LM3S5D91 Datasheet, PDF (343/1340 Pages) Texas Instruments – Stellaris® LM3S5D91 Microcontroller

Stellaris^® LM3S5D91 Microcontroller
7.2.4
7.2.4.1
7.2.4.2
Request Types
The μDMA controller responds to two types of requests from a peripheral: single or burst. Each
peripheral may support either or both types of requests. A single request means that the peripheral
is ready to transfer one item, while a burst request means that the peripheral is ready to transfer
multiple items.
The μDMA controller responds differently depending on whether the peripheral is making a single
request or a burst request. If both are asserted, and the μDMA channel has been set up for a burst
transfer, then the burst request takes precedence. See Table 7-2 on page 343, which shows how
each peripheral supports the two request types.
Table 7-2. Request Type Support
Peripheral
ADC
EPI WFIFO
EPI NBRFIFO
General-Purpose Timer
I²S TX
I²S RX
SSI TX
SSI RX
UART TX
UART RX
USB TX
USB RX
Single Request Signal
None
None
None
Raw interrupt pulse
None
None
TX FIFO Not Full
RX FIFO Not Empty
TX FIFO Not Full
RX FIFO Not Empty
None
None
Burst Request Signal
Sequencer IE bit
WFIFO Level (configurable)
NBRFIFO Level (configurable)
None
FIFO service request
FIFO service request
TX FIFO Level (fixed at 4)
RX FIFO Level (fixed at 4)
TX FIFO Level (configurable)
RX FIFO Level (configurable)
FIFO TXRDY
FIFO RXRDY
Single Request
When a single request is detected, and not a burst request, the μDMA controller transfers one item
and then stops to wait for another request.
Burst Request
When a burst request is detected, the μDMA controller transfers the number of items that is the
lesser of the arbitration size or the number of items remaining in the transfer. Therefore, the arbitration
size should be the same as the number of data items that the peripheral can accommodate when
making a burst request. For example, the UART generates a burst request based on the FIFO trigger
level. In this case, the arbitration size should be set to the amount of data that the FIFO can transfer
when the trigger level is reached. A burst transfer runs to completion once it is started, and cannot
be interrupted, even by a higher priority channel. Burst transfers complete in a shorter time than the
same number of non-burst transfers.
It may be desirable to use only burst transfers and not allow single transfers. For example, perhaps
the nature of the data is such that it only makes sense when transferred together as a single unit
rather than one piece at a time. The single request can be disabled by using the DMA Channel
Useburst Set (DMAUSEBURSTSET) register. By setting the bit for a channel in this register, the
μDMA controller only responds to burst requests for that channel.
January 22, 2012
343
Texas Instruments-Production Data