Motorola PowerQUICC II MPC8280 Series Reference Manual page 1049

Information about the connection can be gained by comparing Nt and Nr:
• If Nt > Nr, the difference indicates the number of lost cells of this block test.
• If Nt < Nr, the difference indicates the number of misinserted cells of this block test.
• When Nt = Nr, no cells are lost or misinserted.
31.7 User-Defined Cells (UDC)
Typical ATM cells are 53 bytes long and consist of a 4-byte header, 1-byte HEC, and
48-byte payload. The MPC8280 also supports user-defined cells with up to 12 bytes of
extra header fields for internal information for switching applications. This choice is made
during initialization by writing to the FPSMR; see Section 31.13.2, "FCC
Protocol-Specific Mode Register (FPSMR)." As shown in Figure 31-19, the extra header
size can vary between 1 to 12 bytes (byte resolution) and the HEC octet is optional.
For AAL5 and AAL1 CES the extra header is taken from the Rx and Tx BDs. The
transmitter reads the extra header from the UDC TxBD and adds it to each ATM cell
associated with the current buffer. At the receive side, the extra header of the last cell in the
current buffer is written to the UDC RxBD.
For AAL0 the extra header is attached to the regular ATM cell in the buffer. The transmitter
reads the extra header and the ATM cell from the buffer. The receiver writes the extra
header and the regular ATM cell to the buffer.
31.7.1 UDC Extended Address Mode (UEAD)
For external CAM accesses, the UDC extra header can be used to supply extra routing
information; see Figure 31-20. If GMODE[UEAD] = 1, two bytes of the UDC header are
used as extensions to the ATM address and the CAM match cycle performs a double-word
access. UEAD_OFFSET in the parameter RAM determines the offset from the beginning
of the UDC extra header to the UEAD entry. The offset should be half-word aligned (even
address). See Section 31.10.1, "Parameter RAM."
MOTOROLA Chapter 31. ATM Controller and AAL0, AAL1, and AAL5
Freescale Semiconductor, Inc.
Extra Header (1–12 Bytes)
ATM Cell Header (4 Bytes) + HEC (optional)
Payload (48 Bytes)
Figure 31-19. Format of User-Defined Cells
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User-Defined Cells (UDC)
31-35