Data Acquisition

The maximum battery life that can be achieved depends on:• How often the input channels are scanned• The number of analog channels and how many are connected to sensors• The number of digital channels and how many are driving outputs• Sensor excitation power draw• Complexity of any calculationsA precise calculation of the battery life is extremely involved, however manufacturerscan provide battery life charts, based on the number of channels and the time betweeneach scan of all the channels.All microprocessor systems need some supervisory functions that are analog rather thandigital in nature. For a typical system, these functions include power reset, batterybackup switching for RAM and real time clock, and the watch dog timer (WDT).The reset circuit ensures that the microprocessor is in the reset state in the absence ofpower. Embedded systems that may need to function in hostile environments requireadvanced reset circuitry that provides voltage threshold detection, independent of the rateof rise of the system power.The battery backup circuit ensures that the RAM and real time clock components receivea constant source of power. It also ensures that the RAM and real time clock are writeprotectedand in the low power mode, when the main power supply fails.If the voltage from the main power supply falls below a preset level, then the batterybackup circuit switches the RAM and real time clock power source to a supplementarybattery supply. Additionally, the battery backup circuit ensures that the RAM and realtime clock are write-protected, and in the low power sleep mode.Two power states, wake and sleep (standby), ensure that the minimum amount of powerwill be used when the unit is not required to perform any data acquisition function. Forexample, the device will wake up when:• RTC periodic interrupt signals a scan of the input channels is due• A memory card is inserted• Characters are received at the communications port• A key is pressed (where fitted)When an internal or external battery is being powered from an AC or DC supply thenthe low power sleep mode is not required to be entered.The watch dog timer (WDT) circuit is intended to detect software-processing errors.During normal operation the software is responsible for periodically resetting the WDT.Failure to reset the WDT on a periodical basis indicates that the software is no longerexecuting the intended sequence, and accordingly the WDT initiates a system reset.