I
2
C Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
32-9
32.5
I
2
C Parameter RAM
The I
2
C controller parameter RAM area, shown in
Table 32-6
, is used for the general I
2
C parameters. It is
similar to the SCC general-purpose parameter RAM. Certain parameter RAM values, such as the
maximum receive buffer length (MRBLR), must be initialized by the user before the I
2
C is enabled, while
other parameters, used by the CPM, do not need initialization. Software usually does not access parameter
RAM entries once they are initialized; they should be changed only when the I
2
C is inactive.
1–6
—
Reserved and should be cleared.
7
M/S
Master/slave. Configures the I
2
C controller to operate as a master or a slave.
0 I
2
C is a slave.
1 I
2
C is a master.
Table 32-6. I
2
C Parameter RAM Memory Map
Offset
1
Name
Width
Description
0x00
RBASE
Hword Rx/TxBD table base address. Indicate where the BD tables begin in the dual-port
RAM. Setting Rx/TxBD[W] in the last BD in each BD table determines how many BDs
are allocated for the Tx and Rx sections of the I
2
C. Initialize RBASE/TBASE before
enabling the I
2
C. Furthermore, do not configure BD tables of the I
2
C to overlap any
other active controller’s parameter RAM.
RBASE and TBASE should be divisible by eight.
0x02
TBASE
Hword
0x04
RFCR
Byte
Rx/Tx function code. The value to appear on AT[1–3] when the associated SDMA
channel accesses memory. Also controls the byte-ordering convention for transfers.
See
Figure 32-11
and
Table 32-7
.
0x05
TFCR
Byte
0x06
MRBLR
Hword Maximum receive buffer length. Defines the maximum number of bytes the I
2
C
receiver writes to a receive buffer before moving to the next buffer. The receiver writes
fewer bytes to the buffer than the MRBLR value if an error or end-of-frame occurs.
Receive buffers should not be smaller than MRBLR.
Transmit buffers are unaffected by MRBLR and can vary in length; the number of
bytes to be sent is specified in TxBD[Data Length].
MRBLR is not intended to be changed while the I
2
C is operating. However it can be
changed in a single bus cycle with one 16-bit move (not two 8-bit bus cycles
back-to-back). The change takes effect when the CP moves control to the next
RxBD. To guarantee the exact RxBD on which the change occurs, change MRBLR
only while the I
2
C receiver is disabled. MRBLR should be greater than zero.
0x08
RSTATE
Word
Rx internal state. Reserved for CPM use.
0x0C
RPTR
Word
Rx internal data pointer
2
is updated by the SDMA channels to show the next address
in the buffer to be accessed.
0x10
RBPTR
Hword RxBD pointer. Points to the next descriptor the receiver transfers data to when it is in
an idle state or to the current descriptor during frame processing for each I
2
C
channel. After a reset or when the end of the descriptor table is reached, the CP
initializes RBPTR to the value in RBASE. Most applications should not write RBPTR,
but it can be modified when the receiver is disabled or when no receive buffer is used.
0x12
RCOUNT Hword Rx internal byte count
2
is a down-count value that is initialized with the MRBLR value
and decremented with every byte the SDMA channels write.
Table 32-5. I2COM Field Descriptions (continued)
Bits
Name Description
Summary of Contents for PowerQUICC MPC870
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