Freescale Semiconductor MK22FN256VDC12, Справочное руководство

Страница: 419 / 1313

Table 22-1. eDMA engine submodules
Submodule Function
Address path This block implements registered versions of two channel transfer control descriptors, channel x
and channel y, and manages all master bus-address calculations. All the channels provide the
same functionality. This structure allows data transfers associated with one channel to be
preempted after the completion of a read/write sequence if a higher priority channel activation is
asserted while the first channel is active. After a channel is activated, it runs until the minor loop is
completed, unless preempted by a higher priority channel. This provides a mechanism (enabled
by DCHPRI
n [ECP]) where a large data move operation can be preempted to minimize the time
another channel is blocked from execution.
When any channel is selected to execute, the contents of its TCD are read from local memory and
loaded into the address path channel x registers for a normal start and into channel y registers for
a preemption start. After the minor loop completes execution, the address path hardware writes
the new values for the TCD
n _{SADDR, DADDR, CITER} back to local memory. If the major
iteration count is exhausted, additional processing is performed, including the final address pointer
updates, reloading the TCD
n _CITER field, and a possible fetch of the next TCD n from memory as
part of a scatter/gather operation.
Data path This block implements the bus master read/write datapath. It includes 16 bytes of register storage
and the necessary multiplex logic to support any required data alignment. The internal read data
bus is the primary input, and the internal write data bus is the primary output.
The address and data path modules directly support the 2-stage pipelined internal bus. The
address path module represents the 1st stage of the bus pipeline (address phase), while the data
path module implements the 2nd stage of the pipeline (data phase).
Program model/channel
arbitration This block implements the first section of the eDMA programming model as well as the channel
arbitration logic. The programming model registers are connected to the internal peripheral bus.
The eDMA peripheral request inputs and interrupt request outputs are also connected to this block
(via control logic).
Control This block provides all the control functions for the eDMA engine. For data transfers where the
source and destination sizes are equal, the eDMA engine performs a series of source read/
destination write operations until the number of bytes specified in the minor loop byte count has
moved. For descriptors where the sizes are not equal, multiple accesses of the smaller size data
are required for each reference of the larger size. As an example, if the source size references 16-
bit data and the destination is 32-bit data, two reads are performed, then one 32-bit write.
The transfer-control descriptor local memory is further partitioned into:
Table 22-2. Transfer control descriptor memory
Submodule Description
Memory controller This logic implements the required dual-ported controller, managing accesses from the eDMA
engine as well as references from the internal peripheral bus. As noted earlier, in the event of
simultaneous accesses, the eDMA engine is given priority and the peripheral transaction is
stalled.
Memory array TCD storage for each channel's transfer profile.
Chapter 22 Enhanced Direct Memory Access (eDMA)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc. 419