Zilog Z16C30 Uživatelský manuál stáhnout pdf (Strana 22)

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Z16C30 USC

USER'S MANUAL

ZILOG

UM97USC0100

Table 1-5. DMA Features of the USC

Flowthrough or Flyby The USC can be used with DMA controllers in a “flowthrough” mode,

in which the REQ line from the USC tells the DMAC when to transfer data

by means of separate accesses to the USC and to memory. Alterna-

tively, the USC and DMA can operate in a “flyby” mode, in which there’s

also an ACK line from the DMAC to tell the USC when to drive data to

the memory or when to capture data from the memory. Flyby operation

requires only one bus cycle per (pair of) character(s).

DMA Requests A USC can provide separate Transmit and Receive DMA Request

outputs from each of its two channels, that become active when the

relevant FIFO reaches a software-selected level of emptiness or

fullness, and stay active until the FIFO is filled or emptied. The Receive

Request for a channel operating in a synchronous block oriented mode

(e.g., HDLC) will also go active when the end of a message or frame is

received.

Separating Receive Frames Chapter 5 ends with a description of how the “Wait for Rx Trigger”

feature can be used to separate received frames into individual

memory buffers, by withholding the Rx DMA Request for the data in a

new frame until after software has read out the length of the frame and/

or programmed the DMA channel with the buffer address for the new

frame.

UM009402-0201

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Zilog Z16C30 Uživatelský manuál Strana 22