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Am79C971
75
General Purpose Serial Interface
The General Purpose Serial Interface (GPSI) provides
a direct interface to the MAC section of the Am79C971
controller. All signals are digital and data is non-en-
coded. The GPSI allows use of an external Manchester
encoder/decoder such as the Am7992B Serial Inter-
face Adapter (SIA). In addition, it allows the Am79C971
controller to be used as a MAC sublayer engine in re-
peater designs based on the IMR+ device
(Am79C981).
GPSI mode is invoked by selecting the interface
through the PORTSEL bits of the Mode register
(CSR15, bits 8-7).
The GPSI interface uses some of the same pins as the
interface to the MII. Simultaneous use of both functions
is not possible.
After an H_RESET, all MII pins are internally config-
ured to function as the MII interface. When the GPSI in-
terface is selected by setting PORTSEL (CSR15, bits
8-7) to 10b, the Am79C971 controller will terminate all
further accesses to the MII.
GPSI signal functions are described in the pin descrip-
tion section under the GPSI subheading.
Note that the XTAL1 input must always be driven with a
clock source, even if GPSI mode is to be used. It is not
necessary for the XTAL1 clock to meet the normal fre-
quency and stability requirements in this case. Any fre-
quency between 8 MHz and 20 MHz is acceptable.
However, voltage drive requirements do not change.
When GPSI mode is used, XTAL1 must be driven for
several reasons:
The default H_RESET configuration for the Am79C971
controller is AUI port selected. Until GPSI mode is se-
lected, the XTAL1 clock is needed for some internal op-
erations (namely, RESET). The XTAL1 clock drives the
EEPROM read operation, regardless of the network
mode selected.
The XTAL1 clock determines the length of the internal
S_RESET caused by the read of the Reset register, re-
gardless of the network mode.
Note:
If a clock slower than 20 MHz is provided at the
XTAL1 input, the time needed for EEPROM read and
the internal S_RESET will increase.
Figure 37.
10BASE-T Interface Connection
Full-Duplex Operation
The Am79C971 controller supports full-duplex opera-
tion on all four network interfaces: AUI, GPSI, 10BASE-
T, and MII. Full-duplex operation allows simultaneous
transmit and receive activity on the TXD
±
and RXD
±
pairs of the 10BASE-T port, the DO
±
and DI
±
pairs of
the AUI port, TXDAT and RXDAT pins of the GPSI port,
and the TXD[3:0] and RXD[3:0] pins of the MII port.
Full-duplex operation is enabled by the FDEN and
AUIFD bits located in BCR9 for all ports. Full-duplex
operation is enabled through Auto-Negotiation when
DANAS (BCR 32, bit 7) is not enabled on the MII port
or when Auto-Negotiation is running on the internal
PHY.
When operating in full-duplex mode, the following
changes to the device operation are made:
Bus Interface/Buffer Management Unit changes:
I
The first 64 bytes of every transmit frame are not
preserved in the Transmit FIFO during transmission
of the first 512 bits as described in the Transmit Ex-
ception Conditions section. Instead, when full-du-
plex mode is active and a frame is being
XMT
Filter
RCV
Filter
RJ45
Connector
Filter &
Transformer
Module
TXP+
TXD-
TXP-
TXD+
RXD+
RXD-
Am79C971
TD+
TD-
RD+
RD-
1
2
3
6
61.9
422
61.9
422
100
1.21 K
1:1
1:1
20550D-40