•
•
•
Set strobe = 0, causing the nStrobe signal to
default to the deasserted state.
Set autoFd = 0, causing the nAutoFd signal
to default to the deasserted state.
OPERATION
Mode Switching/Software Control
Software will execute P1284 negotiation and all
operation prior to a data transfer phase under
programmed I/O control (mode 000 or 001).
Hardware provides an automatic control line
handshake, moving data between the FIFO and
the ECP port only in the data transfer phase
(modes 011 or 010).
Set mode = 011 (ECP Mode)
ECP address/RLE bytes or data bytes may be
sent automatically by writing the ecpAFifo or
ecpDFifo respectively.
Note that all FIFO data transfers are byte wide and
byte aligned.
byte-wide and only allowed in the forward
direction.
Address/RLE transfers are
Setting the mode to 011 or 010 will cause the
hardware to initiate data transfer.
If the port is in mode 000 or 001 it may switch to
any other mode. If the port is not in mode 000 or
001 it can only be switched into mode 000 or 001.
The direction can only be changed in mode 001.
The host may switch directions by first switching to
mode = 001, negotiating for the forward or reverse
channel, setting direction to 1 or 0, then setting
mode = 011. When direction is 1 the hardware
shall handshake for each ECP read data byte and
attempt to fill the FIFO. Bytes may then be read
from the ecpDFifo as long as it is not empty.
Once in an extended forward mode the software
should wait for the FIFO to be empty before
switching back to mode 000 or 001. In this case
all control signals will be deasserted before the
mode switch. In an ecp reverse mode the software
waits for all the data to be read from the FIFO
before changing back to mode 000 or 001. Since
the automatic hardware ecp reverse handshake
only cares about the state of the FIFO it may have
acquired extra data which will be discarded. It may
in fact be in the middle of a transfer when the
mode is changed back to 000 or 001. In this case
the port will deassert nAutoFd independent of the
state of the transfer. The design shall not cause
glitches on the handshake signals if the software
meets the constraints above.
ECP transfers may also be accomplished (albeit
slowly) by handshaking individual bytes under
program control in mode = 001, or 000.
Termination from ECP Mode
Termination from ECP Mode is similar to the
termination from Nibble/Byte Modes. The host is
permitted to terminate from ECP Mode only in
specific well-defined states. The termination can
only be executed while the bus is in the forward
direction. To terminate while the channel is in the
reverse direction, it must first be transitioned into
the forward direction.
ECP Operation
Prior to ECP operation the Host must negotiate on
the parallel port to determine if the peripheral
supports the ECP protocol. This is a somewhat
complex negotiation carried out under program
control in mode 000.
After negotiation, it is necessary to initialize some
of the port bits. The following are required:
•
Set Direction = 0, enabling the drivers.
102
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