Page 148 - Embedded Microprocessor Systems Real World Design

time you have moving objects detected by sensors, you run the risk of unstable
output.

Bus Width
Sometimes an 8-bit device is connected to a 16, 32-, or 64bit processor. As I men-
tioned in Chapter 2, an example would be a 16550 UART connected to an x86
processor. In most cases, the hardware is not designed to decode all the possible
accesses to the device, so the software must not try to perform writes that are wider
than the device. In some cases, this is no problem, as the unused bits will be dis-
carded. However, if the hardware is designed so that the other bits write to a control
register, you can get unexpected results. For instance, if a device is located at
address 03F0 (hex), and the lower 8 bits are connected to a data register while
the upper 8 bits are connected to a status register, a word write will change both
registers. Another case in which this can cause problems is on a read: If you read
a ltkbit word from the %bit device, the unused bits usually will be undefined
because those bits are floating. If you do not mask them off in software, the results
are indeterminate.

Software Architecture

There are only so many ways to connect a flash memory to a processor, but there
are numerous ways to implement almost any software function. However, embed-
ded software usually is built on only a few architectural frameworks, as described
in this section.

Single Polling Loop
In this method, a single piece of polling code loops continuously, checking for
input from interrupt routines and external devices (such as a keypad) and
executing whatever subroutines are necessary to implement the functionality. This
method of coding assumes that all functions are available all the time. For example,
it might check all the key switches all the time, even if some switches are not used
in particular modes. This was the method that I used to implement the protocol
converter described earlier. That design did not even have any interrupts-every-
thing was done in the polling loop.

State Machine

The software is in one state at a time. Only those functions that pertain to the
current state are monitored. I designed a burglar alarm this way once. The system

Software Design 129