AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 48

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

Application Note
48
From this basic equation, the time can be translated into a number of
counts at a given frequency:
where:
From this relationship, the value of each component in the single slope
A/D can be defined at the full-scale voltage input as:
The general process is to begin with the desired full-scale voltage,
resolution, and speed of conversion. This information will give V
and t
remaining variables are the size of the external capacitor and the
effective clock rate which is the product of the prescaler, P, and the
oscillator frequency, f
system factors, but the prescaler can be selected as eight for mode 2 or
mode 3 and greater than eight for software routines using mode 0 or 1.
t
N
P
f
V
Freescale Semiconductor, Inc.
FS
OSC
FS
FS
For More Information On This Product,
FS
. Since the charge current, I
t FS
= Full scale charge time (seconds)
= Full scale counts (counts)
= Prescaler (counts out/counts in)
= Oscillator frequency (Hz)
= Full scale voltage (volts)
N FS
C EXT
V FS
=
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=
P N FS
----------------------
f OSC
=
C EXT V FS f OSC
-------------------------------------------------------- -
=
P N FS I CHG
-------------------------------------------- -
P N FS I CHG
-------------------------------------------- -
C EXT f OSC
f OSC V FS
P I CHG
OSC
=
C EXT V FS
---------------------------------- -
. Usually, the clock frequency is fixed by other
I CHG
CHG
, is fixed at 100 A, the only
MOTOROLA
FS
AN1740
, N
FS
,