22
1
VMIVME-3126A High Resolution, Isolated Analog-to-Digital Converter Board
channels, the
SYS RECONFIG
bit in the BCR will be cleared and
$0000
written to the
BRR. When a VMEbus read indicates
$0000
, the user knows the
Reconfiguration Mode
is
complete and the board is accessible. Figure 1-3 on page 25 shows the reconfiguration
flowchart from the user’s standpoint. Reconfiguration of a single channel takes
approximately 44 msec. Reconfiguring all 16 channels takes approximately 101 msec.
Autozero Mode
The board enters the
Autozero Mode
when the
Autozero
bit in the BCR is set by the
user. An input switch disconnects the field inputs and connects analog ground to the
inputs. The present range of each channel is still active. An average of 1024 samples
are collected by the DSP and this value is compared to the expected reading for a
grounded input. The offset coefficient is adjusted by the difference in the two values.
The advantage of this mode of operation is correcting errors in offset due to
temperature variations without having to invoke a calibration cycle.
The new offset coefficients are used by the DSP and are not written to the E
2
PROM
unless the user sets the
WRITE COEF
bit in the BCR after this mode is complete. The
Autozero
bit in the BCR is reset by the DSP when the new coefficients are determined,
and is an indication that the board has resumed normal operation.
NOTE:
Polling the
Autozero
bit in the BCR will adversely affect calculation of the
new coefficients. The Autozero Operation takes approximately 11 seconds to
complete. After this time elapse, read the BCR to check that the
Autozero
bit has been
reset.
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