Freescale Semiconductor MCF5480 - Manual (Page 663)

Descriptors
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor 22-61
22.13.1.3 Null Fields
On occasion, a descriptor field may not be applicable to the requested service. With seven length/pointer
pairs, it is possible that not all descriptor fields will be required to load the required keys, context, and data.
(Some operations do not require context, others may only need to fetch a small, contiguous block of data.)
Therefore, when processing data packet descriptors, the SEC will skip entirely any pointer that has an
associated length of zero.
22.13.1.4 Next Descriptor Pointer
Following the length/pointer pairs is the ‘Next Descriptor’ field, which contains the pointer to the next
descriptor in memory. Upon completion of processing of the current descriptor, this value, if non-zero, is
used to request a burst read of the next data packet descriptor. This automatic load of the next descriptor
is referred to as descriptor chaining. See Section 22.13.2, “Descriptor Chaining ” for more information.
Figure 22-44 displays the next descriptor pointer field.
Table 22-42 describes the descriptor pointer field mapping.
22.13.2 Descriptor Chaining
Descriptor chaining provides a measure of ‘decoupling’ between host CPU activities and the status of the
SEC. Rather than waiting for the SEC to signal DONE, and arbitrating for the bus in order to write directly
to the fetch register in the crypto-channel, the host can simply create new descriptors in memory, and chain
them to descriptors which have not yet been fetched by the SEC by filling the next descriptor pointer field
with the address of the newly created descriptor. Whether or not processing continues automatically
following next-descriptor fetch and whether or not an interrupt is generated depends on the programming
of the Crypto-Channel’s configuration register.
See
Section 22.7.1.1, “Crypto-Channel Configuration Registers (CCCRn) ,” for additional information on
how the SEC can be programmed to signal and act upon completion of a descriptor.
NOTE
It is possible to insert a descriptor into an existing chain; however, great care
must be taken when doing so.
Figure 22-45 shows a conceptual chain, or ‘linked list’ of descriptors.
31 0
Field NDP
Addr Descriptor Pointer + 0x3C
Figure 22-44. Next Descriptor Pointer Field
Table 22-42. Next Descriptor Pointer Field Mapping
Bits Name Description
31–0 NDP Next descriptor pointer. Contains the address, in global memory space, of the next
descriptor to be fetched if descriptor chaining is enabled. This field should be cleared if
chaining is not required.