NXP Semiconductors PXN2020, Справочное руководство, Страница: 548 / 1376

Enhanced Direct Memory Access Controller (eDMA)
PXN20 Microcontroller Reference Manual, Rev. 1
24-28 Freescale Semiconductor
225–230 /
0x1C [1:6]
BITER
[0:5]
or
BITER.LINKCH[0:5]
Starting major iteration count or link channel number.
If channel-to-channel linking is disabled (TCD.BITER.E_LINK = 0), then
• No channel-to-channel linking (or chaining) is performed after the inner minor loop is
exhausted. TCD bits [225:239] are used to form a 15-bit BITER field.
Otherwise,
• After the minor loop is exhausted, the DMA engine initiates a channel service request
at the channel, defined by BITER.LINKCH[0:5], by setting that channel’s TCD.START
bit.
Note: When the TCD is first loaded by software, this field must be set equal to the
corresponding CITER field. Otherwise, a configuration error is reported. As the
major iteration count is exhausted, the contents of this field is reloaded into the
CITER field.
231–239 /
0x1C [7:15]
BITER
[6:14]
Starting major iteration count. As the transfer control descriptor is first loaded by
software, this field must be equal to the value in the CITER field. As the major iteration
count is exhausted, the contents of this field are reloaded into the CITER field.
Note: If the channel is configured to execute a single service request, the initial values
of BITER and CITER should be 0x0001.
240–241 /
0x1C [16:17]
BWC
[0:1]
Bandwidth control. This two-bit field provides a mechanism to effectively throttle the
amount of bus bandwidth consumed by the eDMA. In general, as the eDMA processes
the inner minor loop, it continuously generates read/write sequences until the minor
count is exhausted. This field forces the eDMA to stall after the completion of each
read/write access to control the bus request bandwidth seen by the system bus crossbar
switch (XBAR).
00 No DMA engine stalls
01 Reserved
10 DMA engine stalls for 4 cycles after each r/w
11 DMA engine stalls for 8 cycles after each r/w
242–247 /
0x1C [18:23]
MAJOR.LINKCH
[0:5]
Link channel number.
If channel-to-channel linking on major loop complete is disabled
(TCD.MAJOR.E_LINK = 0) then,
• No channel-to-channel linking (or chaining) is performed after the outer major loop
counter is exhausted.
Otherwise
• After the major loop counter is exhausted, the DMA engine initiates a channel service
request at the channel defined by MAJOR.LINKCH[0:5] by setting that channel’s
TCD.START bit.
248 /
0x1C [24]
DONE Channel done. This flag indicates the eDMA has completed the outer major loop. It is set
by the DMA engine as the CITER count reaches zero; it is cleared by software or
hardware when the channel is activated (when the DMA engine has begun processing
the channel, not when the first data transfer occurs).
Note: This bit must be cleared to write the MAJOR.E_LINK or E_SG bits.
249 /
0x1C [25]
ACTIVE Channel active. This flag signals the channel is currently in execution. It is set when
channel service begins, and is cleared by the DMA engine as the inner minor loop
completes or if any error condition is detected.
Table 24-20. TCDn Field Descriptions (continued)
Bits /
Word Offset
[n:n]
Name Description