MC68HC705JP7 Datasheet, PDF (122/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
8.6 A/D Conversion Methods
The control bits in the ACR provide various options to charge or
discharge current through the PB0/AN0 pin to perform single-slope A/D
conversions using an external capacitor from the PB0/AN0 pin to V_SS as
shown in Figure 8-7. The various A/D conversion triggering options are
given in Table 8-3.
Charge Time = ^{C x VX}
I
V_DD –1.5 Vdc
VOLTAGE ON
CAPACITOR
CONNECTED
TO (+) INPUT
CHARGE TIME
TO MATCH UNKNOWN
MAXIMUM CHARGE TIME
TO V_DD –1.5 Vdc
UNKNOWN
OR REFERENCE
SIGNALS
RAMP
CAP
PB4/AN4
PB3/AN3
PB2/AN2
PB1/AN1
PB0/AN0
UNKNOWN VOLTAGE ON (–) INPUT
DISCHARGE TIME
TO RESET CAPACITOR
+5V
V_DD
MC68HC705JJ7
MC68HC705JP7
V_SS
Figure 8-7. Single-Slope A/D Conversion Method
The top three bits of the ACR control the charging and discharging
current into or out of the PB0/AN0 pin. These three bits will have no
effect on the PB0/AN0 pin if the ISEN enable bit is cleared. Any clearing
of the ISEN bit will immediately disable both the charge current source
and the discharge device. Since all these bits and the ISEN bit are
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MC68HC705JJ7 • MC68HC705JP7 — REV 4
Analog Subsystem
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