Datasheet

AD9953
Rev. A | Page 23 of 32
nt of 1
he first method of loading is by changing the OSK input pin.
hen the OSK input pin changes state, the ASFR value is
aded into the ramp rate timer, which then proceeds to count
own as normal.
he second method in which the sweep ramp rate timer can be
aded before reaching a count of 1 is if the load OSK timer bit
FR1<26>) is set and an I/O UPDATE is issued.
he third method in which the sweep ramp rate timer can be
oaded before reaching a count of 1 is when going from the
active auto shaped on-off keying mode to the active auto
aped on-off keying mode; that is, when the sweep enable bit is
eing set.
OSK PIN
SYN
TO D
LE
>
OSK Ramp Rate Timer
The OSK ramp rate timer is a loadable down counter, which
generates the clock signal to the 14-bit counter that generates
the internal scale factor. The ramp rate timer is loaded with the
value of the ASFR every time the counter reaches 1 (decimal).
This load and countdown operation continues for as long as the
timer is enabled, unless the timer is forced to load before
reaching a count of 1.
If the load OSK timer bit (CFR1<26>) is set, the ramp rate
timer is loaded upon an I/O UPDATE or upon reaching a value
a cou
T
W
lo
d
T
lo
(C
T
l
in
sh
of 1. The ramp timer can be loaded before reaching
by three methods.
b
DDS CORE
OSK ENAB
CFR<25
AMPLITUDE SCALE
FACTOR REGISTER
(ASF)
LOAD OSK TIMER
CFR1<26>
C_CLK
AUTO DESK
ENABLE
CFR1<24>
AC
0
0
1
03374-0-005
R
RAMP RATE TIMER
CLOCK
AUTO SCALE
CTOR GENERATOFA
01
01
HOLD
EC ENABLEINC/D
OUT
COS(X)
AMPLITUDE RAMP
RATE REGISTER
(ASF)
UP/DN
DATALOAD
EN
f Shaped Key
Figure 20. On-Of ing Block Diagram