4.9 Application Controlled Media Access (ACMA)

Physical Layer Collision Avoidance (PLCA) improves upon traditional CSMA/CD network utilization by eliminating collisions while also providing determinism for packet transmission. A customized media access method may be appropriate in systems requiring more control over latency or bandwidth allocation. The LAN8670/1/2 provides Application Controlled Media Access (ACMA) as an alternative to PLCA or CSMA/CD which allows implementation of custom scheduled access to the medium for applications requiring a fixed, deterministic latency. This could be used to implement a time-division multiple access (TDMA) method that allocates a specific transmit time slot for each station on the shared medium. Such a system avoids collisions, while reserving fixed bandwidth for each station and guarantees access latency to provide deterministic network behavior. A system using ACMA can even allow synchronization of the time slots across a network using the IEEE Std 802.1AS generalized Precision Time Protocol (gPTP).

The ACMA mode is enabled by setting the ACMA Enable (ACMAEN) bit in the ACMA Control (ACMACTL) register. The assertion level of the ACMA input is configured by the ACMA Polarity (ACMAPOL) bit in the Pin Control (PINCTRL) register.
Important: Only the LAN8672 has a dedicated pin for ACMA input. Refer to the Pin Configuration section of this document to ensure proper pin selection and configuration for the LAN8670 or LAN8671.

When enabled, the ACMA signal is used to control transmit access to the medium. If the ACMA input is not asserted, transmission from the MAC is held off by signaling carrier sense through the assertion of the MII CRS or RMII CRSDV pin. Only when the ACMA signal is asserted is the MAC allowed to transmit by negating CRS/CRSDV. If the MAC has a frame to send, it asserts TXEN and begins transmitting. The MAC can send multiple packets if the ACMA assertion time is long enough to permit it. Once transmission of a packet has started, the full packet will be transmitted regardless of the status of ACMA.

The timing of the ACMA signals depends on the bandwidth requirements and numbers of transmitters on the mixing segment. To work with the greatest variety of MACs, the minimum recommended ACMA enable pulse width is 10 μs. This ensures that the device will have more than the inter-packet gap of 9.6 μs to enable TXEN after release of carrier sense. The minimum period between consecutive time slots on the mixing segment must not be less than the time to transmit the largest Ethernet packet or sequence of packets plus 9.6 μs.

Note: Other stations on the network must not assert ACMA until the previous time slot has expired. Failure to meet this constraint may result in collisions on the network or the loss of ability to transmit within the assigned time slot.

Collisions will not occur on properly engineered and synchronized ACMA. However, to aid in development and debug, when ACMA is enabled the device will set the Unexpected Carrier Sense (UNCRS) status bit in the Status 1 (STS1) register if receive carrier is sensed from the network when the ACMA pin is asserted. If needed, IRQ_N can be asserted when this status bit is asserted; this is configured by writing a 0 into the UNCRSM bit of the IMSK1 register.