Re: Starting P21Forth

[Andrew Sieber <asieber@xxxxxxx>]

Louis Frazier wrote:
> I am considering building an adjustable 5v power supply because none of
> the old pc and ttl stuff I have lying around goes more than a couple of
> mv above 5.
> It seems as though I ought to be able to feed the p21 5.5 and the rest
> of the chips 5 if heating cooks the dram too badly. P21 runs only
> slightly warm to the touch at 5v.
I just tried that a few minutes ago.  Had everything but the P21 hooked
to 5V, and the P21 itself hooked to a variable voltage supply.  Tried
everything from 3 to 7 volts, didn't want to go past that because I
don't know exactly how far a P21 will go before it fries.
No luck yet.
I'll respond to the other stuff separately, via email.


Dave Lowry wrote:
>     On my board, I almost always have to do a manual reset to get things going.
>     As I recall, something is broken in the P21 reset design.  I just use a
>     momentary pushbutton.
I have tried manual resets.  No luck.  Well actually I wasn't using a
pushbutton, I just had a 100k resistor to 5V and .1 uF cap to ground
hooked to the reset pin as shown in Dr. Ting's schematics, and for a
manual reset I just short out the capacitor with a screwdriver (the
nearest convenient metal object.)  This is some high-tech breadboarding
I'm doing here.
A few minutes ago while I was redoing my power hookup I decided to do
things right and actually hook it to a pushbutton.

> RE: Running P21 at 5+ volts
> 
>     Most chips are rated for 10% power supply operation, so 5.5 volts should
>     be no problem.  I don't have my P21 set up right now, but if memory serves
>     the video is glitchy at even 5.5 volts.  I think I was running upwards
>     of 6 volts.  This will make the DRAM run hot and probably reduce its
>     life.
See above.  I can run the P21 at any voltage I want and have the other
stuff at 5V, so I don't fry the DRAM, etc.


Jeff Fox wrote:
> P21A5? Which board is that?  P21Forth 1.02 comes in three versions.  One
> for simulator, one for the development system (with the i/o chip I have
> on mine) and one for the kit board with the parallel port.  I don't recall
> the version number of the boards.  It does not run on the latest board
> with only the 82C51 serial port i/o.  I did start debugging a port to
> that board but never finished it.
I am using the P21Forth that comes in your 128kB slow.rom image file,
which is zipped along with fast.rom in your 102roms.zip file, which is
on your web site at
http://www.dnai.com/~jfox/p21forth.html
The schematic that I'm using has a 74HC245 input port and a 74HC574
output port, which are controlled by a 74HC138 which decodes the P21's
address lines when the P21's I/O pin goes low.  No 8255 or 8051.  To
make it as simple as possible (and increase chances that I can get
something to work), I have simplified the circuit to nothing more than a
P21, 128kB EPROM, a 14.3MHz oscillator, and a 70ns 1x36 SIMM.  No I/O. 
I will be delighted to simply get it to throw "P21Forth" up on the
screen after I hit the processor reset pushbutton, even though I will
have no keyboard hooked up with which to talk to it.  That will verify
that everything is working properly.
Since the 128kB eprom of course has only 17 address lines, the P21's
A17, A18, and A19 are not used during a ROM access.  This means that the
ROM image is going to appear eight times across the 1 meg ROM address
space.  I assume P21Forth will run that way.

> P21Forth 1.01 booted w/ either serial or parallel i/o based on what
> it found connected at boot.  P21Forth 1.02 booted in serial i/o mode on
> the parallel port.  Both place "P21Forth" on the video display before
> they read input.
Good.  That means I can still see, without having to hook up I/O chips,
whether the system is booting up properly.

> I would recommend first debugging using the simple OK demo rom.  It is
> a little easier to get 2K of code up and running than 16K of code.  Also
> OK has much less off page program access and therefore seems to run over
> a wider voltage range than P21Forth.  As the documentation for P21Forth
> says it will run from about 4.9 to 6.x volts depending on the brand of
> drams used.  The ones that came with the kit would not run above 5.4.
> The P21Forth documentation recommends running at 5.8 to 6.0 if you
> can as the increased voltage will reduce video noise.
I've tried all of those voltages and everywhere in between, but have not
had any success yet.  I will try out the OK demo ROM if I don't get the
P21Forth ROM working soon.

I guess I should admit that I have pretty much a rat's nest
breadboarding job that I am calling a "system", and that may very well
be the problem.
Has anybody on this MISC list succeeded in getting P21 running on a
breadboard (_not_ with wirewrapping or PC board), with jumper wires
running everywhere?
I kind of assumed that even at the 20MHz+ speeds the P21 accesses the
dram with, I wouldn't have inductance/capacitance problems with a
breadboarding job.

Another problem I had was that I was using 1989 specs on SIMM chips from
Motorola, and the memory book with the pinout list two pins as
no-connects that are actually RAS pins.  I fixed that problem in my
circuit when I saw the discrepancy between the old pinout and a new one
I just got, but the system still doesn't work, so there are other
problems.

Penio Penev wrote:
>We had the following discussion with one of our MuP21 engineers:
>
>> Regarding Andrew's letter we can give the following suggestions:
>>  - larger power decoupling capacitors (220n to 1uF);
>>  - large (>4000 uF) capacitors before and after the 7805 regulator.
>>  - a stable AC source yielding quiet .5 amp at 7.5 V;
>>  - reliable sockets.
>> Also, the 330pF in the video output may be safely removed.
>
>Take it for what it's worth. (And let me know of the result :-) 
I have well-filtered power supplies, but for good measure I added some
more capacitors, both big ones and tiny ones (for high-frequency
suppression).  I have already learned the hard way, with other projects,
that a stable power supply is essential for digital circuitry.  The only
socket I'm using right now is for the SIMM; everything else is plugged
into a breadboard.  You may be right, I might have bum connections.  It
is a well worn breadboard, and some of the connectors have borne very
high currents (eg. from shorted power supplies!)  And I never had the
33pF video cap there to begin with, because I assumed all it was for was
making the video prettier.  I'll be happy to get any video at all.

When I eventually start writing code for P21 that doesn't rely on
P21Forth (ie. I burn my eprom with my own custom programs), of course I
will have to XOR all of the programs' address and data with AAAAA. 
However, I believe I will also have to XOR the address lines leading to
the eprom chip while it's being programmed (or do an equivalent
transformation in software, of course), so that byte zero is at eprom
address AAAAA, byte one is at eprom address AAAAB, byte two is at eprom
address AAAA8, byte three at AAAA9, four at AAAAE, etc.  This is because
P21 will be loading "sequential" bytes in this order due to its bus
interface which XORs everything with AAAAA.
I assume the slow.rom image file already has all necessary
transformations like this already performed.  Ie. I programmed byte 0
into eprom 00000, byte 1 into 00001, byte 2 into 00002, etc. without
doing any XORing.

--Andrew