I
2
C Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
32-12
Freescale Semiconductor
32.7.1
I
2
C Buffer Descriptors (BDs)
Receive and transmit buffer descriptors report information about each buffer transferred and whether a
maskable interrupt should be generated. Each 64-bit BD, shown in
Figure 32-13
and
Figure 32-14
, has the
following structure:
•
The half word at 0 contains status and control bits. The CPM updates the status bits after
the buffer is sent or received.
•
The half word at 2 contains the data length (in bytes) that is sent or received.
— For an RxBD, this is the number of octets the CPM writes into this RxBD’s buffer once the
descriptor closes. The CPM updates this field after the received data is placed into the
associated buffer. Memory allocated for this buffer should be no smaller than MRBLR.
— For a TxBD, this is the number of octets the CPM should transmit from its buffer. Normally,
this value should be greater than zero. The CPM never modifies this field.
•
The word at 4 points to the beginning of the buffer.
— For an RxBD, the pointer must be even and can point to internal or external memory.
— For a TxBD, the pointer can be even or odd. The buffer can reside in internal or external
memory.
32.7.1.1
I
2
C Receive Buffer Descriptor (RxBD)
Using RxBDs, the CPM reports on each buffer received, closes the current buffer, generates a maskable
interrupt, and starts receiving data in the next buffer when the current one is full. It closes the buffer when
a stop or start condition is found on the I
2
C bus or when an overrun error occurs. The core should write
RxBD bits before the I
2
C controller is enabled.
Table 32-9
describes I
2
C RxBD status and control bits.
0
1
2
3
4
5
13
14
15
0
E
—
W
I
L
—
OV
—
2
Data Length
4
RX Buffer Pointer
6
Figure 32-13. I
2
C Receive Buffer Descriptor (RxBD)
Table 32-9. I
2
C RxBD Status and Control Bits
Bits
Name
Description
0
E
Empty.
0 The buffer is full or stopped receiving because of an error. The core can examine or write to any
fields of this RxBD, but the CPM does not use this BD while E = 0.
1 The buffer is empty or reception is in progress. The CPM owns this RxBD and its buffer. Once E
is set, the core should not write any fields of this RxBD.
1
—
Reserved and should be cleared.
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