AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 39

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

AN1740
MOTOROLA
NOTE:
NOTE:
NOTE:
V
limiting voltage and/or current excursions on the pins.
Refer to the application note In-Circuit and Emulation Consideration for
MC68HC05JJ/JP Series Microcontrollers , Motorola document number
AN1741/D, for more information on good PWB layout and limiting these
noise source.
One particularly troublesome high current case occurs when using the
discharge device on PB0 to discharge an external capacitor at the end
of a single slope A/D conversion. While normally there is no need to
make multiple A/D conversion measurements from the same sample, a
diagnostic may try to do this to measure the leakage performance of the
sample capacitor. In this case, the repeated measurements, with
discharges into PB0 at the end of each one, will yield worse results than
actually occur with a single A/D conversion.
Experience has shown that the least sensitivity to such noise sources is
created when the channel select bus is connected to the internal V
after the HOLD or DHOLD multiplexer has been turned off. This also
means that the input channel multiplexers are off. Leaving the channel
bus connected to V
to V
When trying to maintain the sample voltage, avoid discharging
capacitors or other high current loads through the MCU’s V
power supply return wiring.
In cases where severe system noise affects input signal measurements,
it is recommended that the sample and hold method not be used.
Instead, always leave the signal connected to the comparator through
the HOLD or DHOLD and channel multiplexers.
Another source of sample errors can occur if the proper sequencing of
the multiplexers is not followed. Since there is no automatic control of the
multiplexers, the user should first turn off the HOLD or DHOLD
multiplexer before changing any channel multiplexers. If all the
multiplexers are changed on the same write cycle to the AMUX register,
there can occur some charge redistribution due to the tiny variations in
delays between all the multiplexers. The only exception to this is when
SS
Freescale Semiconductor, Inc.
. The user also has some control over substrate current injection by
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ground bounce noise.
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or a low voltage input creates the most sensitivity
Support Circuitry
Application Note
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pin or
DD
39