AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 45

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

Analog Power-Up Considerations
Analog-to-Digital Conversion
AN1740
MOTOROLA
NOTE:
All of the analog subsystem components can be powered down as an
I
current source are contained in the ACR at location $001D. Internally,
there is an analog bias supply which provides bias currents for all of the
analog features. When either comparator or the current source is
powered up, the bias supply is turned on automatically.
The maximum time delay for an analog component to power up and
stabilize is five microseconds. This delay is for individual components. If
several of the analog components are all initially powered up on the first
write cycle to the ACR, then the delay may be as long as 10
microseconds as both the bias supply and the comparators must come
up together.
Before sampling the comparator outputs or clearing the CPF1 or CPF2
flags, be sure to allow sufficient delay after the comparators are powered
up. Otherwise, the outputs may be undefined or the flags may not be
cleared.
Analog-to-digital (A/D) conversion can be accomplished using the JJ/JP
series by allowing the current source to charge up (ramp) an integration
capacitor connected to PB0 and using comparator 2 to detect the time
when the ramp voltage crosses the unknown voltage being measured as
shown in
DD
Freescale Semiconductor, Inc.
current saving technique. The controls for each comparator and the
For More Information On This Product,
Figure
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20.
Analog Power-Up Considerations
Application Note
45