Communications Processor Module Overview
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
14-20 Freescale Semiconductor
unused parameter RAM, such as, in the area made available when a peripheral controller or s ub-block is
not being used.
Microcode can be executed from the first 12 Kbytes. To ensure an uninterrupted instruction stream (one
per cycle), no other agent is allowed to use a RAM bank used by the microcode. Since the first 12 Kbytes
are divided to six 2-Kbyte banks, RAM microcode occupies 2, 4, 6, 8, 10, or 12 Kbytes of RAM,
depending on RCCR[ERAM]; see Section 14.3.7, “RISC Controller Configuration Register (RCCR).”
14.5.1 Buffer Descriptors (BDs)
The peripheral controllers (FCCs, SCCs, SMCs, MCCs, SPI, and I2C) always use BDs for controlling
buffers and their BD formats are all the same, as shown in Table14-9.
If the IDMA is used in the buffer chaining or auto-buffer mode, the IDMA channel also uses BDs. They
are described in Section 19.3, “IDMA Emulation.”
NOTE
The CPM accesses BDs by initiating a DMA cycle on either the 60x or local
bus. If BDs are located in DPRAM, the CPM initiates a cycle on the 60x bus
because DPRAM is a slave device on the 60x bus. Therefore, a system
design should not plan to access the 60x bus simultaneously with DPRAM
BD fetches.
14.5.2 Parameter RAM
The CPM maintains a section of RAM called the parameter RAM, which contains many parameters for
the operation of the FCCs, SCCs, SMCs, SPI, I2C, and IDMA channels. An overview of the parameter
RAM structure is shown in Table 14-10.
The exact definition of the parameter RAM is contained in each protocol subsection describing a device
that uses a parameter RAM. For example, the Ethernet parameter RAM is defined differently in some
locations from the HDLC-specific parameter RAM.
Table14-9. Buffer Descriptor Format
Address Descriptor
Offset + 0 Status and control
Offset + 2 Data length
Offset + 4 High-order buffer pointer
Offset + 6 Low-order buffer pointer