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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).
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.
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.