AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 53

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

Disabled
Conversion A/D
Fault
AN1740
MOTOROLA
The CPF2 flag bit is set by a low-to-high transition on the output of
comparator 2. If the output of comparator 2 cannot return to the low
state, this flag bit cannot get set again. This occurs whenever the level
on the PB0 pin cannot be discharged below the level on the negative
input to comparator 2 as in the following two situations (refer to
Figure
In both these cases, the discharge device cannot pull PB0 below the
level on the negative comparator input, and the output of comparator
never goes low. After once clearing the CPF2 flag bit, it cannot get set
again, and it appears like the infinite conversion fault mentioned before.
However, in this case, the ramp voltage does indeed reach the input
level, but the CPF2 flag is not set because conversion did not discharge
PB0 low enough to make CMP2 go low again. This is referred to also as
the “stuck at zero” condition. This is shown in
input offset case.
•
•
Freescale Semiconductor, Inc.
TOF/OCF
For More Information On This Product,
CMP2
CFR2
The comparator has a positive input offset voltage and the
negative comparator input is more than V
The comparator has a negative input offset voltage and the
negative comparator input is less than V
CHG
PB0
V
7):
IN
Figure 21. Infinite Conversion Time A/D Fault
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V
SS
COMMON-MODE RANGE
Pitfalls in Using Single Slope A/D
Figure 22
IO
IO
MODE 2 OR MODE 3 ONLY
or below V
below V
for the negative
Application Note
SS
SS
.
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