AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 50

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

Application Note
50
NOTE:
All the calculations so far have dealt with variables that are assumed to
be fixed, when in reality there are tolerances on f
The worst-case stack up of these tolerances should be considered when
calculating N
conversion to handle a larger count than would be expected for the ideal
case. Otherwise, the count may reach its maximum value too soon,
causing a loss of top end capabilities, or the count may not produce
enough counts, causing a loss of minimum resolution. A good rule of
thumb is to plan on N+1 bits if using an accurate oscillator (crystal or
ceramic resonator) and standard external capacitor sizes. If the
oscillator is less accurate (internal or external RC) and lower quality
external capacitors are used, then plan on N+2 bits. In other words,
multiple the range by 2 normally and by 4 for poorer tolerances. When
in doubt as to the range of counts possible, figure the minimum and
maximum values of N
where:
With the calculated minimum and maximum counts, the user can
determine if they are going to get the desired results.
When calculating the minimum and maximum values for C
I
changes in supply voltage and temperature.
CHG
N
C
V
f
I
P
Freescale Semiconductor, Inc.
MIN
MIN
MIN
MIN,
MIN
, and f
For More Information On This Product,
,f
, I
, V
, C
MAX
N MAX
MAX
N
N MIN
MAX
MAX
MAX
OSC
Go to: www.freescale.com
FS
, be sure to include any effects caused by expected
=
=
for a given V
C MAX V MAX f MAX
--------------------------------------------------------------- -
C MIN V MIN f MIN
--------------------------------------------------------- -
= Min and max full scale count (counts)
= Min and max value of capacitor ( F)
= Min and max full scale voltage (volts)
= Min and max oscillator frequency (Hz)
= Min and max charge current ( A)
= Prescaler selected (count/count)
FS
P I MIN
P I MIN
using this calculation:
FS
. Provision must be made to allow the
OSC
, C
EXT
EXT
, and I
MOTOROLA
, V
AN1740
FS
CHG
,
.