-- DISCONTINUED PRODUCT --

Flow Control Operation of the GEMAC

R

Receiving a Pause Control Frame

Core Initiated Response to a Pause Request

An error free control frame is a received frame matching the format of Figure 6-2. It must pass all standard receiver frame checks (for example, FCS field checking). In addition, the control frame received must be exactly 64-bytes in length (from destination address through to the FCS field inclusive: this is minimum legal Ethernet MAC frame size and the defined size for control frames).

Any control frame received that does passed to the receiver client with the

not conform to these checks contains an error and is rx_bad_frame signal asserted.

Pause Frame Reception Disabled

When pause control reception is disabled, an error free control frame is received through the client interface with rx_good_frame asserted (see “Flow Control Configuration,” on page 81). In this way, the frame is passed to the client logic for interpretation (see “Client Initiated Response to a Pause Request,” on page 57).

Pause Frame Reception Enabled

When pause control reception is enabled, and an error-free frame is received by the GEMAC core (see “Flow Control Configuration,” on page 81), the following frame decoding functions are performed:

•The destination address field is matched against the IEEE 802.3 globally assigned multicast address or the configurable pause frame MAC address (see “Configuration Registers,” on page 78).

•The length/type field is matched against the MAC control type code.

•The opcode field contents are matched against the Pause opcode.

If any of the previously described checks are false, the frame is ignored by the Flow Control logic and passed up to the client logic for interpretation by marking it with rx_good_frame asserted. It is then the responsibility of the MAC client logic to decode, act on (if required), and drop this control frame.

If all the previously described checks are true, the 16-bit binary value in the MAC Control Parameters field of the control frame is then used to inhibit transmitter operation for the required number of pause_quantum. This inhibit is implemented by delaying the assertion of tx_ack at the transmitter client interface until the requested pause duration has expired. The received pause frame is then passed on to the client with rx_bad_frame asserted to indicate to the client that the pause frame can be dropped.

Note: Any frame in which the length/type field contains the MAC control type should be dropped by the receiver client logic. All control frames are indicated by rx_statistic_vector bit 19 (see “Receiver Statistics Vector,” on page 44).

Client Initiated Response to a Pause Request

For maximum flexibility, flow control logic can be disabled in the core and alternatively implemented in the client logic connected to the core (see “Flow Control Configuration,” on page 81). Any type of error-free control frame is then passed through the core with rx_good_frame asserted. The frame is passed to the client for interpretation. It is then the responsibility of the client to drop this control frame and to act on it by ceasing transmission through the core, if applicable.

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UG144 specifications

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