The most significant bit of the command
indicates whether it is a run-length count (for
compression) or a channel address.
Command/Data
ECP Mode supports two advanced features to
improve the effectiveness of the protocol for
some
applications.
The
features
are
When in the reverse direction, normal data is
transferred when PeriphAck is high and an 8-bit
command is transferred when PeriphAck is low.
The most significant bit of the command is
always zero. Reverse channel addresses are
seldom used and may not be supported in
hardware.
implemented by allowing the transfer of normal
8-bit data or 8-bit commands.
When in the forward direction, normal data is
transferred when HostAck is high and an 8 bit
command is transferred when HostAck is low.
Table 42
Forward Channel Commands (HostAck Low)
Reverse Channel Commands (PeripAck Low)
D7
D[6:0]
0
Run-Length Count (0-127)
(mode 0011 0X00 only)
1
Channel Address (0-127)
Data Compression
The FDC37C669 supports run length encoded
(RLE) decompression in hardware and can
transfer compressed data to a peripheral. Run
length encoded (RLE) compression in hardware
is not supported. To transfer compressed data
in ECP mode, the compression count is written
to the ecpAFifo and the data byte is written to
the ecpDFifo.
indicates that the next byte should be expanded
to 128 bytes. To prevent data expansion,
however, run-length counts of zero should be
avoided.
Pin Definition
The drivers for nStrobe, nAutoFd, nInit and
nSelectIn are open-collector in mode 000
and are push-pull in all other modes.
Compression is accomplished by counting
identical bytes and transmitting an RLE byte
that indicates how many times the next byte is
ISA Connections
to be repeated.
Decompression simply
intercepts the RLE byte and repeats the
following byte the specified number of times.
When a run-length count is received from a
peripheral, the subsequent data byte is
replicated the specified number of times. A
run-length count of zero specifies that only one
byte of data is represented by the next data
byte, whereas a run-length count of 127
The interface can never stall causing the host to
hang. The width of data transfers is strictly
controlled on an I/O address basis per this
specification. All FIFO-DMA transfers are byte
wide, byte aligned and end on a byte boundary.
(The PWord value can be obtained by reading
Configuration Register A, cnfgA, described in
the next section). Single byte wide transfers
105
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