
AMD
P R E L I M I N A R Y
96
Am79C970A
LED and also have an EEPROM, it might be necessary
to buffer the
LED3
circuit from the EEPROM connection.
When an LED circuit is directly connected to the
EEDO/
LED3
/SRD pin, then it is not possible for most
Microwire EEPROM devices to sink enough I
OL
to
maintain a valid low level on the EEDO input to the
PCnet-PCI II controller. In applications where an
EEPROM is not needed, the
LED3
pin may be directly
connected to an LED circuit. The PCnet-PCI II controller
LED3
pin driver will be able to sink enough current to
properly drive the LED circuit.
Each LED can be programmed through a BCR register
to indicate one or more of the following network status or
activities: Collision Status, Full-Duplex Link Status,
Half-Duplex Link Status, Jabber Status, Magic Packet
Status, Receive Match, Receive Polarity, Receive
Status and Transmit Status. The LED pins can be con-
figured to operate in either open-drain mode (active low)
or in totem-pole mode (active high). The output can be
stretched to allow the human eye to recognize even
short events that last only several microseconds. After
H_RESET, the four LED outputs are configured in the
following manner:
Table 13. LED Default Configuration
LED Output
Indication
Driver Mode
Pulse Stretch
LNKST
Link Status
Open Drain – Active Low
Enabled
LED1
Receive Status
Open Drain – Active Low
Enabled
LED2
Receive Polarity
Open Drain – Active Low
Enabled
LED3
Transmit Status
Open Drain – Active Low
Enabled
For each LED register, each of the status signals is
ANDed with its enable signal, and these signals are all
ORed together to form a combined status signal. Each
LED pins combined status signal can be programmed to
run to a pulse stretcher, which consists of a 3-bit shift
register clocked at 38 Hz (26 ms). The data input of each
shift register is normally at logic 0. The OR gate output
for each LED register asynchronously sets all three bits
of its shift register when the output becomes asserted.
The inverted output of each shift register is used to con-
trol an LED pin. Thus the pulse stretcher provides 2–3
clocks of stretched LED output, or 52 ms to 78 ms.
19436A-44
COL
COLE
FDLS
FDLSE
JAB
JABE
LNKST
LNKSTE
RCV
RCVE
RCVM
RCVME
RXPOL
RXPOLE
XMT
XMTE
To
Pulse
Stretch
MFS
MFSE
Figure 41. LED Control Logic