
AD9910
The step interval is controlled by a 16-bit programmable timer
that is clocked at a rate of f
SYSCLK
. The period of the timer sets
the time interval between amplitude steps. The step time interval
(Δt) is given by
M
t
Δ
=
Rev. 0 | Page 27 of 60
SYSCLK
f
4
where
M
is the 16-bit number stored in the amplitude ramp rate
(ARR) portion of the ASF register. For example, if f
SYSCLK
=
750 MHz and M = 23218 (0x5AB2), then Δt ≈
123.8293 μs.
The output of the OSK function is a 14-bit unsigned data bus
that controls the amplitude parameter of the DDS (as long as
the OSK enable bit is set). When the OSK pin is set, the OSK
output value starts at 0 (zero) and increments by the pro-
grammed amplitude step size until it reaches the programmed
maximum amplitude value. When the OSK pin is cleared, the
OSK output starts at its present value and decrements by the
programmed amplitude step size until it reaches 0 (zero).
The OSK output does not necessarily attain the maximum
amplitude value if the OSK pin is switched to Logic 0 before the
maximum value is reached. Nor does the OSK output necessarily
reach a value of zero if the OSK pin is switched to Logic 1
before the zero value is reached.
The OSK output is initialized to 0 (zero) at power-up and reset
whenever the OSK enable bit or the select auto OSK bit is cleared.
The amplitude step size of the OSK output is set by the amplitude
step size bits in the ASF register according to Table 10. The step
size refers to the LSB weight of the 14-bit OSK output. Regardless
of the programmed step size, the OSK output does not exceed
the maximum amplitude value programmed into the ASF
register.
Table 10. OSK Amplitude Step Size
ASF<1:0>
00
01
10
11
Amplitude Step Size
1
2
4
8
As mentioned previously, a 16-bit programmable timer controls the
step interval. Normally, this timer is loaded with the programmed
timing value whenever the timer expires, initiating a new timing
cycle. However, there are three events that can cause reloading of
the timer to have its timing value reloaded prior to the timer
expiring. One such event is when the select auto
OSK bit is
transitioned from cleared to set followed by an I/O update. A
second such event is a change of state in the OSK pin. The third is
dependent on the status of the Load ARR @ I/O Update bit. If this
bit is cleared, then no action occurs, otherwise, when the
I/O_UPDATE pin is asserted (or a profile change occurs), the timer
is reset to its initial starting point.
DIGITAL RAMP GENERATOR (DRG)
DRG Overview
To sweep phase, frequency, or amplitude from a defined start
point to a defined endpoint, a completely digital, digital ramp
generator is included in the AD9910. The DRG makes use of
nine control register bits, three external pins, two 64-bit
registers, and one 32-bit register (see Figure 37).
DIGITAL RAMP LIMIT REGISTER
D
DDS CLOCK
D
D
DIGITAL RAMP RATE REGISTER
DIGITAL RAMP STEP REGISTER
0
TO DDS
SIGNAL
CONTROL
PARAMETER
DIGITAL RAMP ENABLE
DROVER PIN ACTIVE
LOAD LRR AT I/O_UPDATE
CLEAR DIGITAL
RAMP ACCUMULATOR
AUTOCLEAR DIGITAL
RAMP ACCUMULATOR
64
64
DIGITAL RAMP DESTINATION
2
DIGITAL RAMP NO-DWELL
2
32
32
DIGITAL
RAMP
GENERATOR
62
61
63
Figure 37. Digital Ramp Block Diagram
The primary control for the DRG is the digital ramp enable bit.
When disabled, the other DRG input controls are ignored and the
internal clocks are shut down to conserve power.
The output of the DRG is a 32-bit unsigned data bus that can be
routed to any one of the three DDS signal control parameters, as
controlled by the two digital ramp destination bits in Control
Function Register 2 according to Table 11. The 32-bit output
bus is MSB-aligned with the 32-bit frequency parameter, the
16-bit phase parameter, or the 14-bit amplitude parameter, as
defined by the destination bits. When the destination is phase
or amplitude, the unused LSBs are ignored.
Table 11. Digital Ramp Destination
Digital Ramp
Destination Bits
CFR2<21:20>
00
01
1x
1
DDS signal
Control
Parameter
Frequency
Phase
Amplitude
Bits Assigned to
DDS Parameter
31:0
31:16
31:18
1
x = don’t care.
The ramp characteristics of the DRG are fully programmable. This
includes the upper and lower ramp limits, and independent control
of the step size and step rate for both the positive and negative slope
characteristics of the ramp. A detailed block diagram of the DRG
appears in Figure 38.