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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.
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 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.
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.
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. Address/RLE transfers are byte-wide
and only allowed in the forward direction.
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.
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.
Command/Data
ECP Mode supports two advanced features to improve the effectiveness of the protocol for some
applications. The features are implemented by allowing the transfer of normal 8 bit data or 8 bit
commands.
