Tc operation in fcc external rate mode -14 – Freescale Semiconductor MPC8260 User Manual
Page 1192
ATM Transmission Convergence Layer
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
34-14
Freescale Semiconductor
The TC layer requests ATM cells for transmission via the internal UTOPIA interface. Then, when the ATM
cell is passed to the TC layer transmit block, it is stored in the TC layer transmit FIFO. When the ATM cell
is to be transmitted, it is read and processed from the TC layer transmit FIFO.
The scrambling function is performed on the ATM cell payload if TCMODEx[TPS] = 1. The ATM cell
header HEC value is calculated and the coset function is performed on the HEC if TCMODEx[TC] = 1.
The ATM cell is then sent to the PHY via the SI. Once ATM cell transmission is complete, the relevant TC
layer cell counters are updated.
When a TC layer cell counter overflows, an interrupt (TCERx[TOF]) is set (if enabled in the corresponding
TCMRx[TOF]).
The TC layer is responsible for providing the data rate required by the physical medium device (PMD).
On PowerQUICC II there are two ATM transmit modes. Users should refer to
External Rate and Internal Rate Modes,”
for more details. The following text and figures are specific to
TC layer operation.
•
External Rate—In general, the TC would never have its transmit FIFO empty, and thus would not
need to generate idle cells. However, if the CPM is busy and if the transmit FIFO is empty, the TC
layer generates an idle cell and an underrun condition will occur. If TCMODEx[URE] = 1, the
TCERx[UR] interrupt is sent to the host (if not masked). See
.
Figure 34-10. TC Operation in FCC External Rate Mode
•
Internal Rate (Sub Rate)—The TC layer continues to request ATM cells and transmits idle cells.
TCERx[UR] interrupts can by disabled by clearing TCMODEx[URE] until a valid ATM cell is
transmitted via the internal UTOPIA bus from FCC2 to the TC layer FIFO. See
.
FCC
PHY
PowerQUICC II
TC
DPR
CP
UTOPIA
2M Cell Rate
2M Serial Rate
Keep FCC FIFO full
Generate Idle Cells
cell_req
BTM
ATM Channels
Idle Cell
ATM Cell
(External Rate)