AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 30

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AM79C976

Manufacturer Part Number
AM79C976
Description
PCnet-PRO 10/100 Mbps PCI Ethernet Controller
Manufacturer
Advanced Micro Devices
Datasheet
RX_CLK
Receive Clock
RX_CLK is a clock input that provides the timing refer-
ence for the transfer of the RX_DV, RXD[3:0], and
RX_ER signals into the Am79C976 device. RX_CLK
must provide a nibble rate clock (25% of the network
data rate). Hence, an MII transceiver operating at 10
Mbps must provide an RX_CLK frequency of 2.5 MHz
and an MII transceiver operating at 100 Mbps must pro-
vide an RX_CLK frequency of 25 MHz. When the exter-
nal PHY switches the RX_CLK and TX_CLK, it must
provide glitch-free clock pulses.
RXD[3:0]
Receive Data
RXD[3:0] is the nibble-wide MII receive data bus. Data
on RXD[3:0] is sampled on every rising edge of
RX_CLK while RX_DV is asserted. RXD[3:0] is ignored
while RX_DV is de-asserted.
RX_DV
Receive Data Valid
RX_DV is an input used to indicate that valid, received
data is being presented on the RXD[3:0] pins and
RX_CLK is synchronous to the receive data. In order
for a frame to be fully received by the Am79C976 de-
vice on the MII, RX_DV must be asserted prior to the
RX_CLK rising edge, when the first nibble of the Start-
of-Frame Delimiter is driven on RXD[3:0], and must re-
main asserted until after the rising edge of RX_CLK,
when the last nibble of the CRC is driven on RXD[3:0].
RX_DV must then be deasserted prior to the RX_CLK
rising edge which follows this final nibble. RX_DV tran-
sitions are synchronous to RX_CLK rising edges.
RX_ER
Receive Error
RX_ER is an input that indicates that the MII trans-
ceiver device has detected a coding error in the receive
frame currently being transferred on the RXD[3:0] pins.
When RX_ER is asserted while RX_DV is asserted, a
CRC error will be indicated in the receive descriptor for
the incoming receive frame. RX_ER is ignored while
RX_DV is deasserted. Special code groups generated
on RXD while RX_DV is deasserted are ignored (e.g.,
Bad SSD in TX and IDLE in T4). RX_ER transitions are
synchronous to RX_CLK rising edges.
MDC
Management Data Clock
MDC is a non-continuous clock output that provides a
timing reference for bits on the MDIO pin. During MII
management port operations, MDC runs at a nominal
frequency of 2.5 MHz. When no management opera-
tions are in progress, MDC is driven LOW.
30
P R E L I M I N A R Y
Output
Input
Input
Input
Input
Am79C976
If the MII Management port is not used, the MDC pin
can be left floating.
MDIO
Management Data I/O
MDIO is the bidirectional MII management port data
pin. MDIO is an output during the header portion of the
management frame transfers and during the data por-
tions of write transfers. MDIO is an input during the
data portions of read data transfers. When an operation
is not in progress on the management port, MDIO is not
driven. MDIO transitions from the Am79C976 controller
are synchronous to MDC falling edges.
If the PHY is attached through an MII physical connec-
tor, then the MDIO pin should be externally pulled down
to VSS with a 10-k W
nently connected, then the MDIO pin should be exter-
nally pulled up to VCC with a 10-k W
External Address Detection Interface
EAR
External Address Reject
The incoming frame will be checked against the inter-
nally active address detection mechanisms and the re-
sult of this check will be OR’d with the value on the EAR
pin. The EAR pin acts as an external address accept
function. The pin value is OR’d with the internal ad-
dress detection result to determine if the current frame
should be accepted. If EAR remains high while a frame
is being received, the frame will be accepted regard-
less of the state of the internal address matching logic.
The EAR pin must not be left unconnected. If it is not
used, it should be tied to VSS through a 10-k W
sistor.
SFBD
Start Frame-Byte Delimiter
An initial rising edge on the SFBD signal indicates that
a start of valid data is present on the RXD[3:0] pins.
SFBD will go high for one nibble time (400 ns when op-
erating at 10 Mbps and 40 ns when operating at 100
Mbps) one RX_CLK period after RX_DV has been as-
serted and RX_ER is deasserted, and there is the de-
tection of the SFD (Start of Frame Delimiter) of a
received frame.
Data on the RXD[3:0] will be the start of the destination
address field. SFBD will subsequently toggle every nib-
ble time (1.25 MHz frequency when operating at 10
Mbps and 12.5 MHz frequency when operating at 100
Mbps), indicating the first nibble of each subsequent
byte of the received nibble stream. The RX_CLK
should be used in conjunction with the SFBD to latch
the correct data for external address matching. SFBD
will be active only during frame reception.
±
5% resistor. If the PHY is perma-
±
5% resistor.
Input/Output
±
8/01/00
Output
5% re-
Input

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