AM79C973BKC AMD (ADVANCED MICRO DEVICES), AM79C973BKC Datasheet - Page 254

• Manufacturer Part Number: AM79C973BKC
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C973BKC

• Lead Free Status / Rohs Status: Not Compliant

since the frequency of the acknowledgment frames is
very low.
Once the receive activity has ended, the Am79C975
controller will set the MRX_DONE bit in the Interrupt
register and clear the MRX_ENABLE bit in the Receive
Status register. The MIRQ pin will be asserted, if the
global interrupt enable bit MIRQEN in the Command
register is set to a 1 and the receive interrupt mask bit
(MRX_DONEM) is cleared to 0 (default state). Once
MIRQ is asserted, the host can read the MRX_DONE
bit in the Interrupt register to determine that the inter-
rupt was caused by the end of the reception. The read
of the Interrupt register will clear the MRX_DONE bit
and cause the deassertion of the MIRQ pin. The Inter-
rupt register also provides a global interrupt bit MIRQ
that is the OR of the MRX_DONE and MTX_DONE
bits.
Once the MRX_DONE bit indicates that the reception
of the acknowledgment frame has ended, the Receive
Status register provides the error status for the recep-
tion. Two error conditions are reported: Framing Error
and Frame Check Sequence Error. There is also an
error summary bit (MRX_ERR). The receive status bits
remain valid as long as the MRX_ENABLE bit is set to
0. The host has the option to discard an erroneous
frame by simply not reading the receive data and un-
protecting the Receive Data memory by setting the
MRX_ENABLE bit.
The host must read the Receive Message Length reg-
ister in order to determine the length of the frame. The
data from the Receive Data memory is read byte by
byte using one or multiple Block Read commands from
the Receive Data port. The command code of the Block
Read command must be set to 40, the address of the
Receive Data port. The Am79C975 controller will indi-
cate in the byte count field the number of bytes that will
follow. The byte count field will indicate 32 in all but the
last transaction in which the byte count field will indi-
cate the remaining bytes of the frame. The device is ca-
pable of transferring data beyond the 32 byte mark. If
the host does not assert NACK after the 32
Am79C975 controller will continue driving receive data
onto the MDATA line until the host asserts NACK. If the
master does not have enough buffer space for the in-
coming data, it can abort the data transfer after any
byte. The Am79C975 controller will start the next Block
Read command with the remaining data. The location
within the Receive Data memory from where the next
byte is read is controlled by the Receive Address regis-
ter. This register will come up cleared to 0 after
H_RESET. With every byte read the address register
will auto-increment. This allows a FIFO-type access to
the Receive Data memory and the host does not need
to keep track of the location he is reading from. In addi-
tion, MRX_ADR can be set to any address within the
Receive Data memory in order to read a specific loca-
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P R E L I M I N A R Y
nd
Am79C973/Am79C975
byte, the
tion or to start the receive data read from an arbitrary
address inside the Receive Data memory. Note, that
the byte count field in the Block Read command will
only reflect the correct amount of transfer data if the ac-
cess starts at Receive Data memory address 0. If
MRX_ADR is manually changed, the byte count field
should be ignored. The host must use the Receive
Message Length register (MRX_LEN) to determine the
length of the data read operation. Data will be unde-
fined, if the host reads further than the Receive Mes-
sage Length register is indicating.
The Receive Data memory will be protected from over-
writing by another frame, until the host enables the next
receive by setting the MRX_ENABLE bit the Receive
Status register. The operation will also clear the Re-
ceive Address register.
Loopback Operation
The SMIU provides a looback mode for diagnostic pur-
poses. If MLOOP in the Command register is set to 1,
the receive path is not being blocked while the device
is transmitting in half-duplex mode. Receive is never
blocked in full-duplex mode and MLOOP has no effect
in this mode.
For loopback operation, transmit data must be sent
back to the receiver. This is done at the transceiver by
either using an external loopback connector or by pro-
gramming the transceiver for loopback mode. The pro-
gramming must be done using the Am79C975 CSR/
BCR register interface. This limits the SMIU loopback
mode to a debug or manufacturing test environment.
User Accessible Registers
The Serial Management Interface Unit (SMIU) of the
Am79C975 controller provides four types of user ac-
cessible registers: device ID registers, node address
registers, device status registers and control and status
registers. Most registers are accessible via the Read
Byte and/or Write Byte commands. Only the access to
the Transmit and Receive Data port as well as to the
Receive Pattern RAM Data port is performed as a
Block Read or Block Write command. In all commands,
the command code is interpreted as the address of the
register.
Device ID Registers
The following register allow the unique identification of
the Am79C975 device in a system.
SMIU Vendor ID Register 0 (MReg Address 0)
This register is a shadow register of the PCI Vendor ID
Register bits 7:0. The PCI Vendor ID Register is loaded
from the EEPROM.
Bit No. Name and Description
7:0
MVENDOR_ID[7:0]