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GT-96100A Advanced Communication Controller
Revision 1.0
277
5. Resolve the duplex mode of the two link partners in the following manner:
if ((ability == “100BASE-TX Full Duplex”) or (ability == “10BASE-T Full Duplex”)) then
duplex mode = FULL DUPLEX
else
duplex mode = HALF DUPLEX;
Continue to step 6.
6. Update the Port_Status register by writing the correct duplex mode bit. Continue with step 1.
12.4.1.4 Auto-Negotiation for Flow Control
Flow control mode (either enabled or disabled) can be auto-negotiated or set by the CPU. In order to enable auto-
negotiation for flow-control, the CPU should set Port_Configuration_Extend<FCTLen> bit.
If Port_Configuraion_Extend<FCTLen>=1, then auto-negotiation is initiated in the following cases:
After RESET.
After link fail (phy register 1 bit 2).
NOTE: The user may force the port to implement Flow-control by disabling auto-negotiation for flow-control
and programming Port_Configuration_Extend<FCTL>=1.
Auto-negotiation for flow-control is done in two stages:
1. Setting Phy advertise word to support Flow Control.
This is done by writing Phy register 4 in order to set advertise bit 10 (phy-reg4 bit 10 - Enable FC). The
flow of such a cycle is:
- Read Phy register 1. If link_status=1 and was 0 in the last cycle - continue.
- Read Phy register 4.
- Write Phy register 4 with bit 10 set.
2. Reading Phy Flow-Control status and determine result.
This is done by constantly reading PHY’s register 4 and register 5 in order to determine if Flow-control
is supported or not. Only if both link partners support FC (registers 4.10 and 5.10 are both SET),
Port_Status<FCTL> is set to ‘1’, and the port will send PAUSE packets when instructed to do so by the
CPU. Otherwise, Port_Status<FCTL> is set to ‘0’, indicating that the support for 802.3x flow-control is
disabled.
12.4.1.5 Backoff Algorithm Options
The port implements the truncated exponential Backoff algorithm defined by the 802.3 standard. Aggressiveness
of the Backoff algorithm used is controlled by Serial Parameters Register<
Limit4> bit.
Limit4 function controls the number of consecutive packet collisions that will occur before the collision counter
is reset.
When Limit4 feature is disabled, the port resets its collision counter after 16 consecutive retransmit trials and
restarts the Backoff algorithm. Retransmission is done using the data already stored in the FIFO.
When Limit4 feature is enabled, the port will reset its collision counter and restart the Backoff algorithm after 4
consecutive transmit trials. This makes the port more aggressive in getting hold of the media following a colli-
sion. This may result better overall throughput in standardized tests.