Here are some typical symptoms, their possible causes, and suggested resolutions, for problems encountered with X68C75-based boards:
1) Frequent errors with PCBUG11, or failures
with ICC11 and DOS batchfile downloading.
-this can be caused by noisy communications.
To see if this is indeed the case, try removing your board from any breadboard
or other circuit in which it is plugged. If the problem is resolved
by doing this, put a 10K pullup resistor on the TX pin of the 50-pin header
(ie. 10K resistor between pin 5 and +5V). The reason this is necessary
is that the internal pullup resistor of the RS232 interface chip is too
high a value to overcome the stray capacitance of your system, and the
resulting RC circuit is distorting the serial data stream being sent by
the 68HC11 to your PC.
One other possibility is an intermittent connection
in your serial cable. Do a continuity check of the RX, TX, and GND
connections between the 9-pin D-sub connector and the 4-pin Molex.
2) Failure to load new program into X68C75 EEPROM.
-the Xicor chip can be rather troublesome at times,
so a little patience & understanding is required. PCBug11 is
the best tool for diagnosing your problem.
The first thing you should do, once you have PCBUG11
running, is to display the contents of the X68C75 register block.
(ie. type MD $8400). You should see a value of $5E at $8400,
followed by mostly zeroes in the other locations. (Users of Adapt11C75DXE
will examine $0400 instead, and should see $7E). If you see a lot
of other values (typically FF, EF, or BF), it means your Xicor chip's internal
state machine is not properly reset. To reset the state machine,
make sure the WRITE PROT switch is in the PROT position, remove power,
and re-apply power. Re-establish communication with PCBUG11 (by typing
RESTART, if you haven't quit already), switch on the 'HC11 expanded mode
bus (by typing MS $103C $E5), and check the Xicor register block contents
again. Try the same procedure again, if necessary. Once you
see mostly 00, then you can be confident the state machine is properly
reset. (BTW, the improper reset state typically
shows up when you have applied power while the WRITE PROT switch is
in the WRITE position.) A good rule-of-thumb with these boards is
to power down and then back up (with switch at PROT) after you perform
some operation (such as download, turning off SDP, etc.). This puts
the state machine in a reset state.
The next thing to do is establish that the EEPROM
of the X68C75 can be altered (ie. establish that Software Data Protection
(SDP) is not on). To do this,
attempt to modify an EEPROM location. For Adapt11C75, Adapt11C75DX,
and Adapt11C75DXR, any location from $E000 to $FFFF will be an appropriate
candidate. For Adapt11C75DXE, use an address between $6000 and
$7FFF. For the purposes of this example, we will assume an Adapt11C75DX
board, and
use $E000. For the memory to be alterable, you should now move
the switch to the WRITE position. Enter MM $E000, to which PCBUG11
responds with the
contents of that location. Enter a new value (other than what
is displayed), and hit ENTER. If you receive an error message, you
can conclude that SDP is on, or
the X68C75 Block Protect bit has inadvertently been set (a very remote
possibility), or that there is a communication error (see item 1, above).
If you concluded that SDP is on, you need to use
the DOS batchfile SDPOFF1 or SDPOFF2 to disable it (as per your Starter
Package Manual). Keep in mind the rule-of-thumb mentioned above concerning
powering up and powering down with the switch at PROT. Once you've
run the batchfile, get back into PCBUG11 and go through the above steps
of checking the register block contents at $8400, and again attempting
to modify an EEPROM location. If the contents still won't
change, go through the SDPOFF batchfile routine once more. This
may sound like a real pain, but once you have SDP off, the chip behaves
rather well.