Hardware manual

Microcomputer Technical Q&A
80
Q&A No.: QAH8S-208
Category: DMAC
Subject: Activation Request Signal Detection
Question
When the DREQ pin is designated falling-edge input, why is the first transfer request performed at
the low level?
Answer
If a falling edge at the DREQ pin were used for the first activation, it would not be possible to
recognize a request at first when transfer is enabled (DTE = 1) after the DREQ signal falling edge.
For this reason, the first activation after transfer is enabled is by low level detection.
Transfer group A
Transfer group B
Invalid recognition
DREQ normal low level
DREQ normal high level
DREQ
DTE
DREQ
DTE
DREQ and DTE Timing Chart