QADC64E Legacy Mode Operation
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
13-35
13.4.1.1
Conversion Cycle Times
Total conversion time is made up of initial sample time, final sample time, and resolution time. Initial
sample time refers to the time during which the selected input channel is coupled through the buffer
amplifier to the sample capacitor. This buffer is used to quickly reproduce its input signal on the sample
capacitor and minimize charge sharing errors. During the final sampling period the amplifier is bypassed,
and the multiplexer input charges the sample capacitor array directly for improved accuracy. During the
resolution period, the voltage in the sample capacitor is converted to a digital value and stored in the SAR.
Initial sample time is fixed at two QCLK cycles. Final sample time can be 2, 4, 6, 8, or 16 QCLK cycles,
depending on the value of the IST field in the CCW. Resolution time is ten QCLK cycles.
Therefore, conversion time requires a minimum of 14 QCLK clocks (7 µs with a 2.0-MHz QCLK). If the
maximum final sample time period of 16 QCLKs is selected, the total conversion time is 28 QCLKs or 14
µs (with a 2.0-MHz QCLK)
illustrates the timing for conversions.
Figure 13-21. Conversion Timing
13.4.1.2
Amplifier Bypass Mode Conversion Timing
If the amplifier bypass mode is enabled for a conversion by setting the amplifier bypass (BYP) bit in the
CCW, the timing changes to that shown in
.
The buffered sample time is eliminated, reducing
the potential conversion time by two QCLKs. However, due to internal RC effects, a minimum final
sample time of four QCLKs must be allowed. This results in no savings of QCLKs. When using the bypass
mode, the external circuit should be of low source impedance, typically less than 10 k
Ω
. Also, the loading
effects of the external circuitry by the QADC64E need to be considered, since the benefits of the sample
amplifier are not present.
NOTE
Because of internal RC time constants, a sample time of two QCLKs in
bypass mode for high frequency operation is not recommended.
BUFFER
Sample
Time
Final Sample
Time
Resolution
Time
Sample TIME
Successive Approximation Resolution
Sequence
2 cycles
N cycles:
10 cycles
QCLK
(2, 4, 8, 16)
Summary of Contents for MPC561
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