Analog-to-Digital (A/D) Converter
Data Sheet
M68HC11E Family — Rev. 5
66
Analog-to-Digital (A/D) Converter
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MOTOROLA
3.2.6 Conversion Sequence
A/D converter operations are performed in sequences of four conversions each. A
conversion sequence can repeat continuously or stop after one iteration. The
conversion complete flag (CCF) is set after the fourth conversion in a sequence to
show the availability of data in the result registers.
Figure 3-3
shows the timing of
a typical sequence. Synchronization is referenced to the system E clock.
Figure 3-3. A/D Conversion Sequence
3.3 A/D Converter Power-Up and Clock Select
Bit 7 of the OPTION register controls A/D converter power-up. Clearing ADPU
removes power from and disables the A/D converter system. Setting ADPU
enables the A/D converter system. Stabilization of the analog bias voltages
requires a delay of as much as 100
μ
s after turning on the A/D converter. When the
A/D converter system is operating with the MCU E clock, all switching and
comparator operations are inherently synchronized to the main MCU clocks. This
allows the comparator output to be sampled at relatively quiet times during MCU
clock cycles. Since the internal RC oscillator is asynchronous to the MCU clock,
there is more error attributable to internal system clock noise. A/D converter
accuracy is reduced slightly while the internal RC oscillator is being used
(CSEL = 1).
0
32
64
96
128 — E CYCLES
SAMPLE ANALOG INPUT
SUCCESSIVE APPROXIMATION SEQUENCE
MSB
4
CYCLES
BIT 6
2
CYC
BIT 5
2
CYC
BIT 4
2
CYC
BIT 3
2
CYC
BIT 2
2
CYC
BIT 1
2
CYC
LSB
2
CYC
2
CYC
END
R
S
CONVERT FIRST
CHANNEL, UPDATE
ADR1
CONVERT SECOND
CHANNEL, UPDATE
ADR2
CONVERT THIRD
CHANNEL, UPDATE
ADR3
CONVERT FOURTH
CHANNEL, UPDATE
ADR4
12 E CYCLES
W
E CLOCK
Address: $1039
Bit 7
6
5
4
3
2
1
Bit 0
Read:
ADPU
CSEL
IRQE
(1)
DLY
(1)
CME
CR1
(1)
CR0
(1)
Write:
Reset:
0
0
0
1
0
0
0
0
1. Can be written only once in first 64 cycles out of reset in normal modes or at any time in special modes
= Unimplemented
Figure 3-4. System Configuration Options Register (OPTION)
F
Freescale Semiconductor, Inc.
n
.