Datasheet

AD9957 Data Sheet
Rev. C | Page 32 of 64
START ADDRESS
RAM
ADDRESS
END ADDRESS
1
1 PDCLK CYCLE
OR
M DDS CLOCK CYCLES
Δ
t
I/O_UPDATE OR
RT TRANSITION
2
3
1
Δ t
06384-025
Figure 45. Continuous Bidirectional Ramp Timing Diagram
RAM Continuous Bidirectional Ramp Mode
In continuous bidirectional ramp mode, upon assertion of an
I/O update or a state change on the RT pin, the RAM begins
playback operation using the parameters programmed into the
selected RAM segment register. Data is extracted from RAM
over the specified address range contained in the start address
and end address. The data is delivered at the appropriate rate
and to the destination as specified by the RAM playback
destination bit.
The playback rate is governed by the timer internal to the RAM
state machine and its period (Δt) is determined by the state of
the RAM playback destination bit as detailed in the RAM
Playback Operation section.
After initialization, the internal state machine begins extracting
data from the RAM at the start address of the active RAM segment
register and increments the address counter until it reaches the
end address, at which point the state machine reverses the direc-
tion of the address counter and begins decrementing through
the address range. Whenever one of the terminal addresses is
reached, the state machine reverses the address counter; the
process continues indefinitely.
Note that a change in state of the RT pin aborts the current
waveform and the newly selected RAM segment register is used
to initiate a new waveform.
A graphic representation of the continuous bidirectional ramp
mode is shown in Figure 45. The circled numbers in Figure 45
indicate specific events, explained as follows:
Event 1—an I/O update or state change on the RT pin has
activated the RAM continuous bidirectional ramp mode. The
state machine initializes to the start address of the active RAM
segment register. The state machine begins incrementing
through the specified address range.
Event 2—the state machine reaches the end address of the active
RAM segment register.
Event 3—the state machine reaches the start address of the
active RAM segment register.
The continuous bidirectional ramp continues indefinitely until
the next I/O update or state change on the RT pin.