Page 70 - The Art of Designing Embedded Systems

Real Time Means Right Now! 57

more commonly it’s set up by the firmware. Forget to initialize the
device and the system will crash as the device supplies a bogus
number.
Some peripherals and interrupt inputs will skip the acknowledge
cycle because they have predetermined vector addresses.
9 All CPUs let you disable interrupts via a specific instruction (DI,
CLI, or something similar). Further, you can generally enable and
disable interrupts from specific devices by appropriately setting
bits in peripheral or interrupt control registers.
9 Before invoking the ISR the hardware disables or reprioritizes in-
terrupts. Unless your code explicitly reverses this, you’ll see no
more interrupts at this level.

At first glance the vectoring seems unnecessarily complicated. Its
great advantage is support for many varied interrupt sources. Each device
inserts a different vector; each vector invokes a different ISR. Your UART
Data-Ready ISR is called independently of the UART Transmit-
Buf fer-Full routine. The vectoring scheme also limits pin counts,
since it requires just one dedicated interrupt line.
Some CPUs sometimes directly invoke the ISR without vectoring.
This greatly simplifies the code, but unless you add a lot of manual pro-
cessing, it limits the number of interrupt sources a program can con-
veniently handle.

Interrupt Design Guidelines
While crummy code is just hard to debug, crummy ISRs are virtually
undebuggable. The software community knows it’s just as easy to write
good code as it is to write bad. Give yourself a break and design hardware
and software that eases the debugging process.
Poorly coded interrupt service routines are the bane of our industry.
Most ISRs are hastily thrown together, tuned at debug time to work, then
tossed in the “Oh my God, it works” pile and forgotten. A few simple rules
can alleviate many of the common problems.
First, don’t even consider writing a line of code for your new em-
bedded system until you lay out an interrupt map (Figure 4-3). List each
interrupt and give an English description of what the routine should do. In-
clude your estimate of the interrupt’s frequency. Figure the maximum,
worst-case time available to service each. This is your guide: exceed this
number, and the system stands no chance of functioning properly.
The map is a budget. It gives you an assessment of where interrupt-
ing time will be spent. Just as your own personal financial budget has a