3D7523 Datasheet, PDF (3/5 Pages) Data Delay Devices, Inc. – MONOLITHIC MANCHESTER ENCODER/DECODER

3D7523
APPLICATION NOTES (CONT’D)
DECODER
The Manchester decoder subsystem samples the
input at precise pre-selected intervals to retrieve
the data and to recover the clock from the
received data stream. Its architecture comprises
finely tuned delay elements and proprietary
circuitry which, in conjunction with other circuits,
implement the data decoding and clock recovery
function.
Typically, the encoded data transmitted from a
source arrives at the decoder corrupted. Such
corruption of the received data manifests itself as
jitter and/or pulse width distortion at the decoder
input. The instantaneous deviations from
nominal Baud Rate and/or Pulse Width (high or
low) adversely impact the data extraction and
clock recovery function if their published limits
are exceeded. See Table 4, Allowed Baud
Rate/Duty Cycle. The decoder, being a self-
timed device, is tolerant of frequency modulation
(jitter) present in the input data stream, provided
that the input data pulse width variations remain
within the allowable ranges.
The decoder presents at its outputs the decoded
data (inverted) and the recovered clock. The
decoded data is valid at the rising edge of the
clock.
The clock recovery function operates in two
modes dictated by the input data stream bit
sequence. When a data bit is succeeded by its
inverse, the clock recovery circuit is engaged and
forces the clock output low for a time equal
to one over twice the baud rate. Otherwise, the
input is presented at the clock output unchanged,
shifted in time. Therefore, the clock duty cycle is
strongly dependent on the baud rate, as this will
affect the clock-high duration.
The clock output falling edge is not operated on
by the clock recovery circuitry. It, therefore,
preserves more accurately the clock frequency
information embedded in the transmitted data. It
can therefore be used, if desired, to retrieve clock
frequency information.
INPUT SIGNAL CHARACTERISTICS
The 3D7523 inputs are TTL compatible. The
user should assure him/herself that the 1.5
volt TTL threshold is used when referring to all
timing, especially to the input clock duty cycle
(encoder) and the received data (decoder).
POWER SUPPLY AND
TEMPERATURE CONSIDERATIONS
CMOS integrated circuitry is strongly dependent
on power supply and temperature. The
monolithic 3D7523 Manchester encoder/decoder
utilizes novel and innovative compensation
circuitry to minimize timing variations induced by
fluctuations in power supply and/or temperature.
Nevertheless, optimum performance is achieved
by providing a stable power supply and a clean
ground plane, and by placing a bypass capacitor
(0.1uf typically) as close to the device as
possible.
ENCODED
0
1
0
1
1
0
0
1
RECEIVED
(RX)
CLOCK
(CLK)
DATA
(DATB)
DECODED
t_C
t_CL
t_{CW L}
t_CD
1
0
1
1
0
0
1
Figure 2: Timing Diagram (Decoder)
Doc #06003
DATA DELAY DEVICES, INC.
3
5/8/2006
3 Mt. Prospect Ave. Clifton, NJ 07013