NJ8821
PROGRAMMING
Timing is generated externally, normally from a
microprocessor, and allows the user to change the data in
selected latches as defined by the data map Fig.5. The PE pin
is used as a strobe for the data: taking PE high causes data to
be transferred from the data pins (D0-D3) into the addressed
latch. Following the falling edge of PE, the data is retained in
the addressed latch and the data inputs are disabled. Data
transfer from all internal latches into the counters occurs
simultaneously with the transfer of data into latch 1, which
would therefore normally be the last latch addressed during
each channel change. Timing information for this mode of
operation is given in Fig. 6.
When re-programming, a reset to zero state is followed by
reloading with the new counter values. This means that the
synthesiser loop lock-up time is well defined and less than
10ms. If shorter lock-up times are are required when making
only small changes in frequency, the GPS NJ8823 (with non-
resettable counters) should be considered.
WORD DS2
1
2
3
4
5
6
7
8
0
0
0
0
1
1
1
1
DS1
0
0
1
1
0
0
1
1
DS0
0
1
0
1
0
1
0
1
D3
M1
M5
M9
A3
-
R3
R7
-
D2
M0
M4
M8
A2
A6
R2
R6
R10
D1
-
M3
M7
A1
A5
R1
R5
R9
D0
-
M2
M6
A0
A4
R0
R4
R8
Fig. 5 Data map
DS0-DS2
PE
D0 - D3
t
DS
t
SE
t
W(ST)
t
DH
t
HE
Fig. 6 Timing diagram
PHASE COMPARATORS
The digital phase/frequency detector drives a three-state
output, PDB, which provides a ‘coarse’ error signal to enable
fast switching between channels. The PDB output is active
until the phase error is within the sample and hold phase
detector, PDA, window, when PDB becomes high impedance.
Phase-lock is indicated at this point by a low level on LD. The
sample and hold phase detector provides a ‘fine’ error signal
to give further phase adjustment and to hold the loop in lock.
An internally generated ramp, controlled by the digital
output from both the reference and main divider chains, is
sampled at the reference frequency to give the ‘fine’ error
signal, PDA. When in phase lock, this output would be typically
at (V
DD
2V
SS
)/2 and any offset from this would be proportional
to phase error. The relationship between this offset and the
phase error is the phase comparator gain, which is
programmable with an external resistor, RB. An internal 50pF
capacitor is used in the sample and hold comparator.
CRYSTAL OSCILLATOR
When using the internal oscillator, the stability may be
enhanced at high frequencies by the inclusion of a resistor
between pin 8 (OSC OUT) and the other components. A value
of 150-270Ω is advised.
PROGRAMMING/POWER UP
Data and signal input pins should not have input applied to
them prior to the application of V
DD
, as otherwise latch-up may
occur.
4
GEC [ GENERAL ELECTRIC COMPANY ]
