New instructions used in this chapter:

We all know what interrupts are and we don't like being interrupted. We are busy doing something and the phone rings or someone arrives at the door.

If we are expecting someone, we could look out of the window every now and again to see if they had arrived or we could carry on with what we are doing until the doorbell rings. These are two ways of receiving an interrupt. The first when we keep checking in software terms is called polling, the second when the bell rings is equivalent to the hardware interrupt.

We have looked at polling when we used the keypad to see if any keys had been pressed. We will now look at the interrupt generated by the hardware.

Before moving onto an example of an interrupt consider the action of the door in a washing machine. The washing cycle does not start until the door is closed, but after that the door does not take any part in the program. But what if a child opens the door, water could spill out or worse!! We need to switch off the outputs if the door is opened. To keep looking at the door at frequent intervals in the program (software polling) would be very tedious indeed, so we use a hardware interrupt. We carry on with the program and ignore the door. But if the door is opened the interrupt switches off the outputs – spin motor etc. If the door had been opened accidentally then closing the door would return back to the program for the cycle to continue.

This suggests that when an interrupt occurs we need to remember what the contents of the files were. i.e. the STATUS register, W register, TMR0 and PORT settings so that when we return from the interrupt the settings are restored. If we did not remember the settings, we could not continue where we left off, because the interrupt switches off all the outputs and the W register would also be altered, at the very least.

Interrupt sources

The 16F84 has 4 interrupt sources.

The 16F818/9 has 9 interrupt sources, and of course need extra bits in the interrupt registers to handle them. The additional interrups used in the 16F818/9 are

These interrupts can be enabled or disabled as required by their own interrupt enable/disable bits. These bits can be found in the interrupt control register INTCON for the 16F84 and also on the Peripheral Interrupt Enable Register1, PIE1 on the 16F818/9.

In this section we will be looking at the interrupt caused by a rising or falling edge on PORTB,0.

Interrupt control register

The Interrupt Control Register INTCON, file 0Bh is shown in Figure 14.1.

Figure 14.1 The interrupt control register, INTCON of 16F84

Bit 6 in this register is designated as the Peripheral Interrupt Enable Bit, PEIE for the 16F818/9.

Before any of the individual enable bits can be switched ON, the Global Interrupt Enable (GIE) bit 7 must be set, i.e. a 1 enables all unmasked interrupts and a 0 disables all interrupts.

Program using an interrupt

As an example of how an interrupt works consider the following example:

Suppose we have 4 lights flashing consecutively for 5 seconds each. A switch connected to B0 acts as an interrupt so that when B0 is at a logic 0 an interrupt routine is called. This interrupt routine flashes all 4 lights ON and OFF twice at 1 second intervals and then returns back to the program providing the switch on B0 is at a logic1.

I have used the 16F818 for this application.

The circuit diagram for this application is shown in Figure 14.2.

Figure 14.2 Interrupt demonstration circuit

One thing to note from the circuit the 16F818 chip has internal pull-up resistors on PORTB so B0 does not need a pull up resistor on the switch.

The interrupt we are using is a change on B0, we are therefore concerned with the following bits in the INTCON register, i.e. INTE bit4 the enable bit and INTF bit1 the flag showing B0 has changed, and of course GIE bit7 the Global Interrupt Enable Bit.