AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 52

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

Application Note
Pitfalls in Using Single Slope A/D
Infinite
Conversion Time
52
The simplicity of single slope A/D conversion also presents some
unwanted faults due to the limits of its input voltage range, signal noise,
and timing with respect to the software. The user should consider their
application to see if any of the following faults are possible. Four A/D
fault conditions can occur in all the modes of operation:
Two additional A/D fault conditions only occur in mode 2 or mode 3:
All six of these faults are described in detail in the following paragraphs.
When the unknown input signal is above the common-mode range of
comparator 2, the integration ramp on the PB0 pin can never reach the
level to trip comparator 2 and set the CPF2 flag. This is shown in
Figure
This fault can be eliminated by making sure that the input signal is
clamped within the common-mode range. Another means is to use the
divided input which can limit the input to the comparator.
If it does occur, this fault can be detected by making a software check
for the maximum conversion time. When this time is reached, the CMP2
flag will still be low, indicating that the ramp never reached the input
level. Therefore, the input is at or above the maximum conversion level,
the conversion can be terminated, and the conversion result can be
forced to its maximum value.
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Infinite conversion time
Disabled conversions
Extra conversions
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AN1740