
120
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
3.10.14.3
Enabling Traffic Regulation
MAC Control Pause frame generation or half-duplex back pressure assertion is controlled by the host
CPU when it sets or clears the Flow Control Command (FCCMD) bit. When the FCCMD bit is set,
the receiver is considered to be in the ‘congested’ state. When the FCCMD bit is cleared, the receiver
is not considered to be congested.
If the controller is in the half-duplex mode and the BKPRS_EN bit is set, back pressure is asserted as
long as the receiver is in the congested state. If the controller is in the full-duplex mode, a MAC
Control Pause frame is automatically transmitted when the receiver enters the congested state, and
optionally a second Pause frame is transmitted when the receiver leaves the congested state. In half-
duplex mode the network controller does not respond to received pause frames.
When the device is operating in full-duplex mode, the content of the Pause frames and the number of
Pause frames transmitted are controlled by the Fixed Length Pause control bit (FIXP). When FIXP is
set to 1, a single Pause frame is transmitted when the receiver enters the congested state. The contents
of the request_operand field of the Pause frame are taken from the Pause Length field of the Flow
Control Register (PAUSE_LEN). This frame causes the link partner to halt transmissions for a
predetermined length of time that corresponds to the value read from the PAUSE_LEN field.
If FIXP is 0, when the receiver enters the congested state, a Pause frame is transmitted with the
request_operand field set to FFFFh, so that the link partner stops transmitting for a very long time.
When the receiver exits the congested state, a second Pause frame is transmitted with the
request_operand field cleared to 0000h, so that the link partner resumes transmissions immediately.
Traffic regulation is affected by the following:
Duplex mode
Flow Control Command (FCCMD) bit
Fixed Length Pause (FIXP) bit
16-bit PAUSE Length field (PAUSE_LEN)
Negotiate Pause Ability (NPA) bit
Negotiate Asymmetric Pause Ability (NAPA) bit
Force Receive Pause Enable (FRPE) bit
Force Transmit Pause Enable (FTPE) bit
3.10.14.4
Software Control of Traffic Regulation
The host CPU uses the Flow Control Command bit (FCCMD) to cause the controller to transmit flow
control frames automatically or to enable back pressure.
In half-duplex mode, back pressure is enabled when FCCMD is set to 1, and it is disabled when
FCCMD is cleared to 0.
In full-duplex mode, the act of setting FCCMD to 1 causes a pause frame to be sent. The contents of
the request_operand field of the frame depend on the state of the FIXP bit. If FIXP is 1, the contents