Page 178 - Embedded Microprocessor Systems Real World Design

Table 5.2
Pool Timer Flags

Flag Function

ONFLAG Set when the ON key is pressed by the user
OFFLAG Set when the OFF key is pressed by the user
SEFLAG Set when the SET key is pressed by the user
FCFLAG Set when the FCN key is pressed by the user
MTFLAG Set when the water low switch is closed
TFLAG Set when time rolls over to 0 : 0

BACKGROUND STOPS EXECUTING FOR
THIS LONG
c
CODE BEING
EXECUTED BACKGROUND I ISR1 I ISR2 I ISR3 I BACKGROUND

INTERRUPT 1 OCCURS HERE
INTERRUPT 2 OCCURS HERE
AND IS SERVICED HERE

INTERRUPT 3 OCCURS HERE
AND IS SERWCED HERE

Figure 5.8
Interrupt Stackup.

Some Real World Dos and Don’ts

Interrupt Stackup
Figure 5.8 shows a system with three interrupts and three ISRs. The code is
executing in the background when interrupt 1 occurs. During execution of ISR 1,
interrupt 2 occurs. As soon as ISR 1 is finished, ISR 2 will be executed. During ISR
2, interrupt 3 occurs, so ISR 2 is followed by ISR 3. The background code stops
execution for the sum of the time it takes ISR 1, ISR 2, and ISR 3 to execute. Note
that the interrupts do not have to occur simultaneously to stack up and appear simul-
taneous to the non-ISR code.

In any system with more than one active interrupt, eventually, the inter-
rupts will stack up. Count on it.

Interrupts in Embedded Systems 159