Datasheet

Data Sheet AD9910
Rev. D | Page 31 of 64
DRG OUTPUT
LOWER LIMIT
UPPER LIMIT
DRCTL
DRHOLD
AUTOCLEAR DIGITAL
RAMP ACCUMULATOR
CLEAR DIGITAL
RAMP ACCUMULATOR
I/O_UPDATE
POSITIVE
STEP SIZE
NEGATIVE
STEP SIZE
P DDS CLOCK CYCLES N DDS CLOCK CYCLES
1 DDS CLOCK CYCLE
DIGITAL RAMP ENABLE
DROVER
06479-020
CLEAR
RELEASE
AUTO
CLEAR
–Δ
t +Δ t
1 2 3
4 5 6
7 8 9
10
11
12
13
Figure 39. Normal Ramp Generation
Event 1—The digital ramp enable bit is set, which has no effect
on the DRG output because the bit is not effective until an I/O
update.
Event 2—An I/O update registers the enable bit. If DRCTL = 1
is in effect at this time (the gray portion of the DRCTL trace),
then the DRG output immediately begins a positive slope (the
gray portion of the DRG output trace). Otherwise, if DRCTL =
0, the DRG output is initialized to the lower limit.
Event 3—DRCTL transitions to a Logic 1 to initiate a positive
slope at the DRG output. In this example, the DRCTL pin is
held long enough to cause the DRG to reach its programmed
upper limit. The DRG remains at the upper limit until the ramp
accumulator is cleared, DRCTL = 0, or the upper limit is
reprogrammed to a higher value. In the last case, the DRG
immediately resumes its previous positive slope profile.
Event 4—DRCTL transitions to a Logic 0 to initiate a negative
slope at the DRG output. In this example, the DRCTL pin is
held long enough to cause the DRG to reach its programmed
lower limit. The DRG remains at the lower limit until DRCTL = 1,
or until the lower limit is reprogrammed to a lower value. In the
latter case, the DRG immediately resumes its previous negative
slope profile.
Event 5—DRCTL transitions to a Logic 1 for the second time,
initiating a second positive slope.
Event 6—The positive slope profile is interrupted by DRHOLD
transitioning to a Logic 1. This stalls the ramp accumulator and
freezes the DRG output at its last value.
Event 7—DRHOLD transitions to a Logic 0, releasing the ramp
accumulator and reinstating the previous positive slope profile.
Event 8—The clear digital ramp accumulator bit is set, which
has no effect on the DRG because the bit is not effective until an
I/O update is issued.
Event 9—An I/O update registers that the clear digital ramp
accumulator bit is set, resetting the ramp accumulator and
forcing the DRG output to the programmed lower limit. The
DRG output remains at the lower limit until the clear condition
is removed.
Event 10—The clear digital ramp accumulator bit is cleared,
which has no effect on the DRG output because the bit is not
effective until an I/O update is issued.
Event 11—An I/O update registers that the clear digital ramp
accumulator bit is cleared, releasing the ramp accumulator, and
the previous positive slope profile restarts.
Event 12—The autoclear digital ramp accumulator bit is set,
which has no effect on the DRG output because the bit is not
effective until an I/O update is issued.
Event 13—An I/O update registers that the autoclear digital
ramp accumulator bit is set, resetting the ramp accumulator.
However, with an automatic clear, the ramp accumulator is only
held reset for a single DDS clock cycle. This forces the DRG
output to the lower limit, but the ramp accumulator is immedi-
ately made available for normal operation. In this example, the
DRCTL pin remains a Logic 1; therefore, the DRG output
restarts the previous positive ramp profile.