Freescale Semiconductor PowerQUICC MPC870, Справочное руководство

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ATM Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor 39-5
Note that if SRSTATE[MCF] is set (see Section 39.2, “Management Cell Filter (MCF)” ), PTI bits are not
checked to be zero, although they may not be part of the FLMASK (so OAM and other management cells
are not considered to be misinserted cells); otherwise, if MCF=0 and CUMB is set, the user should also
include the PTI bits in FLMASK so that cells marked as congested (EFCI = 1) or last (PTI[1] =1) in the
PTI are not received into the default raw cell queue.
39.1.2.5 OAM Screening
OAM/management cells are indicated by having their most significant PTI bit set. If OAM/management
cells are not a part of the address mapping process (the most significant PTI bit in FLMASK is cleared),
OAM cells are passed to the default raw cell queue instead of being routed to the buffer otherwise assigned
to this incoming channel. This treatment keeps the OAM cells of an AAL5 connection from interfering
with the SAR process and allows OAM cells to be processed separately. Note, however, if the management
cell filter (MCF) is active, the OAM screening is done according to the MCF programming instead.
39.1.3 CAM Address Mapping (SRSTATE[EXT,ACP] = 10)
The CAM address mapping method uses dual DMA accesses to an external CAM-based look-up table.
When a cell is received, the incoming header is masked with HMASK in the parameter RAM (see
Table 38-12 ). The CP then performs a DMA write access to the CAM address (CAMADD) with the
address generated from the masked header as its data operand. The second access is a read DMA access
to CAMADD. During the read access, the CAM should drive a “match successful” indication on the data
bus D[0] signal, and the matched channel number on the data bus D[16–31] signals. The “match
successful” indication driven on data bus D[0] is active low, (a 0 indicates a successful match). If there is
no “match successful” signal driven, the cell is passed to the default raw cell queue.
39.2 Management Cell Filter (MCF)
OAM and other management cells which have reached their termination point can be screened out of the
main cell flow using the management cell filter (MCF). The filter is activated when SRSTATE[MCF] is
set, and the actual extraction condition depends on the role of the application in the ATM network
(endpoint, segment endpoint, or pass-all element). The network role is programmed per VC in the
connection’s RCT[FT] (filter type) field.
In the overall receive flow, the MCF filtering follows the address mapping mechanism. Therefore, the
MCF applies only to the cell streams passed by the address mapping mechanism. As shown in Figure 39-3,
the main cell flow of a particular PHY can be divided into several cell streams according to the type of
address mapping. The cell streams then enter the management cell filter and are either passed through or
redirected to the raw cell queue (cell termination) based on their cell headers and each channel’s filter type.