AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 3

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Analog Subsystem Overview
Voltage
Comparators
Constant Current
Source
Discharge Device
AN1740
MOTOROLA
The block diagram of the analog subsystem is shown in
analog subsystem is structured around three basic components:
An assortment of channel multiplexers, control logic, and control/status
registers are provided in order to use these basic building blocks in
typical analog interfaces.
Each voltage comparator can compare voltages which are more than 1.5
volts below the V
millivolts and typical response time is less than a microsecond. Both
comparators have identical performance and differ only in how they are
connected to external pins. Each comparator can have its internal
circuitry “flipped” so that any input offset can be referenced to either
input. The value of this is included in the discussion of A/D techniques.
Each comparator also can be powered down to conserve supply current
when its voltage comparison function is not needed.
A constant current source is provided to an external pin and sources
typically 100 A as long as the voltage on the pin is more than 1.5 volts
below the V
controlled by a bit that also turns off its internal V
it is not being used. The primary purpose of the current source is to
construct an integrating A/D using voltage comparator 2. But, this
current source can be used for other purposes if an external device can
utilize the nominal 100- A current.
A discharge device is also attached to the constant current source
output. Either the current source is ON, the discharge device is ON, or
both are OFF. This allows control of an external charge storage device,
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Two voltage comparators
A constant current source
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supply voltage. The current being sourced can be
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supply voltage. The input offset is typically 10
Analog Subsystem Overview
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supply current when
Figure
Application Note
1. The
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