Zilog Z16C30 User Manual
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Z16C30 USC
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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