MPC8540 datasheet(822/1302 Pages) FREESCALE

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MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

15-44

Freescale Semiconductor

DMA Controller

15.5.1.2 DMA to Ethernet

The Ethernet controllers cannot serve as DMA targets because they have no suitable internal

memory for this purpose. The Ethernet controllers have dedicated DMA channels to move data

between the external transmit and receive buffers and the internal packet buffer. This dedicated

channel is the only DMA service to the internal packet buffers.

However, Ethernet ports can serve as DMA targets by using a general-purpose DMA controller to

access the transmit and receive buffers defined by the Ethernet buffer descriptors. Because

Ethernet data buffers are located in RAM outside of the Ethernet controllers, general-purpose

DMA engines can move data to or from these memory regions. Also, because Ethernet controllers

automatically read buffer descriptors and send (or load) data buffers, a DMA transfer into (or out

of) these buffers is effectively a transfer into (or out of) the Ethernet ports.

15.5.1.3 DMA to Configuration and Control Registers

Because any internal register can be addressed with the four-channel DMA controller,

configuration and control registers throughout the device are valid DMA targets. However, the

primary purpose of DMA—to reduce processor load by moving large blocks of data— is not

served by DMA transfers of configuration data. For example, while it is possible to DMA into the

2C controller or programmable interrupt controller (PIC), doing so is extremely inefficient and is

seldom beneficial in normal operation. The overhead of creating DMA descriptors far exceeds any

savings in CPU cycles.

15.5.1.4 DMA to I2C

The I2C controller is not transparent to DMA transfers. Observe the caveats listed in

Section 15.5.1.3, “DMA to Configuration and Control Registers,” when accessing any I

15.5.1.5 DMA to DUART

The DUART provides complete and sophisticated DMA support which is described in Chapter 12,

“DUART,” specifically Section 12.4.5, “FIFO Mode.”

MPC8540 Datasheet(PDF) 822 Page - Freescale Semiconductor, Inc