Elan Digital Systems Ltd.
17
HD717 USER’S
GUIDE
3.1.4 SCC TRANSMIT PROCEDURE
The HD717 is intended to operate in a mode where interrupts from
the SCC only occur on special conditions. All data transfers are
performed by the SCC DMAing the data out of the TX 4K FIFO. In
this way the PC is able to load the 4K FIFO with TX data, configure
the SCC, HD717 and finally enable the HD717 DMA controller.
The TX data will automatically be sent without further intervention
from the PC unless the message length is greater than 4K. The
following flow diagram summarises the steps to transmit data:
DISABLE HD717
TX DMA
⇓⇓
CONFIGURE SCC AS
REQUIRED
WITH TX DMA ENABLED
AND
TX ENABLED
⇓⇓
LOAD TX 4K FIFO
WITH
TX DATA
⇓⇓
ENABLE HD717 TX DMA
⇓⇓
⇓⇓
The above process could generate an interrupt request for three
reasons:
1. The SCC signals that it has completed the message (EOM)
2. The HD717 signals that the TX 4K FIFO has fallen below half full
(only applies if original message was 2048 or more bytes long)
3. The HD717 signals that the TX 4K FIFO has fallen to empty
The HD717 will cease to acknowledge TX DMA requests from the
SCC when the TX 4K FIFO is empty. This will force the SCC to
terminate the message and so complete the transfer.
