27-74
MCF5282 User’s Manual
MOTOROLA
Signal Connection Considerations
• The external capacitor is perfect (no leakage, no significant dielectric absorption
characteristics, etc.).
• All parasitic capacitance associated with the input signal is included in the value of
the external capacitor.
• Inductance is ignored.
• The “on” resistance of the internal switches is 0 ohms and the “off” resistance is
infinite.
27.9.7.1 Settling Time for the External Circuit
The values for R
SRC
, R
F
, and C
F
in the user's external circuitry determine the length of time
required to charge C
F
to the source voltage level (V
SRC
). At time t = 0, V
SRC
changes in
Figure 27-53 while S1 is open, disconnecting the internal circuitry from the external
circuitry. Assume that the initial voltage across C
F
is 0. As C
F
charges, the voltage across
it is determined by the equation, where t is the total charge time:
As t approaches infinity, V
CF
will equal V
SRC
. (This assumes no internal leakage.) With
10-bit resolution, 1/2 of a count is equal to 1/2048 full-scale value. Assuming worst case
(V
SRC
= full scale), Table 27-24 shows the required time for C
F
to charge to within 1/2 of
a count of the actual source voltage during 10-bit conversions. Table 27-24 is based on the
RC network in Figure 27-53.
NOTE
The following times are completely independent of the A/D
converter architecture (assuming the QADC is not affecting the
charging).
The external circuit described in Table 27-24 is a low-pass filter. Measurements of an AC
component of the external signal must take the characteristics of this filter into account.
Table 27-24. External Circuit Settling Time to 1/2 LSB
Filter Capacitor (CF)
Source Resistance (R
F
+ R
SRC
)
100
Ω
1 k
Ω
10 k
Ω
100 k
Ω
1
µ
F
760
µ
s
7.6 ms
76 ms
760 ms
0.1
µ
F
76
µ
s
760
µ
s
7.6 ms
76 ms
0.01
µ
F
7.6
µ
s
76
µ
s
760
µ
s
7.6 ms
0.001
µ
F
760 ns
7.6
µ
s
76
µ
s
760
µ
s
100 pF
76 ns
760 ns
7.6
µ
s
76
µ
s
VCF = VSRC (1 –e–t/(RF + RSRC) CF)
Summary of Contents for ColdFire MCF5281
Page 124: ...3 20 MCF5282 User s Manual MOTOROLA EMAC Instruction Set Summary ...
Page 141: ...MOTOROLA Chapter 5 Static RAM SRAM 5 5 SRAM Programming Model ...
Page 142: ...5 6 MCF5282 User s Manual MOTOROLA SRAM Programming Model ...
Page 168: ...6 26 MCF5282 User s Manual MOTOROLA Interrupts ...
Page 186: ...7 18 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 228: ...9 22 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 246: ...10 18 MCF5282 User s Manual MOTOROLA Low Power Wakeup Operation ...
Page 254: ...11 8 MCF5282 User s Manual MOTOROLA Memory Map and Registers ...
Page 264: ...12 10 MCF5282 User s Manual MOTOROLA Chip Select Registers ...
Page 280: ...13 16 MCF5282 User s Manual MOTOROLA Misaligned Operands ...
Page 314: ...14 34 MCF5282 User s Manual MOTOROLA MCF5282 External Signals ...
Page 339: ...MOTOROLA Chapter 15 Synchronous DRAM Controller Module 15 25 SDRAM Example ...
Page 340: ...15 26 MCF5282 User s Manual MOTOROLA SDRAM Example ...
Page 356: ...16 16 MCF5282 User s Manual MOTOROLA DMA Controller Module Functional Description ...
Page 408: ...17 52 MCF5282 User s Manual MOTOROLA Buffer Descriptors ...
Page 446: ...20 24 MCF5282 User s Manual MOTOROLA Interrupts ...
Page 474: ...22 18 MCF5282 User s Manual MOTOROLA Programming Model ...
Page 510: ...23 36 MCF5282 User s Manual MOTOROLA Operation ...
Page 526: ...24 16 MCF5282 User s Manual MOTOROLA I2C Programming Examples ...
Page 672: ...28 12 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 718: ...29 46 MCF5282 User s Manual MOTOROLA Motorola Recommended BDM Pinout ...
Page 750: ...32 8 MCF5282 User s Manual MOTOROLA Ordering Information ...