am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 40

• Manufacturer Part Number: am79c940
• Description: Media Access Controller For Ethernet Mace
• Manufacturer: Advanced Micro Devices

below. The differential input impedance, Z
common-mode input impedance, Z
that the Ethernet specification for cable termination im-
pedance is met using standard 1% resistor terminators.
Collision Detection
A transceiver detects the collision condition on the net-
work and generates a differential signal at the CI in-
puts. This collision signal passes through an input stage
which detects signal levels and pulse duration. When
the signal is detected by the MENDEC it sets the CLSN
line HIGH. The condition continues for approximately
1.5 bit times after the last LOW-to-HIGH transition on
CI .
Jitter Tolerance Definition
The Receive Timing-Start of Reception Clock Acquisi-
tion waveform diagram shows the internal timing rela-
tionships implemented for decoding Manchester data in
the SIA module. The SIA utilizes a clock capture circuit
to align its internal data strobe with an incoming bit
stream. The clock acquisition circuitry requires four valid
bits with the values 1010. Clock is phase locked to the
negative transition at the bit cell center of the second “0”
in the pattern.
Since data is strobed at 1/4 bit time, Manchester transi-
tions which shift from their nominal placement through
1/4 bit time will result in improperly decoded data. With
this as the criteria for an error, a definition of “Jitter Han-
dling” is:
The peak deviation approaching or crossing 1/4 bit cell
position from nominal input transition, for which the SIA
section will properly decode data.
Attachment Unit Interface (AUI)
The AUI is the PLS (Physical Signaling) to PMA (Physi-
cal Medium Attachment) interface which effectively con-
nects the DTE to the MAU. The differential interface
provided by the MACE device is fully compliant to Sec-
tion 7 of ISO 8802-3 (ANSI/IEEE 802.3).
40
AMD
MACE
DI+
DI-
ICM
, are specified so
Differential Input Termination
40.2
IDF
, and the
Am79C940
0.01 F
If SIP devices are used, 39 ohms is also a suitable value.
The CI differential inputs are terminated in exactly the
same way as the DI pair.
After the MACE device initiates a transmission it will ex-
pect to see data looped-back on the DI pair (AUI port
selected). This will internally generate a carrier sense ,
indicating that the integrity of the data path to and from
the MAU is intact, and that the MAU is operating cor-
rectly. This carrier sense signal must be asserted during
the transmission when using the AUI port (DO trans-
mitting). If carrier sense does not become active in re-
sponse to the data transmission, or becomes inactive
before the end of transmission, the loss of carrier
(LCAR) error bit will be set in the Transmit Frame Status
(bit 7) after the packet has been transmitted.
Digital Attachment Interface (DAI )
The Digital Attachment Interface is a simplified electrical
attachment specification which allows MAUs which do
not require the DC isolation between the MAU and DTE
(e.g. devices compatible with the 10BASE-T Standard
and 10BASE-FL Draft document) to be implemented. All
data transferred across the DAI port is Manchester En-
coded. Decoding and encoding is performed by the
MENDEC.
The DAI port will accept receive data on the basis that
the RXCRS input is active, and will take the data pre-
sented on the RXDAT input as valid Manchester data.
Transmit data is sent to the external transceiver by the
MACE device asserting TXEN and presenting compli-
mentary data on the TXDAT pair. During idle, the
MACE device will assert the TXDAT+ line high, and the
TXDAT line low, while TXEN is maintained inactive
(high). The MACE device implements logical collision
detection and will use the simultaneous assertion of
TXEN and RXCRS to internally detect a collision condi-
tion, take appropriate internal action (such as abort the
current transmit or receive activity), and provide exter-
nal indication using the CLSN pin. Any external
40.2
AUI Isolation
Transformer
16235C-6
16907A-009A