Multiple External Interrupts

One of the practical applications of the port-B line-change interrupt is in handling several different interrupt sources; for example, a circuit containing four push-button switches that activate four different circuit responses. If the switches are wired to the corresponding pins in Port-B (RB4 to RB7) and the line-change interrupt is enabled, the interrupt takes place when any one of the four switches changes level, that is, when any one of the interrupt lines go from high to low or from low to high. The interrupt handler software can easily determine which of the switches changed state and if the change took place on the signal’s rising or falling edge. The corresponding software routines then handle each case.

Later in this chapter we develop a sample program that uses the Port-B line-change interrupt to respond to action on four pushbutton switches.

11.1.4 EEPROM Data Write Interrupt

The origin of this interrupt relates to the relative slowness of the EEPROM data write operation, which is of 10 ms. The interrupt serves no other function than to allow the microcontroller to continue execution while the data write operation is in progress. The interrupt service routine informs the microcontroller when writing has ended through the EEIF bit located in the EECON1 register. The use of this interrupt is considered in Chapter 15, in the context of EEPROM data memory access and programming.

11.2 Interrupt Handlers

The interrupt handler, also called the interrupt service routine or the ISR, is the code that receives control upon occurrence of the interrupt. Most of the programming that goes into the service routine is specific to the application; however, there are certain housekeeping operations that should be included. The following list describes the structure of an interrupt service routine for the mid-range PICs:

1.Preserve the value in the w register.

2.Preserve the value of the STATUS register.

3.Execute the application-specific operations.

4.Restore the value of the STATUS register at the time of the interrupt.

5.Restore the value of the w register at the time of the interrupt.

6.Issue the RETFIE instruction to end the interrupt handler.

In the PIC 16F84, the interrupt service routine must be located at offset 0x004 in code memory. A simple org directive takes care of ensuring this location, as in the following code fragment:

ered in the chapter on Serial Communications and the one on EEPROM Data Operations, respectively.

11.3.1 Programming the External Interrupt

Port-B, line 0, is referred to as the External Interrupt source. The name is not the most adequate since other interrupts can also have external sources. One of the important uses of this interrupt source is to wake the processor from the SLEEP mode. This allows developing applications that can run on a small power source (such as batteries) since the program uses almost no power until some action associated with the interrupt source wakes up the PIC. A sample program using the RB0 interrupt is developed later in this chapter. Our first sample program is a simple demonstration of the installation and action of the interrupt. The program is based on the circuit in Figure 11-4.

RA1 RA0 OSC1 OSC2 Vdd RB7 RB6 RB5 RB4

16F84

RA2 RA3 T0Tkl MCLR Vss RB0/INT RB1 RB2 RB3

+5 V

10K Ohms

4.7K Ohms

+5 V

Figure 11-4 Circuit for RB0 Interrupt Demonstration

In the circuit of Figure 11-4, a pushbutton switch is wired to the RB0 port. It is this switch which produces the interrupt when pressed. A red LED is wired to port RB1 and a green LED to port RB2. The main program flashes the green LED on and off at a rate of approximately one-half second. The red LED is toggled on and off when the pushbutton switch is pressed. The switch contains a 4.7K Ohm resistor that keeps the port high until the contact is made and sent to ground. This makes the

switch active low and the interrupt is programmed on the falling edge of the signal, which takes place when the contact is made.

RB0 Interrupt Initialization

In order to initialize the RB0 interrupt, the following operations must take place:

1.Port-B, line 0, must be initialized for input.

2.The interrupt source must be set to take place either on the falling or the rising edge of the signal.

3.The external interrupt flag (INTF in the INTCON Register) must be initially cleared.

4.Global interrupts must be enabled by setting the GIE bit in the INTCON Register.

5.The External Interrupt on RB0 must be enabled by setting the INTE bit in the INTCON Register.

The following code fragment, from the program RB0Int in the book’s online software package, performs these operations:

;============================= ; interrupt handler ;=============================

org 0x04 goto IntServ

;=============================

;main program ;============================= main:

;Set up interrupt on falling edge

;by clearing OPTION register bit 6 movlw b’10111111’

option

;setup interrupts ;============================

;Clear external interrupt flag (intf = bit 1)

;Enable global interrupts (gie = bit 7)

;Enable RB0 interrupt (inte = bit 4)

;============================

; Program flashes LED wired to Port-B, line 2 lights:

RB0 Interrupt Service Routine

The Service Routine for the RB0 interrupt depends on the specific application. Nevertheless, the following processing steps should be considered:

1.Determine if the source is an RB0 interrupt.

