FINAL
Am7992B
Serial Interface Adapter (SIA)
DISTINCTIVE CHARACTERISTICS
s
Compatible with lEEE 802.3/Ethernet/Cheapernet
specifications
s
Crystal/TTL oscillator-controlled Manchester
encoder
s
Manchester decoder acquires clock and data
within four bit times with an accuracy of
±
3 ns
s
Guaranteed carrier and collision detection
squelch threshold limits
— Carrier/collision detected for inputs greater than
–275 mV
— No carrier/collision for inputs less than –175 mV
s
Input signal conditioning rejects transient noise
— Transients <10 ns for collision detector inputs
— Transients <20 ns for carrier detector inputs
s
Receiver decodes Manchester data with worst
case
±
19 ns of clock jitter (at 10 MHz)
s
TTL-compatible host interface
s
Transmit accuracy +0.01% (without adjustments)
GENERAL DESCRIPTION
The Am7992B Serial Interface Adapter (SIA) is a
Manchester encoder/decoder compatible with IEEE
802.3, Cheapernet, and Ethernet specifications. In an
IEEE 802.3/Ethernet application, the Am7992B inter-
faces the Am7990 Local Area Network Controller for
Ethernet (LANCE) to the Ethernet transceiver device,
acquires clock and data within four bit times, and de-
codes Manchester data with worst case
±
19 ns phase
jitter at 10 MHz. SIA provides both guaranteed signal
threshold limits and transient noise suppression cir-
cuitry in both data and collision paths to minimize false
start conditions.
BLOCK DIAGRAM
Receive Data (RX)
Receive Clock (RCLK)
Manchester
Decoder
Data
Receiver
Receive+
Controller Interface
Collision (CLSN)
Collision
Detect
Noise
Reject
Filter
Collision+
Collision–
Transmit+
Transmit–
Transmit Data (TX)
Transmit Enable (TENA)
Transmit Clock (TCLK)
XTAL
1
20 MHz
XTAL
2
Publication#
03378
Rev:
I
Issue Date:
May 1993
Amendment/0
Manchester
Encoder
Crystal
OSC
03378I-1
Transceiver Interface
Carrier Present (RENA)
Carrier
Detect
Noise
Reject
Filter
Receive–
1
PIN DESCRIPTION
CLSN
Collision (Output, TTL Active HIGH)
Signals at the Collision
±
terminals meeting threshold
and pulse-width requirements will produce a logic
HIGH at CLSN output. When no signal is present at
Collision
±
, CLSN output will be LOW.
TCLK
Transmit Clock (Output)
MOS/TTL output. TCLK provides symmetrical HIGH
and LOW clock signals at data rate for reference timing
of data to be encoded. It also provides clock signals for
the controller chip (Am7990—LANCE) and an internal
timing reference for receive path voltage-controlled
oscillators.
RX
Receive Data (Output)
A MOS/TTL output, recovered data. When there is no
signal at Receive
±
and TEST is HIGH, RX is HIGH. RX
is actuated with RCLK and remains active until RENA
is deasserted at the end of the message. During recep-
tion, RX is synchronous with RCLK and changes after
the rising edge of RCLK. When TEST is LOW, RX is
enabled.
Transmit+, Transmit–
Transmit (Outputs)
A differential line output. This line pair is intended to op-
erate into terminated transmission lines. For signals
meeting setup and hold time to TCLK at TENA and TX,
Manchester clock and data are outputted at Transmit+/
Transmit–. When operating into a 78
Ω
terminated
transmission line, signaling meets the required output
levels and skew for both Ethernet and IEEE 802.3 drop
cables.
RENA
Receive Enable (Output, TTL Active HIGH)
When there is no signal at Receive+, RENA is LOW.
Signals meeting threshold and pulse-width “on” re-
quirements will produce a logic HIGH at RENA. When
RENA is HIGH, Receive+ signals meeting threshold
and pulse-width “off” requirements will produce a LOW
at RENA.
Receive+, Receive–
Receiver (Inputs)
A differential input. A pair of internally biased line re-
ceivers consisting of a carrier detect receiver with offset
threshold and noise filtering to detect the line activity,
and a data recovery receiver with no offset for
Manchester data decoding.
RCLK
Receive Clock (Output)
A MOS/TTL output, recovered clock. When there is no
signal at Receive
±
and TEST is HIGH, RCLK is LOW.
RCLK is activated 1/4 bit time after the second negative
Manchester preamble clock transition at Receive
±
and
remains active until after an end of message. When
TEST is LOW, RCLK is enabled and meets minimum
pulse-width specifications.
Collision+, Collision–
Collision (Inputs)
A differential input. An internally biased line receiver
input with offset threshold and noise filtering. Signals at
Collision
±
have no effect on data-path functions.
TSEL
Transmit Mode Select (Output, Open Collector;
Input, Sense Amplifier)
s
TSEL LOW: Idle transmit state Transmit+ is positive
with respect to Transmit–.
s
TSEL HIGH: Idle transmit state Transmit+ and
Transmit– are equal, providing “zero” differential to
operate transformer-coupled loads.
When connected with an RC network, TSEL is held
LOW during transmission. At the end of transmission
the open collector output is disabled, allowing TSEL to
rise and provide a smooth transmission from logic
HIGH to “zero” differential idle. Delay and output return
to zero are externally controlled by the RC network at
TSEL and Transmit
±
load inductance.
TX
Transmit (Input)
TTL-compatible input. When TENA is HIGH, signals at
TX meeting setup and hold time to TCLK will be
encoded as normal Manchester at Transmit+ and
Transmit–.
s
TX HIGH: Transmit+ is negative with respect to
Transmit– for first half of data bit cell.
s
TX LOW: Transmit+ is positive with respect to
Transmit– for first half of data bit cell.
TENA
Transmit Enable (Input)
TTL-compatible input. Active HIGH data encoder
enable. Signals meeting setup and hold time to TCLK
will allow encoding of Manchester data from TX to
Transmit+ and Transmit–.
4
Am7992B
X
1
, X
2
Biased Crystal Oscillator (Input)
X
1
is the input and X
2
is the bypass port. When con-
nected for crystal operation, the system clock that ap-
pears at TCLK is half the frequency of the crystal
oscillator. X
1
may be driven from an external source of
two times the data rate.
TEST
Test Control (Input)
A static input that is connected to V
CC
for Am7992B/
Am7990 operation and to ground for testing of
Receive
±
path threshold and RCLK output HIGH
parameters. When TEST is grounded, RX is enabled
and RCLK is enabled except during clock acquisition,
when RCLK is HIGH.
RF
Frequency Setting Voltage-Controlled Oscillator
(V
CO
) Loop Filter (Output)
This loop filter output is a reference voltage for the re-
ceive path phase detector. It also is a reference for tim-
ing noise immunity circuits in the collision and receive
enable path. Nominal reference V
CO
gain is 1.25 TCLK
frequency MHz/V.
GND1
High Current Ground
GND2
Logic Ground
GND3
Voltage-Controlled Oscillator Ground
PF
Receive Path V
CO
Phase-Locked Loop Filter (Input)
This loop filter input is the control for receive path loop
damping. Frequency of the receive V
CO
is internally lim-
ited to transmit frequency
±
12%. Nominal receive V
CO
gain is 0.25 reference V
CO
gain MHz/V.
V
CC1
High Current and Logic Supply
V
CC2
Voltage-Controlled Oscillator Supply
Am7992B
5
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