MC68HC11F1 Datasheet, PDF (54/68 Pages) Motorola, Inc – Technical Summary 8-Bit Microcontroller

11.1 Input Pins
Port E pins can also be used as digital inputs. Reads of port E pins are not recommended during the
sample portion of an A/D conversion cycle, when the gate signal to the N-channel input gate is on. Be-
cause no P-channel devices are directly connected to either input pins or reference voltage pins, volt-
ages above V_DD do not cause a latchup problem, although current should be limited according to
maximum ratings. Figure 15 is a functional diagram of an input pin.
ANALOG
INPUT
PIN
< 2 pF
+ ~20V
– ~0.7V
INPUT
PROTECTION
DEVICE
+ ~12V
– ~0.7V
DUMMY N-CHANNEL
OUTPUT DEVICE
DIFFUSION/POLY
COUPLER
≤ 4 KΩ
400 nA
JUNCTION
LEAKAGE
*
~ 20 pF
DAC
CAPACITANCE
V_RL
* THIS ANALOG SWITCH IS CLOSED ONLY DURING THE 12-CYCLE SAMPLE TIME.
Figure 15 Electrical Model of an Analog Input Pin (Sample Mode)
11.2 Conversion Sequence
A/D converter operations are performed in sequences of four conversions each. A conversion sequence
can be repeated 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 16
shows the timing of a typical sequence. Synchronization is referenced to the system E clock.
E CLOCK
12 E CYCLES
SAMPLE ANALOG INPUT
MSB
4
CYCLES
BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 LSB 2
2
2
2
2
2
2
2 CYC
CYC CYC CYC CYC CYC CYC CYC END
SUCCESSIVE APPROXIMATION SEQUENCE
CONVERT FIRST
CONVERT SECOND
CONVERT THIRD
CONVERT FOURTH
CHANNEL, UPDATE
CHANNEL, UPDATE
CHANNEL, UPDATE
CHANNEL, UPDATE
0
ADR1
32
ADR2
64
ADR3
96
ADR4
128 — E CYCLES
Figure 16 A/D Conversion Sequence
MOTOROLA
54
MC68HC11F1/FC0
MC68HC11FTS/D