Analog-to-Digital (A/D) Converter
Data Sheet
M68HC11E Family — Rev. 5
70
Analog-to-Digital (A/D) Converter
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MOTOROLA
system is configured to perform a conversion on each of four channels where
each result register corresponds to one channel.
NOTE:
When the multiple-channel continuous scan mode is used, extra care is needed in
the design of circuitry driving the A/D inputs. The charge on the capacitive DAC
array before the sample time is related to the voltage on the previously converted
channel. A charge share situation exists between the internal DAC capacitance
and the external circuit capacitance. Although the amount of charge involved is
small, the rate at which it is repeated is every 64
μ
s for an E clock of 2 MHz. The
RC charging rate of the external circuit must be balanced against this charge
sharing effect to avoid errors in accuracy. Refer to M68HC11 Reference Manual,
Motorola document order number M68HC11RM/AD, for further information.
CD:CA — Channel Selects D:A Bits
Refer to
Table 3-2
. When a multiple channel mode is selected (MULT = 1), the
two least significant channel select bits (CB and CA) have no meaning and the
CD and CC bits specify which group of four channels is to be converted.
Table 3-2. A/D Converter Channel Selection
Channel Select
Control Bits
CD:CC:CB:CA
0000
0001
0010
0011
0100
0101
0110
0111
10XX
Channel Signal
Result in ADRx
if MULT = 1
AN0
AN1
AN2
AN3
AN4
AN5
AN6
AN7
ADR1
ADR2
ADR3
ADR4
ADR1
ADR2
ADR3
ADR4
—
Reserved
1100
V
RH(1)
1. Used for factory testing
ADR1
1101
V
RL(1)
ADR2
1110
(V
RH
)/2
(1)
ADR3
1111
Reserved
(1)
ADR4
F
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n
.