August 27, 1998
GENERAL RELEASE SPECIFICATION
NOTE
Sufficient time should be allowed to discharge the external capacitor or
subsequent charge times will be shortened with resultant errors in timing
conversion.
NOTE
If the unknown voltage applied to the comparator is greater than its common-
mode range (V –1.5 volts) the external capacitor will try to charge to the same
DD
level. This will cause both comparator inputs to be above the common-mode
range and the output will be indeterminate. All A/D conversion software methods
should have a maximum time check to determine if this case is occurring.
Table 15-4. A/D Conversion Signals and Definitions
Name
Function
Conditions
I
Charging current on external ramping capacitor
Discharge current on external ramping capacitor
Voltage on external ramping capacitor
I
= 80 - 120 µA
CHG
CHG
I
I
> 1 mA
DIS
DIS
V
V
< V
< (V – 1.5)
CAP
SS
CAP DD
V
Voltage of unknown on (–) input of voltage comparator
Maximum voltage on external ramping capacitor
TIme to charge external capacitor
V
< V < (V –1.5)
X
SS X DD
V
V
= V – 1.0
MAX
CHG
MAX DD
t
∆t from V to V
SS X
t
Time to discharge external capacitor
∆t from V
to V
DIS
MAX SS
C
Capacitance of external ramping capacitor
0.001 to 1.000 µF
N
Number of counts for I
to charge C to V
0 to 65536
CHG
X
f
÷ loop time for software timing
OSC
P
Prescaler into timing function
Clock source frequency (excluding any prescaling)
f
÷ 8 for Core Timer
OSC
f
÷ 8 for Programmable TImer
OSC
f
0 to 4.2 MHz
OSC
Table 15-5 gives examples of voltage ranges, resolution, ramp times and capaci-
tor sizes for various conversion methods.
MC68HC05SB7
REV 2.1
ANALOG SUBSYSTEM
MOTOROLA
15-21
FREESCALE [ Freescale ]
