August 27, 1998
GENERAL RELEASE SPECIFICATION
15.4 A/D CONVERSION METHODS
The control bits in the ACR provide various options to charge or discharge current
through the CAP pin in order to perform single-slope A/D conversions using an
external capacitor from the CAP pin to V as shown in Figure 15-7. The various
SS
A/D conversion triggering options are given in Table 15-3.
V
– 1.5 VDC
DD
Charge Time = C x VX
I
UNKNOWN VOLTAGE ON (–) INPUT
VOLTAGE ON
CAPACITOR
CONNECTED
TO (+) INPUT
CHARGE TIME
TO MATCH UNKNOWN
DISCHARGE TIME
TO RESET CAPACITOR
MAXIMUM CHARGE TIME
TO V – 1.5 VDC
DD
+5V
V
DD
TM
PB7/AN0
Using V
as Voltage
Reference
DD
UNKNOWN
SIGNALS
PB6/AN1
PB5/AN2
PB4/AN3
CAP
MC68HC05SB7
V
SS
RAMP
CAP
+5V
V
DD
TM
PB7/AN0
UNKNOWN
SIGNALS
Using an
PB6/AN1
PB5/AN2
PB4/AN3
CAP
External Voltage
Reference
MC68HC05SB7
REFERENCE
VOLTAGE
V
SS
RAMP
CAP
Figure 15-7. Single-Slope A/D Conversion Method
The top three bits of the ACR control the charging and discharging current into or
out of the CAP pin. These three bits will have no affect on the CAP pin if the ISEN
enable bit is cleared. Any clearing of the ISEN bit will immediately disable both the
MC68HC05SB7
REV 2.1
ANALOG SUBSYSTEM
MOTOROLA
15-19
FREESCALE [ Freescale ]