2.Clear the RB0 interrupt flag (INTF bit) in the INTCON Register.

3.Save the context. Which registers and variables need to be saved depends on the specific application.

4.Perform the interrupt action.

5.Restore the context.

6.Return from the interrupt with the retfie instruction.

In addition, the interrupt handler may have to perform operations that are specific to the application. For example, debounce a switch or initialize local variables. The following Interrupt Service routine is from the program RB0Int in the book’s online software:

;=======================================================

;=======================================================

;Service routine receives control when there is

;action on pushbutton switch wired to port-B, line 0 IntServ:

;First test if source is an RB0 interrupt

;=========================

;interrupt action ;=========================

;Debounce switch

;Logic:

;Debounce algorithm consists in waiting until the

;same level is repeated on a number of samplings of the

;switch. At this point the RB0 line is clear since the

;interrupt takes place on the falling edge. The routine

;waits until the low value is read several times.

wait:

;Check to see that port-B bit 0 is still 0

;If not, wait until it changes

;Interrupt action consists of toggling bit 2 of

;port-B to turn LED on and off

;=========================

;exit ISR ;========================= exitISR:

;Restore context

Note that the interrupt handler listed previously contains a debouncing routine that cleans the switch’s signal. In this particular implementation the detection of a signal of the wrong value determines that the interrupt is aborted. For the particular switch used in the test circuit this approach seemed to work better. Alternatively, the routine can be designed so that if a wrong edge is detected, execution continues in the wait loop. In any case, the entire complication of software debouncing can be avoided by debouncing the switch in hardware.

11.3.2 Wakeup from SLEEP Using the RB0 Interrupt

The PIC microcontroller sleep mode provides a useful mechanism for saving power. It is particularly useful in battery-operated devices.

The sleep mode is activated by executing the SLEEP instruction; it suspends all normal operations and switches of the clock oscillator.

The sleep mode is suitable for applications that are not required to run continuously. For example, a device that records temperature at daybreak can be designed so that a light-sensitive switch generates an interrupt that turns the device on each morning. Once the data is recorded, the device goes into the sleep mode until the next daybreak.

Several events can make the device wake up from the sleep mode:

1.A device reset on the !MCLR pin

2.Watchdog timer wake-up signal, if WDT is enabled

3.Interrupt on RB0 line

4.Port change interrupt on RB4 to RB7 lines

5.EEPROM write complete interrupt

In the sleep mode, the device is placed on a power-down state that generates the lowest power consumption. The system clock is turned off in the sleep mode so signals that depend on the clock cannot be used to terminate the sleep. If enabled, the Watchdog Timer is cleared by the sleep instruction but keeps running. The PD bit in the STATUS register is also cleared and the TO bit is set. The ports maintain the status they had before the SLEEP instruction was executed.

The TO and PD bits in the STATUS register can be used to determine the cause of wake-up, since the TO bit is cleared if a Watchdog Timer wake-up took place. The corresponding interrupt enable bit must be set for the device to wake-up up due to an interrupt. Wake-up takes place regardless of the state of the General Interrupt Enable (GIE) bit. If the bit is clear, the device continues execution at the instruction following SLEEP. Otherwise, the device executes the instruction after the SLEEP instruction and then branches to the interrupt address. If the execution of the instruction following SLEEP is undesirable, the program should contain a NOP instruction after the SLEEP instruction.

The SleepDemo Program

The program named SleepDemo in the book’s online software package is a trivial demonstration of using the RB0 interrupt to wake the processor from sleep mode. The program can be tested using the circuit in Figure 11-4. SleepDemo flashes the green LED at ½ second intervals during 20 iterations and then goes into sleep mode. Pressing the pushbutton switch on line RB0 generates an interrupt that wakes the processor from sleep mode. The following code fragment shows the coding of the main loop in the program:

;============================

;flash LED 20 times ;============================ wakeUp:

;Program flashes LED wired to port-B, line 2

;20 times before entering the sleep state

In the SleepDemo program the Interrupt Service Routine does nothing. Its coding is as follows:

;=======================================================

;Interrupt Service Routine ;=======================================================

;The interrupt service routine performs no operation IntServ:

retfie

The initialization of the RB0 interrupt is identical to the one in the RB0Int program previously listed.

