Elan Digital Systems Ltd.
21
HD717 USER’S
GUIDE
3.1.5 SCC RECEIVE 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 in to the RX 4K FIFO. In
this way the PC is able to unload RX data from the 4K FIFO each
time an interrupt (of the correct type) occurs. The following flow
diagram summarises the steps to receive data:
DISABLE HD717
RX DMA
⇓⇓
CONFIGURE SCC AS
REQUIRED
WITH RX DMA ENABLED
AND
RX ENABLED
⇓⇓
ENABLE HD717
RX DMA
⇓⇓
⇓⇓
The above process could generate an interrupt request for three
reasons:
1. The SCC signals that it has completed the message (EOM) i.e.
message is ready in the RX 4K FIFO
2. The SCC signals that it has a special condition or error
3. The HD717 signals that the RX 4K FIFO has risen above half full
and needs to be emptied of at least 2K of data.
The HD717 will cease to acknowledge RX DMA requests from the
SCC when the RX 4K FIFO is full. This will force the SCC to
generate an interrupt indicating an RX overrun condition.
The Interrupt Service Routine in the PC should use the 80286 “REP
INSB” command to rapidly copy the RX data to its local buffer from
the RX 4K FIFO. The amount of data to copy can be at most 2048
