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Rabbit 5000 Microprocessor User’s Manual
21.3.6 DMA with Peripherals
When the DMA is directed towards an internal I/O address, the DMA transfer request
signals will be connected as appropriate for that peripheral. For example, when a DMA
transfer is performed to Serial Port D’s data register, the transfer request will be enabled
whenever the serial port transmit buffer is empty, and will be disabled whenever it is not.
21.3.6.1 DMA with HDLC Serial Ports
The HDLC serial ports receive special handing by the DMA. When the DMA destination
is Serial Port E’s or Serial Port F’s data register (SxDR), the final byte of the transfer will
be written to the appropriate last data register (SxLDR) as required to complete an HDLC
packet and append the CRC value. In addition, the value in the appropriate status register
(SxSR) will be written to the status byte in the buffer descriptor pointed to by the initial
address registers (not necessarily the buffer descriptor that is currently being used). These
features allow an application to automatically send and receive packets via DMA, only
requiring direct handling of a packet when an error occurs.
21.3.6.2 DMA with Ethernet
The Ethernet network peripheral also receives special handing by the DMA. When the DMA
destination is the network data register (NBDR), the final byte of the transfer will be written
to the last data register (NBLDR) as required to complete an Ethernet packet and append
the CRC value. The Ethernet network peripheral also has support for DMA fly-by transfers
between the peripheral and external memory.
21.3.6.3 DMA with Wi-Fi
The Wi-Fi network peripheral has support for DMA fly-by transfers between the peripheral
and external memory. However, the Wi-Fi peripheral has a minimum access time of 75 ns,
and typically requires wait states, so fly-by DMA may not be the most efficient access
method.
21.3.6.4 DMA with PWM and Timer C
The PWM and Timer C peripherals have special support for DMA — the block access and
pointer registers in each of these peripherals provide a means for the DMA to update the
settings of these peripherals at some desired rate. This allows complex PWM waveforms
to be generated by using the DMA timed request to update the PWM duty cycles at regular
intervals.
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