SP5654
FUNCTIONAL DESCRIPTION
The SP5654 contains all the elements necessary, with the
exception of reference crystal, loop filter and external high
voltage transistor, to control a voltage controlled local
oscillator, so forming a PLL frequency synthesised source.
The system is controlled by a microprocessor via a
standard data, clock and enable three–wire bus. The data load
consists of a single word, which contains the frequency and
port information, and is only transferred to the internal data
shift register during an enable high period. The clock is
disabled during low periods. New data words are only
accepted by the internal data buffers from the shift register on
a negative transition of the enable, so giving improved fine
tuning facility for digital AFC etc.
The device has 5 modes of operation, as defined in Table
1, and each of these modes can accept either 18–bit or 19–bit
data entry. The format of the data entry is shown in Fig. 4, and
consists of 4–bits for port switching, plus 14/15 bits to control
the 15–bit programmable divider. For 18–bit data entry (4+14),
the MSB of the 15–bit programmable divider is internally set to
logic ‘0’ effectively making the divider 14–bits. The device
recognises the data entry as 18–bit when a falling edge at the
enable input occurs during the 18th clock period. The device
associates falling enable edges during the 19th clock period
with 19–bit data entry. A falling edge at the enable input before
the 18th clock period constitutes invalid data entry to the
device.
The frequency is set by first selecting the required mode of
operation as detailed in Table 1, and then by loading the
programmable divider with the required 14/15–bit divisor
word. The output of this divider, F
PD
, is fed to the phase
comparator where it is compared in phase and frequency to
the internally generated comparison frequency, F
COMP
.
The comparison frequency F
COMP
is obtained by dividing
the output of the on–chip crystal controlled oscillator. The
crystal frequency generally used is 4MHz, giving an F
COMP
of
7.8125kHz in mode 4, which when multiplied back up to the LO
gives a minimum step size of 125kHz.
The programmable divider is preceded by an input RF
preamplifier and high speed low radiation prescaler. The
preamplifier is arranged to be self oscillating, so giving
excellent input sensitivity. The input impedance and sensitivity
are shown in Fig. 2 and 6 respectively.
The device contains a lock detect circuit which generates
a flag when the loop has attained lock. The ‘in lock’ condition
is indicated by a high impedance state.
The charge pump current is initially set to
150mA.
When
the device attains frequency lock, the charge pump current is
switched to
50mA,
so improving the local oscillator short term
jitter.
The device also contains four general purpose open
collector output ports P0–P3. These outputs are each capable
of sinking a minimum of 10mA, when the appropriate bits
P0–P3 of the programming data, see Fig. 4 are set to a logic
‘1’.
PIN and PROGRAMMING COMPATIBILITY
The SP5654 may be used in SP5655 applications which
require 3–wire bus as opposed to I
2
C bus data format. In
SP5655 applications where the reference crystal is grounded
to pin 3, a small modification is required to ground the crystal
as shown in Fig. 5.
Appropriate connections must also be to the mode select
input (see Table 1). For each mode of operation, the SP5654
is programming and step size compatible with Toshiba
devices as shown in Table 3.
TEST FEATURES
Charge pump disable
The charge pump may be disabled by sourcing current
from the data input, i.e. by forcing a negative input voltage.
Varactor line disable
The charge pump amplifier drive output may be disabled by
sourcing current from the enable input, i.e. by forcing a
negative voltage.
Device test mode
Further test modes can be invoked by sourcing current
from the clock input, i.e. by forcing a negative input voltage.
These test modes when invoked are determined by the data
held in the P1, P2 and P3 internal registers as detailed in Table
2.
MODE
‘MODE SELECT’
INPUT VOLTAGE
PROGRAMMABLE
DIVIDER BIT
LENGTH
14/15
14/15
14/15
14/15
14/15
REFERENCE
DIVIDER RATIO
*FREQUENCY
STEP SIZE
(kHz)
125
50
62.5
31.25
100
*MAXIMUM
OPERATING
FREQUENCY (GHz)
14 bit
15 bit
2.7000
1.6383
2.0479
1.0239
2.7000
2.0479
0.8191
1.0239
0.5119
1.6383
4
3
2
1
0
0.85 V
CC
– V
CC
0.65 V
CC
– 0.75V
CC #
OPEN CIRCUIT
0.25 V
CC –
0.35V
CC
[
0 – 0.15 V
CC
512
1280
1024
2048
640
*When used with a 4MHz crystal
# Selected by connecting a 15kW resistor to V
CC
[
Selected by connecting a 15kW resistor to V
EE
Table
1. Modes of operation
5
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
