Motorola MPC8260 - Manual (Page 733)

MOTOROLA Chapter 27. Multi-Channel Controllers (MCCs) 27-3
Part IV. Communications Processor Module
Figure 27-1. BD Structure for One MCC
27.3 Global MCC Parameters
The global MCC parameters are described in Table 27-1.
Table 27-1. Global Multiple-Channel Parameters
Offset 1 Name Width Description
0x00 MCCBASE Word Multi-channel controller base pointer. User-initialized parameter points to the starting
address of a 512-Kbyte BD segment in external memory.
0x04 MCCSTATE Hword Multi-channel controller state, used by the CP for global state deÞnition (reserved for
the user)
0x06 MRBLR Hword Maximum receive buffer length (user-initialized). DeÞnes the maximum number of
bytes written to a receive buffer before moving to the next buffer for this channel.
This value must be a multiple of 8.
0x08 GRFTHR Hword Global receive frame threshold. Used to reduce interrupt overhead that can occur
when many short HDLC frames arrive that each cause an RXF interrupt. Setting all
bits enables every interrupt event. Setting a GRFTHR value can limit the frequency
of RXF interrupts. Note that an RXF event is written to the interrupt queue on each
received frame but GINT is set only when the number of RXF events (by all
channels) reaches the GRFTHR value. This parameter does not need to be reset
after an interrupt.
0x0A GRFCNT Hword Global receive frame count. A decrementor counter used to implement the GRFTHR
feature. It should be initialized to the GRFTHR value. Setting all bits enables every
interrupt event. The CP writes an entry in a circular interrupt table and decrements
GRFCNT each time a frame is received. When GRFCNT underßows the CP
generates an interrupt and copy GRFTHR to GRFCNT. This parameter does not
need to be reset after an interrupt.
Channel j TxBD
Table
Channel 0 Parameter
Channel 1 Parameter
Channel j Extra RBASE
TBASE
Global MCC MCCBASE
DPR
Buffer Descriptor
+
+
Channel j RxBD
Table
External Memory
512 Kbytes
x8
x8
DPR_base
Table Base Address
Parameter
Parameters