MC68HC908LJ24 Datasheet, PDF (339/464 Pages) Motorola, Inc – Microcontrollers

Analog-to-Digital Converter (ADC)
Functional Descriptions
Finally, 8-bit truncation mode will place the eight MSBs in ADC data
register low (ADRL). The two LSBs are dropped. This mode of operation
is used when compatibility with 8-bit ADC designs are required. No
interlocking between ADRH and ADRL is present.
NOTE: Quantization error is affected when only the most significant eight bits
are used as a result. See Figure 16-3.
8-BIT 10-BIT
RESULT RESULT
003
00B
IDEAL 8-BIT CHARACTERISTIC
WITH QUANTIZATION = ±1/2
10-BIT TRUNCATED
TO 8-BIT RESULT
00A
IDEAL 10-BIT CHARACTERISTIC
009 WITH QUANTIZATION = ±1/2
002
008
007
006
005
001
004
003
002
WHEN TRUNCATION IS USED,
ERROR FROM IDEAL 8-BIT = 3/8 LSB
DUE TO NON-IDEAL QUANTIZATION.
001
000
000 _1/2
2 1/2
4 1/2
6 1/2
8 1/2
INPUT VOLTAGE
1 1/2
3 1/2
5 1/2
7 1/2
9 1/2
REPRESENTED AS 10-BIT
INPUT VOLTAGE
1/2
1 1/2
2 1/2
REPRESENTED AS 8-BIT
Figure 16-3. 8-Bit Truncation Mode Error
16.4.6 Monotonicity
The conversion process is monotonic and has no missing codes.
MC68HC908LJ24/LK24 — Rev. 2.1
Freescale Semiconductor
Analog-to-Digital Converter (ADC)
Data Sheet
339