MC908JL16CFAE FREESCALE [Freescale Semiconductor, Inc], MC908JL16CFAE Datasheet - Page 127
MC908JL16CFAE
Manufacturer Part Number
MC908JL16CFAE
Description
Microcontrollers
Manufacturer
FREESCALE [Freescale Semiconductor, Inc]
Datasheet
1.MC908JL16CFAE.pdf
(230 pages)
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The maximum total conversion time for a single conversion or the first conversion in continuous
conversion mode is determined by the clock source chosen and the divide ratio selected. The clock
source is selectable by the ADICLK and ACLKEN bits, and the divide ratio is specified by the ADIV bits.
For example, if the alternate clock source is 16 MHz and is selected as the input clock source, the input
clock divide-by-8 ratio is selected and the bus frequency is 4 MHz, then the conversion time for a single
10-bit conversion is:
9.3.4 Sources of Error
Several sources of error exist for ADC conversions. These are discussed in the following sections.
9.3.4.1 Sampling Error
For proper conversions, the input must be sampled long enough to achieve the proper accuracy. Given
the maximum input resistance of approximately 15 kΩ and input capacitance of approximately 10 pF,
sampling to within
1/4
maximum ADCK frequency) provided the resistance of the external analog source (R
kΩ. Higher source resistances or higher-accuracy sampling is possible by setting ADLSMP (to increase
the sample window to 23.5 cycles) or decreasing ADCK frequency to increase sample time.
9.3.4.2 Pin Leakage Error
Leakage on the I/O pins can cause conversion error if the external analog source resistance (R
If this error cannot be tolerated by the application, keep R
1/4
9.3.4.3 Noise-Induced Errors
System noise which occurs during the sample or conversion process can affect the accuracy of the
conversion. The ADC10 accuracy numbers are guaranteed as specified only if the following conditions
are met:
Freescale Semiconductor
•
•
•
•
•
•
LSB
LSB
There is a 0.1µF low-ESR capacitor from V
There is a 0.1µF low-ESR capacitor from V
If inductive isolation is used from the primary supply, an additional 1µF capacitor is placed from
V
V
The MCU is placed in wait mode immediately after initiating the conversion (next instruction after
write to ADCSC).
There is no I/O switching, input or output, on the MCU during the conversion.
(at 10-bit resolution) can be achieved within the minimum sample window (3.5 cycles / 2 MHz
leakage error (at 10-bit resolution).
DDA
SSA
Maximum Conversion time =
and V
to V
The ADCK frequency must be between f
maximum to meet A/D specifications.
SSA
REFL
(if available).
(if available) is connected to V
Number of bus cycles = 11.25 µs x 4 MHz = 45 cycles
MC68HC908JL16 Data Sheet, Rev. 1.1
21 ADCK cycles
16 MHz/8
NOTE
REFH
DDA
SS
to V
ADCK
to V
AS
at a quiet point in the ground plane.
SSA
lower than V
REFL
+
minimum and f
(if available).
3 bus cycles
(if available).
4 MHz
ADVIN
ADCK
/ (4096*I
= 11.25 µs
Functional Description
AS
Leak
) is kept below 10
) for less than
AS
) is high.
127
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