EVAL-ADUC831QSZ Datasheet, PDF (25/76 Pages) Analog Devices – MicroConverter®, 12-Bit ADCs and DACs with Embedded 62 kBytes Flash MCU

ADuC831
The DMA logic operates from the ADC clock and uses
pipelining to perform the ADC conversions and access the
external memory at the same time. The time it takes to perform
one ADC conversion is called a DMA cycle. The actions per-
formed by the logic during a typical DMA cycle are shown in
the following diagram.
CONVERT CHANNEL READ DURING PREVIOUS DMA CYCLE
WRITE ADC RESULT
CONVERTED DURING
PREVIOUS DMA CYCLE
READ CHANNEL ID
TO BE CONVERTED DURING
NEXT DMA CYCLE
DMA CYCLE
Figure 16. DMA Cycle
From the previous diagram, it can be seen that during one DMA
cycle the following actions are performed by the DMA logic:
1. An ADC conversion is performed on the channel whose ID
was read during the previous cycle.
2. The 12-bit result and the channel ID of the conversion per-
formed in the previous cycle is written to the external memory.
3. The ID of the next channel to be converted is read from
external memory.
For the previous example, the complete flow of events is shown
in Figure 16. Because the DMA logic uses pipelining, it takes
three cycles before the first correct result is written out.
Micro Operation during ADC DMA Mode
During ADC DMA mode the MicroConverter core is free to
continue code execution, including general housekeeping and
communication tasks. However, note that MCU core accesses
to Ports 0 and 2 (which of course are being used by the DMA
controller) are gated “OFF” during ADC DMA mode of
operation. This means that even though the instruction that
accesses the external Ports 0 or 2 will appear to execute, no data
will be seen at these external ports as a result. Note that during
DMA the internally contained XRAM Ports 0 and 2 are
available for use.
The only case in which the MCU will be able to access XRAM
during DMA, is when the internal XRAM is enabled and the
section of RAM to which the DMA ADC results are being writ-
ten to lies in an external XRAM. Then the MCU will be able to
access the internal XRAM only. This is also the case for use of
the extended stack pointer.
The MicroConverter core can be configured with an interrupt to
be triggered by the DMA controller when it had finished filling
the requested block of RAM with ADC results, allowing the
service routine for this interrupt to postprocess data without any
real-time timing constraints.
ADC Offset and Gain Calibration Coefficients
The ADuC831 has two ADC calibration coefficients, one for
offset calibration and one for gain calibration. Both the offset and
gain calibration coefficients are 14-bit words, and are each stored
in two registers located in the Special Function Register (SFR)
area. The offset calibration coefficient is divided into ADCOFSH
(six bits) and ADCOFSL (eight bits) and the gain calibration
coefficient is divided into ADCGAINH (six bits) and ADCGAINL
(eight bits).
The offset calibration coefficient compensates for dc offset errors
in both the ADC and the input signal. Increasing the offset coeffi-
cient compensates for positive offset, and effectively pushes the
ADC transfer function down. Decreasing the offset coefficient
compensates for negative offset, and effectively pushes the ADC
transfer function up. The maximum offset that can be compensated
is typically ± 5% of V_REF, which equates to typically ± 125 mV
with a 2.5 V reference.
Similarly, the gain calibration coefficient compensates for dc gain
errors in both the ADC and the input signal. Increasing the gain
coefficient compensates for a smaller analog input signal range
and scales the ADC transfer function up, effectively increasing
the slope of the transfer function. Decreasing the gain coefficient,
compensates for a larger analog input signal range and scales the
ADC transfer function down, effectively decreasing the slope of
the transfer function. The maximum analog input signal range
for which the gain coefficient can compensate is 1.025 Ï« V_REF
and the minimum input range is 0.975 Ï« V_REF which equates
to typically ± 2.5% of the reference voltage.
CALIBRATING THE ADC
There are two hardware calibration modes provided which can
be easily initiated by user software. The ADCCON3 SFR is
used to calibrate the ADC. Bit 1 (TYPICAL) and the CS3 to
CS0 (ADCCON2) set up the calibration modes.
Device calibration can be initiated to compensate for significant
changes in operating conditions frequency, analog input range,
reference voltage and supply voltages. In this calibration mode,
offset calibration uses internal AGND selected via ADCCON2
register bits CS3–CS0 (1011) and gain calibration uses internal
V_REF selected by CS3–CS0 (1100). Offset calibration should be
executed first, followed by gain calibration.
System calibration can be initiated to compensate for both inter-
nal and external system errors. To perform system calibration
using an external reference, tie system ground and reference to
any two of the six selectable inputs. Enable external reference
mode (ADCCON1.6). Select the channel connected to AGND
via CS3–CS0 and perform system offset calibration. Select the
channel connected to V_REF via CS3–CS0 and perform system
gain calibration.
The ADC should be configured to use settings for an ADCCLK
of divide by 16 and 4 acquisition clocks.
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