ST90158 - ON-CHIP DIRECT MEMORY ACCESS (DMA)
5 ON-CHIP DIRECT MEMORY ACCESS (DMA)
5.1 INTRODUCTION
5.2 DMA PRIORITY LEVELS
The ST9 includes on-chip Direct Memory Access
(DMA) in order to provide high-speed data transfer
between peripherals and memory or Register File.
Multi-channel DMA is fully supported by peripher-
als having their own controller and DMA chan-
nel(s). Each DMA channel transfers data to or
from contiguous locations in the Register File, or in
Memory. The maximum number of bytes that can
be transferred per transaction by each DMA chan-
nel is 222 with the Register File, or 65536 with
Memory.
The 8 priority levels used for interrupts are also
used to prioritize the DMA requests, which are ar-
bitrated in the same arbitration phase as interrupt
requests. If the event occurrence requires a DMA
transaction, this will take place at the end of the
current instruction execution. When an interrupt
and a DMA request occur simultaneously, on the
same priority level, the DMA request is serviced
before the interrupt.
An interrupt priority request must be strictly higher
than the CPL value in order to be acknowledged,
whereas, for a DMA transaction request, it must be
equal to or higher than the CPL value in order to
be executed. Thus only DMA transaction requests
can be acknowledged when the CPL=0.
The DMA controller in the Peripheral uses an indi-
rect addressing mechanism to DMA Pointers and
Counter Registers stored in the Register File. This
is the reason why the maximum number of trans-
actions for the Register File is 222, since two Reg-
isters are allocated for the Pointer and Counter.
Register pairs are used for memory pointers and
counters in order to offer the full 65536 byte and
count capability.
DMA requests do not modify the CPL value, since
the DMA transaction is not interruptable.
Figure 28. DMA Data Transfer
REGISTER FILE
REGISTER FILE
OR
MEMORY
DF
REGISTER FILE
GROUP F
PERIPHERAL
PAGED
COUNTER
ADDRESS
PERIPHERAL
DATA
REGISTERS
0
COUNTER VALUE
TRANSFERRED
DATA
START ADDRESS
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