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ARCHITECTURAL OVERVIEW
2.5.7 Analog-to-digital Converter
The analog-to-digital (A/D) converter converts an analog input voltage to a digital equivalent.
Resolution is either 8 or 10 bits; sample and convert times are programmable. Co nversions can
be performed on the analog ground and reference voltage, and the results can be used to calculate
gain and zero-offset errors. The internal zero-offset compensation circuit enables automatic zero-
offset adjustment. The A/D also has a threshold-detection mode, which can be used to generate
an interrupt when a programmable threshold voltage i s crossed in either direction. The A/D scan
mode of the PTS facilitates automated A/D conversions and result storage. See Chapter 12, “An-
alog-to-digital (A/D) Converter,” for more information.
2.5.8 Watchdog Timer
The watchdog timer is a 16-bit internal timer that resets the microcontroller if the software fail s
to operate properly. See Chapter 13, “Minimum Hardware Considerations,” for more informa-
tion.
2.6 SPECIAL OPERATING MODES
In addition to the normal execution mode, the microcontroller operates in several special-purpose
modes. Idle and powerdown modes conserve power when the microcontroller is inactive. On-
circuit emulation (ONCE) mode electrically isolates the microc ontroller from the system, and
several other modes provide programming options for nonvolatile memory. See Chapter 14,
“Special Operating Modes,” for more information about idle, powerdown, and ONCE modes, and
see Chapter 16, “Programming the Nonvolatile Me mory,” for details about programm ing options.
2.6.1 Reducing Power Consumption
In idle mode, the CPU stops executing instructions, but the peri pheral clocks remain active. Pow-
er consumption drops to about 40% of normal execution mode consumption. Either a hardware
reset or any enabled interrupt source will bring the microco ntroller out of idle mode.
In powerdown mode, all internal clocks are frozen at logic state zero and the internal oscillator is
shut off. The register file and most peri pherals retain their data if VCC is m aintained. Power con-
sumption drops into the µW range.
2.6.2 Testing the Printed Circuit Board
The on-circuit emulation (ONCE) mode electrically isolates the microcont roller from the system.
By invoking the ONCE mode, you can test the printed circuit board while the microcontroller is
soldered onto the board.