am79c864a Advanced Micro Devices, am79c864a Datasheet - Page 30

• Manufacturer Part Number: am79c864a
• Description: Physical Layer Controller With Scrambler Plc-s
• Manufacturer: Advanced Micro Devices

Interrupt Mask Register (INTR_MASK)
The Interrupt Mask Register (INTR_MASK) has ad-
dress 02 (hex). It is readable and writeable. It allows the
disabling of interrupts caused by specific events. The
INTR_MASK contains a bit that corresponds to each bit
of the INTR_EVENT register that, when clear, prohibits
that condition from causing an interrupt to the node
Addr
(Hex)
Built In Self Test Register
In addition to a bit in the PLC_CNTRL_A register and a
bit in the INTR_EVENT register, Built In Self Test re-
quires one register.
Built In Self Test Signature Register
(BIST_SIGNATURE)
The Built In Self Test Signature register (BIST_SIGNA-
TURE) has address 15 (hex). It is a 16 bit, read-only reg-
ister that contains the resultant signature after execution
of the chip’s self test. After BIST has been completed
(signaled by the INT pin being asserted), this register
should be read and its contents compared against the
known good signature for the PLC-S to determine
whether the chip has passed its self test. The value of
the BIST signature is 5B6B (hex).
Framer
The Framer accepts five bit wide parallel data as well as
the recovered clock from the Physical Data Receiver
(PDR) chip. Generally, data received by the Framer is
not framed into proper FDDI symbols. The Framer is
used to align the incoming data to form proper symbols
before the data is passed onto the Elasticity Buffer. A
starting delimiter that is used at the beginning of each
frame is detected by the Framer and used to determine
proper symbol boundaries for the data. The Framer has
been designed such that the starting delimiter (the JK
symbol pair) can be detected independent of previous
framing.
Elasticity Buffer
The purpose of the Elasticity Buffer is to perform the
necessary buffering in order to allow the passing of data
between different FDDI stations with independent sta-
tion clocks. The Elasticity Buffer consists of an 80-bit
buffer and some control circuitry. The buffer is used to
compensate for the differences in the transmit and re-
ceive clock frequencies in the station. Data is clocked
into the buffer by the recovered byte clock and clocked
out of the buffer by the byte clock. The recovered clock is
3-32
02
AMD
NP
ERR
15
LSDO
14
IE
CTR
13
MINI
CTR
12
The SUPERNET 2 Family for FDDI 1994 Data Book
VSYM
CTR
11
PHYINV
10
EBUF
ERR
P R E L I M I N A R Y
9
INTR-MASK
TNE
EXPIRED
8
TPC
EXPIRED
processor. For each set bit, the setting of the corre-
sponding bit in the INTR_EVENT will generate an inter-
rupt to the node processor via the INT pin of the PLC-S.
Note however, that the operation of a bit in the
INTR_EVENT remains unchanged by the state of the
corresponding bit in the INTR_MASK. All bits of this reg-
ister are cleared with the assertion of RST.
also used to drive all the input circuitry including the in-
put controller and the local input pointer. The byte clock
is used to drive the output circuitry including the output
pointer, the output controller, the overflow/underflow
detection circuitry and the output buffer. Note that the
Elasticity Buffer uses a different version of the byte clock
than the rest of the PLC-S chip. This version is gener-
ated on chip from LSCLK.
Smoother Operation
The Smoother resides in the Elasticity Buffer. The main
purpose of the Smoother is to add and delete Idle sym-
bols into the data stream when the Smoother detects an
inadequate or a surplus number of Idles between
frames.
The Smoother function is necessary because the Elas-
ticity Buffer may delete symbols from the preamble of a
frame. If multiple PHY Elasticity Buffers delete symbols
from the same preamble, then the number of Idle sym-
bols in that preamble can reach a value resulting in a
loss of that frame. This may happen because according
to the ANSI PHY document,
The Smoother absorbs surplus symbols from longer
preambles and redistributes them into shorter pream-
bles. The smoothing function is capable of inserting ad-
ditional preamble symbols into repeated preambles
shorter than fourteen symbols. The Smoother attempts
to maintain 7 Idle bytes (or 14 Idle symbols) between
frames. If there are less than 7 Idle bytes the smoother
may inject Idle bytes onto the data path. If there are
more than 7 Idle bytes in the preamble the Smoother
may delete at least one Idle byte of the longer preamble.
7
An Elasticity Buffer is not required to recenter on
preambles shorter than four symbols
MAC is not required to repeat frames with pream-
bles shorter than two symbols
MAC is not required to copy frames with pream-
bles shorter than twelve symbols
PCM
ENABLED
6
PCM
BREAK
5
SELF
TEST
4
TRACE
PROP
3
PCM
CODE
2
LS
MATCH
1
15535B-13
PARITY
ERR
0