the configuration Set all unused ports as outputs. If GPIO.3 is not used as MCLR, connect to either Vss or Vdd Tie the unused ports to Vdd using the internal weak pull-up (see page 19) If using the POT command, set the ‘POT’ port ‘low’ before going to sleep, bring high on awakening Turn off the ADC reference voltage by using the statement : VRCON=0 turn off watchdog timer is using @SLEEP ie WDT_OFF in configuration input data must not be changing. • • • •
• • •
HINT: Continually changing data entering an INPUT port will prevent the 12F765 from reaching minimum SLEEP current. Depending upon the input circuitry, you might convert the port in question to an OUTPUT during the SLEEP part of the program. Be sure to change it back to an INPUT on awakening. eg TRISIO.4=0..then..TRISIO.4=1
Ian Burn
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January 31, 2005
V5.0
9. Interrupt-on-Change A useful way to wake up a sleeping PIC is by using an interrupt-on-change. For example if a battery powered 12F675 is meant to start some sequence of outputs after receiving an incoming pulse, it is likely a good idea for it to be in SLEEP mode until the pulse arrives. The Interrupt-on-Change (IOC) can be set for either a positive or negative going pulse and can be allocated to any GPIO pins by setting the IOCB Register . The GPIE variable must be set in the INCON Register and the ‘falling edge’ option is set in the OPTION Register. +
BAT1
4V5
Here’s how to do it. C1 0.1
First let’s assume we have a project where a switch closure, SW1, causes a LED, D1, to flash 10 times. A very simple objective.
U1 1
R2
GPIO.3 and 4 will be set as inputs and GPIO.3 is not used and will be connected to Vdd.
2
Vdd/+
Vss/Gnd
GP5
GP0/AN0
GP4/AN3
GP1/AN1
GP3/MCLR
GP2/AN2
8
7
10k 3
4
R2 will hold GPIO.4 high until SW1 closes. The interrupt on change at GPIO.4 will be ‘negative going’.
6
5
R1 1k
D1
PIC12F675 SW1
Interrupt on Change
The next step is to tell the IOCB Register which ports are using the IOC function.
From Microchip’s PIC12F629/675 Datasheet - Page 20
Using the information above, we will set IOCB= 00010000 or Ian Burn
16
IOCB = 16 January 31, 2005
V5.0
Next we have to set the Interrupt Control Register, INTCON
Question - will SLEEP 65535 wake up with interrupt on change?
From Microchip’s PIC12F629/675 Datasheet - Page 13
We aren’t dealing with a lot of the information here. We only need to enable the GPIE variable. Bit 7 = 0 Bit 6, 5 and 4 = 0 Bit 3 = 1 Bit 2 and 1 = 0 Bit 0 = 0
Must be 0 in this application Not used in this case Must Enable GPIE Not used in this case Initially set at 0 and again just before @SLEEP
Therefore INTCON = 00001000 or
Ian Burn
INTCON = 8
17
or you could use INTCON.3=1
January 31, 2005
V5.0
One other key item is to turn off the GPIF, Bit 0, of INTCON just prior to the @SLEEP command. This can be done by using INTCON.0=0
'Example of Interrupt on Change 'It will then blink a LED times and go back to sleep
January 26, 2005
From Microchip’s PIC12F629/675 Datasheet - Page 12
Two items of interest are in this register - Bit 6 and Bit 7 both of which need to set at 0 by using:
OPTIONS_REG.6=0 OPTIONS_REG.7=0
This set interrupt on the falling edge of the pulse. This enable the weak pull up resistors needed to stabilize the unused ports
However, all the bits could be set at 0 without a problem so we could also use:
OPTION_REG=0
Ian Burn
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January 31, 2005
V5.0
From Microchip’s PIC12F629/675 Datasheet - Page 12
GPIO ports 1, 2 and 5 are not used and can be tied to Vdd using the weak pull-up resistors therefore WPU=%00100110 or WPU = 38
After all this nonsense, here is the program! January 26, 2005 'Example of Interrupt on Change 'A change from Vdd to Vss on GPIO.4 will cause the 12F675 to wake up 'It will then blink a LED times and go back to sleep
