this is a raspberry pi pico it's not
like the other raspberry pi's it doesn't
run linux or any desktop operating
system but it does have otg usb and can
generate a clean vga signal
so this four dollar microcontroller
board is only a few parts away from
being a full computer comparable with
something from the 1980s
all you do is plug a usb hub in here and
wire up a vga connector with some
resistor dividers to the gpios
i thought
what if you attach this to a
retroprocessor like the 6502
with the pi pico in the design we no
longer need a clock generator reset
timers address logic gpio chips or roms
we don't need any chip programmers
either the pi pico is programmed with
uf2 which is simply copying a file over
a usb cable
you don't even need special software
just the copy command from your
operating system
i suppose a case could be made that we
don't need the 6502 either
but then we wouldn't have a 6502
i eventually got the design down to a pi
pico 6502 64k of ram and 12 glue chips
this is for a retro computer with vga
video pwm audio expansion slots and
support for usb keyboards mice joysticks
and mass storage devices
you can see everything on the breadboard here
here
these are all through-hole parts but
available in surface mount packaging as well
well
there's two pi picos here
the second one is acting as a debug
probe and console port but isn't
strictly necessary
it's connected to my development system
over usb which also powers everything
the vga cable is actually the end of a
vga2 hdmi converter
the pi pico is able to provide accurate
vga timings so hdmi output is perfect
with no added frame buffer lag even on
cheap adapters
but the real gem here is the usb host controller
controller
no more grungy ps2 keyboards and clumsy
sd cards just plug ordinary usb devices
straight into this hub
there's nothing special about the hub
but you might need an adapter cable to
fit it to the pipe picot
but with usb and vga wired up i can
write a lot of the pi pico software so i did
did
computers of this era typically had
software called a monitor not to be
confused with the monitor of your
terminal which was a cathode ray tube
monitors replaced panels of blinking
lights and switches as many computers
transitioned from core memory to the ram
and rom we used today
typically the monitor runs on the main
cpu but the pico computer has its
monitor on the dma controller the serial
port on the pi pico gives us access to
the monitor at all times
here it is in minicom
the primary function of a monitor is to
inspect and modify memory so having it
functional while bringing up the dma
controller should be handy
typing in an address will show the
memory contents
here my keyboard is recognized for some
reason it indicates it can send mouse
anyhow
okay uh mouse
for some reason this mouse indicates it
can send keyword reports but i've never
seen it do so
i'm just putting events to standard out
for testing
left click right click and
and
yep looks great
the analog data is all 8-bit so looks
like the mouse reports are going to be
i was able to see an sd card in a card
reader too i haven't tried reading files
now let's look at vga output what you're
looking at here is the terminal emulator
running on the pi pico
so there's two ways to access the
monitor over the serial port or with the
usb keyboard and vga display
i think this is the first color ansi
terminal emulator for the pi pico this
isn't the primary input and output for
the main computer though it's just a
conveniently built in terminal pressing
scroll lock on the keyboard switches the
terminal on and off
turning it off we can see a mock-up of
the low-res 40 column mode with the
commodore petsky font
of course the final video system will
have graphics sprites and parallax
scrolling but a character mode is
sufficient to keep things visually
interesting during this part of development
development
you've seen both graphics resolutions a
high-res 640 by 480 and the low-res 320
by 240.
either of these can be letterboxed to
16x9 and output 720p
720p
it doesn't matter if you have an ibm
8513 from 1987
or modern widescreen lcd the pico
computer output is very easy to work with
however it does not and will not support
old televisions to be clear it could but
that's a lot of work i won't be doing
because there's already plenty of retro
video hardware for the 6502 that
supports televisions that's what the
let's bring up the monitor and graphics
system simultaneously for the next bit
this is easy enough since the monitor is
always available on the serial port
video memory has its own 64k separate
from the 6502 64k of memory
the dma controller will be able to move
data between usb storage
video memory and 6502 memory
although dma is not working yet we can
manually modify video memory with the
monitor i've worked out an address and
hello world
now imagine using the same technique to
load a 6502 program into ram
that's all for now some of my design
notes are included in the github
repository link in the description i'll
make videos explaining each section as i
bring it up
if you want to learn about the design
and operation or see how i deal with the
unexpected hit that subscribe button
