GENERAL RELEASE SPECIFICATION
August 27, 1998
charge current source and the discharge device. Since all these bits and the ISEN
bit are cleared when the device is reset, the MC68HC05SB7 starts with the
charge and discharge function disabled.
The length of time required to reach the maximum voltage to be measured will
determine the resolution of the reading. The time to ramp the external capacitor
voltage to match the maximum voltage is dependent on:
•
•
•
•
•
Desired resolution.
Clock rate for timing function.
Any prescaling of the clock to the timing function.
Charging current to external capacitor.
Value of the external capacitor.
The values of each parameter are related by the general equation:
C × VMAX
tCHG = ----------------------
ICHG
Each parameter can also be expressed by the following equations:
P × N
tCHG = -------------
fOSC
ICHG × P × N
VMAX = -----------------------------
C × fOSC
V
MAX × C × fOSC
N = ---------------------------------------
CHG × P
I
I
CHG × N × P
C = -----------------------------
MAX × fOSC
V
where the signal names and parameters used are given in Table 15-4.
NOTE
Noise on the system ground or the external ramping capacitor can cause the
comparator to trip prematurely. Therefore in any given application it is best to use
the fastest possible ramp rate (shortest possible time).
NOTE
The value of any capacitor connected directly to the CAP pin should be limited to
less than 2µF. Larger capacitances will create signal noise.
MOTOROLA
15-20
ANALOG SUBSYSTEM
MC68HC05SB7
REV 2.1
FREESCALE [ Freescale ]
