Link Layer Controller LinkMonitor
Functional Description
The LinkMonitor module converts messages based on Valid-Ready handshakes into L-Credit-based handshakes while maintaining the power states of the TX and RX links. For details, refer to the CHI Spec Link Handshake chapter.
Feature 1: Decoupled Handshake to L-Credit Handshake
The Decoupled handshake from the three TX channels is converted into an LCredit handshake via the Decoupled2LCredit module. The Decoupled2LCredit module records the number of LCredits received by the downstream ICN (lcreditPool). Only when lcreditPool is greater than 0 can it accept upstream Decoupled requests; upon successful Decoupled handshake, the lcreditPool count decreases by one.
Impact of TX Link State: When the TX link state is STOP or ACTIVATE, the reception of Decoupled messages should be halted. If the TX link state is STOP, the reception of LCredit should also be stopped, and the lcreditPool should remain unchanged even if the downstream lcrdv signal pulls the lcreditPool high.
Feature 2: L-Credit Handshake to Decoupled Handshake
The LCredit handshake received from the three RX channels is converted into a Decoupled handshake via the LCredit2Decoupled module. The LCredit2Decoupled module maintains a default 4-entry queue (configurable as lcreditNum, with lcreditNum ≤ 15) to temporarily store messages, meaning an RX channel can send up to lcreditNum outstanding LCredits downstream. It also maintains a counter (lcreditPool) initialized to lcreditNum, tracking the maximum number of LCredits the channel can currently send. When lcreditPool > the number of valid queue entries (queueCnt), it indicates the channel has sent fewer outstanding LCredits than the queue can receive, allowing the channel to send LCredits downstream. When lcreditPool < lcreditNum, the channel should unconditionally accept valid downstream requests, i.e., those with flitv high and flitpending high in the previous cycle.
Impact of RX Link State: If the RX link state is not RUN, the channel should not send LCredits downstream, even if lcreditPool > the number of valid queue entries.
Feature 3: TXSACTIVE and RXSACTIVE
TXSACTIVE is always held high. RXSACTIVE is currently unused.
Feature 4: Interface activation and deactivation
TXLINKACTIVEREQ remains high after reset. RXLINKACTIVEACK is set to true in the next cycle after RXLINKACTIVEREQ is set to true; starting from the cycle after RXLINKACTIVEREQ is set to false, it monitors the states of the three RX channels. Once all outstanding LCredits are reclaimed (i.e., lcreditPool equals lcreditNum), RXLINKACTIVEACK can be set to false.
Overall Block Diagram