11.3.3 Port-B Bits 4-7 Status Change Interrupt

In the PIC 16F84 microcontroller, a change of input signal on Port-B, lines 4 to 7, generates an interrupt. This interrupt sets the RBIF bit in the INTCON Register to indicate that at least one of the ports have changed value. The port change takes place when the port’s previous value changes from logic one to logic zero or vice versa. In order for port pins to recognize this interrupt, they must have been defined as input. If any one of the port pins (4 to 7) is defined as output the interrupt takes place. The status change of the ports is in reference to the last time port-B was read.

The principal application of this interrupt source is in detecting several different interrupt sources. Its principal disadvantage is that it forces the declaration of four port-B lines as input, although during processing not all lines need be recognized as interrupt sources. The conclusion is that applications that only need a single external interrupt source should use the RB0 interrupt described in previous sections. Only applications that require more than one external interrupt should use the Port-B lines 4 to 7 interrupt on change source.

Since the interrupt takes place on any status change (high-to-low or low-to-high) the service routine executes on both signal edges. If interrupt processing is required on only one edge, that is, either when the port goes high or low, then the filtering must be performed in software. The circuit in Figure 11-5 allows testing the Port-B Status Change Interrupt.

2x470 Ohm

+5 V

10K Ohm

Figure 11-5 Circuit for Testing the Port-B Status Change Interrupt

In the circuit of Figure 11-5, a pushbutton switch is wired to the RB7 port and another one to RB4. Both of these switches produce the interrupt when pressed. A red LED is wired to port RA1 and a green LED to port RA0. The red and green LEDs are toggled on and off when the corresponding pushbutton switches are pressed. The switches contain a 4.7K Ohm resistor that keeps the port high until the contact is made and sent to ground. This makes both switches active low and the interrupt is programmed on the falling edge of the signal.

RB4-7 Interrupt Initialization

In order to initialize the RB4-7 change interrupt the following operations must take place:

1.Port-B lines 4 to 7 must be initialized for input.

2.The interrupt source must be set to take place either on the falling or the rising edge of the signal.

3.The RB port change interrupt flag (RBIF in the INTCON Register) must be initially cleared.

4.Global interrupts must be enabled by setting the GIE bit in the INTCON Register.

5.The RB port change interrupt must be enabled by setting the RBIE bit in the INTCON Register.

6.Internal pull-ups on port-B should be disabled in the OPTION register.

The following code fragment from the program RB4to7Int in the book’s online software package shows the required processing:

;=============================

;main program ;============================= main:

;Disable port-B internal pull-ups

;Interrupts on falling edge of pushbutton action

;============================

;setup interrupts ;============================

;Clear external interrupt flag (intf = bit 1)

;Enable global interrupts (gie = bit 7)

;Enable RB0 interrupt (inte = bit 4)

RB4-7 Change Interrupt Service Routine

The Service Routine for the RB4-7 change interrupt depends on the specific application. Nevertheless, the following processing steps should be considered:

1.Determine if the source is an RB4-7 change interrupt.

2.Clear the RBIF interrupt flag in the INTCON Register.

3.Save the context. Which registers and variables need to be saved depends on the specific application.

4.Perform the interrupt action.

5.Restore the context.

6.Return from the interrupt with the retfie instruction.

In addition, the interrupt handler may have to perform operations that are specific to the application; for example, debounce a switch or initialize local variables. The following Interrupt Service routine is from the program RB4to7Int in the book’s online software:

;=======================================================

;=======================================================

;Service routine receives control whenever any of

;port-B lines 4 to 7 change state

IntServ:

; First test: make sure source is an RB4-7 interrupt

;=========================

;interrupt action ;=========================

;The interrupt occurs when any of port-B bits 4 to 7

;have changed status.

; Test each meaningful bit (4 and 7 in this example)

;========================

;bit 4 change routine ;========================

;Check for signal falling edge, ignore if not bit4Chng:

;bit 7 change routine ;========================

;Check for signal falling edge, ignore if not bit7Chng:

;At this point all port-B pushbuttons are released ;=========================

;exit ISR