On 10/15/20 10:13 AM, Josh Malone wrote:
That sounds awesome, Brian! I'm excited to build one of these once the
PCB is ready. Is the flash version able to be written by the WP-2? Or
is this just for running z80 programs on the WP-2 which you flash
externally?
-Josh
On Thu, Oct 15, 2020 at 12:45 AM Brian White <b.kenyo...@gmail.com> wrote:
The last piece I as waiting for came in today and the ram card works both
mechanically and functionally.
Ok finished the next round of iterations on these WP-2 expansion cards.
Now waiting for the boards to come it to verify it's all still good.
Lot's of cleanup and a couple new boards.
Now there 3 main types of cards and a programing adapter.
v003_RAM - is a 128K SRAM board that snaps into the 3d-printed carrier.
It has a cap, resistors and a diode to provide backup power for about 10
minutes.
v004_RAM_B - is a large board that fills the slot itself instead of
using a 3d-printed carrier. It has the same 128K SRAM and cap as
v003_RAM, and also adds a coin cell battery, which should preserve the
memory for about 10 years vs about 10 minutes. The cap is still there
mostly because "why not?" and it allows you to change the battery
without losing the contents.
v003_ROM - is a 128K or 256K "ROM" (flash) board that snaps into the
3d-printed carrier, and only has a single pullup resistor aside from the
flash chip.
programming adapter for the ROM board - simple pcb with a set of regular
2.54mm pitch DIP pins on the bottom to go into a TL866 or similar, and
one row of 1.27mm pitch pins on the top which you stick the ROM board
onto to program it.
https://github.com/bkw777/WP-2_IC_Card
I could use help making less naive backfeed prevention and reverse
polarity protection circuits on the two ram boards..
On the regular ram board with only a cap, no battery, I don't need to
worry about reverse polarity, just backfeeding from the cap to the WP-2
VDD rail when you turn the WP-2 off with the card plugged in.
Right now I just have a Schottky diode between the the WP-2 VDD and the
rest of the card. This drops the working voltage on the card from 5.1v
to 4.7-4.8, which drops the run-time of the cap. This isn't *so* bad,
but I believe something better can be made without getting too crazy
with too many parts. On this board the diode just handles the case when
you turn the machine off with the card still plugged in. If you just
pluck the card out while it's powered on, the cp alone (no diode)
preserves the ram for at least 10 minutes. That's not really long enough
to be good for much, but it is a good safety against accidents. The
power button is too easy to hit, so even a few seconds grace period is
worth having. Several minutes is way more than needed for accidents, but
not enough to say, pack up, drive home and download stuff later in the day.
The real problem is the battery-backed board.
I decided it really should have reverse polarity protection from the
battery holder. It's just too easy to put the thing in backwards. So I
added another of the same schottky diodes on the + from the battery.
This does double duty and also takes care of protecting the 3v battery
from the 5v active running power. But there, the small voltage drop
hurts more, because the battery is only 3v to begin with. That could
really benefit from a proper pmos circuit or maybe just an ordinary
switching or rf transistor? Or I can actually fit 2 battery holders on
the board to get 6v which I can then drop to 5, but I shouldn't have to
do that.
But the simple diode circuit will at least work reasonably. It just
won't hold memory for as many years, but still "several" regardless. So
the battery-backed ram board should work like a nice removable "flash"
drive.
Pics (renders), and viewable svg prints of the schematics all in the
readme on github. (link above)
--
bkw
