Texas Instruments TMS320DM646x manual Buffer Length, Packet Length, Start of Packet SOP Flag

www.ti.com

Architecture

2.5.5.4Buffer Length

This 16-bit field is used for two purposes:

∙Before the descriptor is first placed on the receive queue by the application software, the buffer length field is first initialized by the software to have the physical size of the empty data buffer pointed to by the buffer pointer field.

∙After the empty buffer has been processed by the EMAC and filled with received data bytes, the buffer length field is updated by the EMAC to reflect the actual number of valid data bytes written to the buffer.

2.5.5.5Packet Length

This 16-bit field specifies the number of data bytes in the entire packet. This value is initialized to zero by the software application for empty packet buffers. The value is filled in by the EMAC on the first buffer used for a given packet. This is signified by the EMAC setting a start of packet (SOP) flag. The packet length is set by the EMAC on all SOP buffer descriptors.

2.5.5.6Start of Packet (SOP) Flag

When set, this flag indicates that the descriptor points to a packet buffer that is the start of a new packet. In the case of a single fragment packet, both the SOP and end of packet (EOP) flags are set. Otherwise, the descriptor pointing to the last packet buffer for the packet has the EOP flag set. This flag is initially cleared by the software application before adding the descriptor to the receive queue. This bit is set by the EMAC on SOP descriptors.

2.5.5.7End of Packet (EOP) Flag

When set, this flag indicates that the descriptor points to a packet buffer that is last for a given packet. In the case of a single fragment packet, both the start of packet (SOP) and EOP flags are set. Otherwise, the descriptor pointing to the last packet buffer for the packet has the EOP flag set. This flag is initially cleared by the software application before adding the descriptor to the receive queue. This bit is set by the EMAC on EOP descriptors.

2.5.5.8Ownership (OWNER) Flag

When set, this flag indicates that the descriptor is currently owned by the EMAC. This flag is set by the software application before adding the descriptor to the receive descriptor queue. This flag is cleared by the EMAC once it is finished with a given set of descriptors, associated with a received packet. The flag is updated by the EMAC on SOP descriptor only. So when the application identifies that the OWNER flag is cleared on an SOP descriptor, it may assume that all descriptors up to and including the first with the EOP flag set have been released by the EMAC. (Note that in the case of single buffer packets, the same descriptor will have both the SOP and EOP flags set.)

2.5.5.9End of Queue (EOQ) Flag

When set, this flag indicates that the descriptor in question was the last descriptor in the receive queue for a given receive channel, and that the corresponding receiver channel has halted. This flag is initially cleared by the software application prior to adding the descriptor to the receive queue. This bit is set by the EMAC when the EMAC identifies that a descriptor is the last for a given packet received (also sets the EOP flag), and there are no more descriptors in the receive list (next descriptor pointer is NULL).

The software application can use this bit to detect when the EMAC receiver for the corresponding channel has halted. This is useful when the application appends additional free buffer descriptors to an active receive queue. Note that this flag is valid on EOP descriptors only.

SPRUEQ6–December 2007 Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) 29

Submit Documentation Feedback