Page 307 - Embedded Microprocessor Systems Real World Design

while in the override mode. When the override time expires, time keeping and display will
revert to normal mode. Pressing OFF will activate the OFF override, with the same charac-
teristics as the ON override.
Pressing OFF while in ON override or pressing ON while in OFF override will terminate
override mode. Time keeping and display revert to normal mode. ON override may be used
while the pump is on normally to extend the ON time to up to 24 hours. Similarly, the OFF
override may be used while the pump is off normally.

Setting Time

When user presses SET, the timer enters the time set mode. The set LED goes on. Pressing
ON after SET will light the ON LED and show the current ON time, not the time remain-
ing. Each press of ON will increment the time by 30 minutes until the time reaches 24 hours;
the time will then roll over to 30 minutes. Pressing SET terminates the time set mode and
stores the time, and the SET LED goes off. OFF time setting works in the same way as ON
time setting.
While setting ON time, pressing OFF will save the ON time and change to the OFF set
mode. Similarly, pressing ON while setting OFF time will save the OFF time and switch to
the ON time set mode.

Water Low

E a low-water condition is detected by closure of the water low switch, the pump will turn
off if it is on. If the pump is already off, it will not be permitted to turn on. Any time that
the low-water condition is detected and the pump should be on, the ON LED will flash to
indicate the problem. The water low switch input will be filtered to prevent spurious
transitions.

Example System Hardware Specifications

Initial Hardware Specification (Predesign)

Display:
Four seven-segment LED displays (hours, minutes)
ON LED (high intensity)
SET LED (high intensity)
OVERRIDE LED (high intensity)
Keys:
SET: Enables time set
ON: On override, on time set
OFF: Off override, off time set
FCN: Undefined

288 Appendix A