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Enhanced Queued Analog-to-Digital Converter (EQADC)
MPC5644A Microcontroller Reference Manual, Rev. 6
1126
Freescale Semiconductor
25.6.6.3
ADC Sampling Delay after Power-Up
To guarantee accuracy specifications, a delay of at least 8 ms must be present between the power-up of the
VDDA supply and the start of the first ADC conversion. This delay allows internal ADC references to
settle. The accuracy of a conversion during the first 8 ms is not guaranteed by the specifications, however
conversion within the first 8 ms will be possible on the eQADC if this delay is not implemented in
software.
25.6.6.4
Time Stamp Feature
The on-chip ADCs can provide a time stamp for the conversions they execute. A time stamp is the value
of the time base counter latched when the EQADC detects the end of the analog input voltage sampling.
A time stamp for a conversion command is requested by setting the TSR bit in the corresponding
command. When TSR is negated, that is a time stamp is not requested, the ADC returns a single result
message containing the conversion result. When TSR is asserted, that is a time stamp is requested, the
ADC returns two result messages; one containing the conversion result, and afterwards another containing
the time stamp for that conversion. The result messages are sent in this order to the RFIFOs and both
messages are sent to the same RFIFO was specified in the MESSAGE_TAG field of the executed
conversion command.
The time stamp can be provided by an external source using the STAC bus interface (more details in
Section 25.6.6.4.1, STAC Client Submodule (REDLC)
) or by the internal time base counter. The selection
between the two sources is done by field ADC0/1_TBSEL in the ADC0/1_CR register or by field ATBSEL
in registers ADC_ACR1-8. Refer to
and
for selection details.
The time base counter is a 16-bit up counter that wraps after reaching 0xFFFF. It is disabled after reset and
it is enabled according to the setting of TBC_CLK_PS field in
Section 25.5.3.2, ADC Time Stamp Control
. TBC_CLK_PS defines if the counter is enabled or disabled, and, if enabled, at
what frequency it is incremented. The time stamps are returned regardless of whether the time base counter
is enabled or disabled. The time base counter can be reset by writing 0x0000 to the
Time Base Counter Registers (ADC_TBCR)
, with a write configuration command.
0b11101
0
60
2.0 MHz
133 Ksps
125 Ksps
1
61
1.97 MHz
131 Ksps
123 Ksps
0b11110
0
62
1.94 MHz
129 Ksps
121 Ksps
1
63
1.90 MHz
127 Ksps
119 Ksps
0b11111
0
64
1.88 MHz
125 Ksps
117 Ksps
1
65
1.85 MHz
123 Ksps
115 Ksps
Table 25-65. ADC Clock Configuration Example (System Clock Frequency=120 MHz) (continued)
ADC0/1_CLK_PS [0:4]
ADC0/1_
ODD_PS
System Clock
Divide Factor
ADC Clock
(System Clock =
120 MHz)
Differential
Conversion Speed
with Default
Sampling Time (2
cycles)
Single-Ended
Conversion Speed
with Default
Sampling Time (2
cycles)
Содержание MPC5644A
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