'Front end setup @ Device PIC12F675,WDT_OFF,PWRT_ON,PROTECT_OFF,MCLR_OFF,BOD_OFF 'sets configuration TRISIO=24 'sets GPIO.3 and GPIO.4 as inputs and all others as outputs (or %00011000) CMCON=7 'turns off comparator function ADCON0=0 'no ADC - doesn't seem necessary but may be a good idea ANSEL=0 'no ADC input - IMPORTANT if using ports in digital mode VRCON=0 'turns off voltage reference to minimize sleep current INTCON=8 ‘Enables port change interrupt ie GPIE IOCB=16 'sets GPIO.4 for Interrupt on change OPTION_REG.6=0 'INT on falling edge OPTION_REG.7=0 '00000000 WPU enabled, WPU=38 '00100110 = 38 set unused ports 1,2 and 5 tied to weak pull-up resistors 'Define Variables wakeup VAR GPIO.4 LED VAR GPIO.0 x VAR BYTE
'input, interrupt on change. 'LED output
'Initial settings LED=0
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'Program START: INTCON.0=0 Pause 100 @ Sleep Pause 100 For x= 1 TO 10 LED=1 Pause 300 LED=0 Pause 300 Next x GoTo start End '90 Words
HINT: When building a circuit with a sleep component, removing the power from the circuit doesn’t necessarily ‘turn off’ the PIC. The charged 0.1 mfd bypass capacitor can keep a sleeping PIC operating for awhile after the power is removed. To be sure it has reset, short out Vdd to Vss once the power is removed. The other option is to use the MCLR with a reset button.
HINT: Using the internal weak pull-up resistors can eliminate ex ternal pull-up resistor on ‘switched’ inputs.
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10. Lost Calibration code In the last flash memory slot of a new 12F675 there is a calibration code that is placed during chip manufacture. The code has a hex value of 34xx where the xx part varies from chip to chip. This code is retrieved using the DEFINE OSCCAL_1K 1 statement. The last two hex digits, xx, of the code are then placed in the OSCCAL register and the chip’s internal oscillator is adjusted to operate at 4 MHz. If serial data, for example, is not being sent by the 12F675 then an exact 4 MHz may not be necessary and the DEFINE statement can be omitted. It is possible to erase the calibration code, in which case the “DEFINE OSCCAL_1K 1” command finds nothing to forward to the OSCCAL register and the chip will not operate. If you do erase the calibration code, you have three options: •
•
•
Use the 12F675 in applications where timing isn’t critical and omit the DEFINE OSCCAL_1K 1 statement. IF you know the last two hex digits of the calibration code you can insert them directly into the OSCCAL register using: OSCCAL=$xx For example if the code is 34a8, use: OSCCAL=$a8 DO NOT use the “DEFINE OSCCAL_1K 1” statement as there is no calibration code to read and then transfer to the OSCCAL register. If you DO NOT know the calibration code (as in my case, with two 12F675's!!!) use OSSCAL $80 as a starting point. (do not use DEFINE OSCCAL_1K 1 ) Connect a scope or frequency counter to Pin 3 (GPIO.4/OSC2/CLKOUT). When programming the chip, set the Oscillator Configuration to "INTRC Clockout". This must be done within the programmer software not the @DEVICE codes. You will get a square wave at 1/4 of the oscillator’s frequency ie 1 MHz on Pin 3 Here are my results: OSSCAL= $80 OSSCAL= $90 OSSCAL= $85 OSSCAL= $89 OSSCAL= $8c OSSCAL= $8d
Freq=966 kHz Freq=1001 kHz Freq=977 kHz Freq=991 kHz Freq=998 kHz Freq=1000 kHz
I have now labeled this PIC as an "8d" and will use OSSCAL= $8d when I need accurate timing!
INTRC Signal There is obviously some merit in recording the calibration number on the back of each 12F675 chip!
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11. Feedback and Acknowledgements If you have found this useful and/or have any suggestions, please let me know: Ian Burn Edmonton, Alberta, Canada
[email protected]
Special thanks to Melinda Burn and Angus Cameron in assisting with PDF conversions and editing suggestions.
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