Page 134 - Embedded Microprocessor Systems Real World Design

Timer Design Considerations When working with timers, be sure to consider
all the following:
Does the timer have sufficient resolution for the application?
Does the timer have sufficient range? Will it overflow in the worst-case scenario?
If the timer can overflow, is there a hardware/software mechanism to handle that
condition?
Are the accuracy and drift of the timer clock adequate for the application?
Can the software keep up with the selected timer period, capture rates, and so
on?
If interrupts are used, can the software keep up with the interrupt rate over the
full range of input/output values?

Example System

Appendix A contains the schematic of the pool pump timer system I mentioned in
the last chapter. An 80C31 microcontroller is used, with external ROM and 1/0
ports. Because there is only one read and one write port, no address decoding is
needed. The read buffer (a 74HC244) is directly enabled by -RD, and the write
register is clocked by -wR. A 74LS123 one-shot provides a watchdog timer (WDT) .
The user key inputs connect to 52 and are switch closures to ground. Switch inputs
are debounced in software by the 250Hz interrupt code. The display, not shown,
consists of four seven-segment displays and three high-intensity LEDs. The display
is multiplexed in software so only one seven-segment drive register is required.
Input power and the pump relay coil are connected via a four-terminal barrier
strip.

Hardware Specifications Outline

The following is a generic outline for the hardware specifications.
Overview. A brief description of what the outline covers.
Related Documents. ANSI and IEEE specs. May also include a reference to the
product requirements document.
Board Description. A brief description of the hardware and what it does. For
example, The xyz board controls the three-axis robotic motors for the robotic
arm. This includes the stepper and DC servo control and the related limit
switches. The xyz board is controlled by a 10MHz 80C188 microprocessor. DC

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