Starting with my first SBC Project, Plans and Confusions

[Proxy]

GARTHWILSON wrote:

Proxy wrote:

so basically i should make the button active-high with a debouce capacitor and then invert the signal before sending it to the RST pin?
and is it nessesary to be a Schmitt trigger Inverter? if not i can just use 2 extra pins on my GALs to do the job.

The Schmitt trigger input is the whole reason for using that gate (an inverter in this case). A standard gate won't have the desired behavior. Its purpose is to take a slowly rising or falling voltage (like what's on a capacitor that's charging or discharging), and ignore noise on the line and suddenly flip the output when it reaches the right threshold. The threshold for going the other direction will be different. If you don't have any Schmitt-trigger gates in your personal stock and don't want to order and wait for one to arrive, you can make one by adding positive feedback with resistors. I show this kind of think in the last diagram at the bottom of http://6502.org/users/garth/projects.php?project=7, but you won't need the open-collector transistor at the output like I needed there to work with the other RST\ sources in the C64. That transistor inverts the signal again; so the rest of your circuit before that would be done in the opposite polarity.

to be honest since i need to order the CPU, RAM, FT240X, Sockets, etc. i'd rather just also order a schmitt trigger Inverter as well instead of hacking something together.

GARTHWILSON wrote:

Quote:

speaking of which i also need a power-up reset circuit, so the CPU doesn't start execusing random garabge from RAM before reaching the ROM.
so that'll probably require another IC.

No; if I understand you correctly, you're talking about the same circuit. After the power is up and stable, and the oscillator is stabilized, the reset circuit brings the RST\ line up with a single, fast rising edge, and the processor reads address $FFFC and $FFFD to get the address of where your reset routine in your ROM starts.

maybe i worded it wrong but with "speaking of which i also need a power-up reset circuit" i meant "speaking of reset circuits i also need an additional circuit that brings the reset line low automatically on power up". so don't worry i know what a power-up reset circuit is. just i mentioned that i also needed one since i don't have one yet.
anyways, i saw people online use an NE555 timer for this exact thing (including the primer but i think the circuit on there is a manual reset not a power-on reset). opun power up it would keep the output low for a second or something before putting it high and staying at that state until the next power off. LINK

[GARTHWILSON]

The reset circuits in the 6502 primer also work on power-up, not just pushing the button. It gives the reset pulse either way.

[Proxy]

GARTHWILSON wrote:

The reset circuits in the 6502 primer also work on power-up, not just pushing the button. It gives the reset pulse either way.

oh.... well i'm blind.

but i also found this online where someone implements a different Reset circuit to the Primer.
I think I can just copy this one, and it seems like that's a regular inverter instead of a schmitt trigger one, so i should be able to use one of the GALs instead of having to add yet another IC.

[GARTHWILSON]

Proxy wrote:

but i also found this online where someone implements a different Reset circuit to the Primer.

That one is also in the 6502 primer's reset page, at the bottom.

[Proxy]

GARTHWILSON wrote:

Proxy wrote:

but i also found this online where someone implements a different Reset circuit to the Primer.

That one is also in the 6502 primer's reset page, at the bottom.

but how? the Primer's reset circuit is said to be from the Sym-1 while the one i linked is from the C64.
and both have differences between eachother so they aren't the exact same circuit. (though they are very similar)
which is also why i don't really know which to copy.
i went with the one i linked since the Primer one has a Diode in it and i didn't want to bother with that

[GARTHWILSON]

Yes, they're very slight variations on the same circuit. The diode is to discharge the 10uF capacitor when the power goes down, to make sure you really are starting fresh when it comes up. (The diode used for the same function in the second circuit from the top, which I use, is just the one that's built into the CMOS gate for input protection. The time constant of the capacitor and the 5.6K resistor is only about 25ms.)

[Proxy]

GARTHWILSON wrote:

Yes, they're very slight variations on the same circuit. The diode is to discharge the 10uF capacitor when the power goes down, to make sure you really are starting fresh when it comes up. (The diode used for the same function in the second circuit from the top, which I use, is just the one that's built into the CMOS gate for input protection. The time constant of the capacitor and the 5.6K resistor is only about 25ms.)

So i guess i should also include a Diode and just copy the primer Circuit. ok. so what diode should i choose? i know there are different types and that they all have different values like breakdown voltage, voltage drop, etc. but i'm not sure if any of those are really important

[Proxy]

ok so i added the Reset circuit from the Primer in the bottom right. i've hooked the output up to U2 (GAL22V10) as Inverter, which should hopefully work. the diode i put in is very generic... I have some Diodes at home, they are labeled "1N400X" where the X is different from 1 to 6. looking at a datasheet they seem all pretty much the same except for the blocking and reverse voltage.

IMAGE (Edited)

if there are any other noticeable errors, let me know. now i need to make a footprint for the RAM, since that's the only component i had to make myself and didn't find a symbol/footprint for online.
can't i just copy a standard 28 pin DIP footprint from something else?

[cjs]

Proxy wrote:

now i need to make a footprint for the RAM, since that's the only component i had to make myself and didn't find a symbol/footprint for online.
can't i just copy a standard 28 pin DIP footprint from something else?

There are JEDEC standard pinouts for 8-bit wide static ROM (and perhaps RAM as well), and most 24-pin ROMs and RAMs are going to be pretty darn close to the standard. So yeah, you can just copy and tweak. Just confirm everything carefully with the data sheet.

[Proxy]

cjs wrote:

Proxy wrote:

now i need to make a footprint for the RAM, since that's the only component i had to make myself and didn't find a symbol/footprint for online.
can't i just copy a standard 28 pin DIP footprint from something else?

There are JDEC standard pinouts for 8-bit wide static ROM (and perhaps RAM as well), and most 24-pin ROMs and RAMs are going to be pretty darn close to the standard. So yeah, you can just copy and tweak. Just confirm everything carefully with the data sheet.

the pinout is not the problem, i already have the symbol, just not the footprint for the PCB. i worded that wrong again didn't i?
I just meant that i had to make my the symbol myself since there was no download for it on Mouser.

also while i'm in the process of assigning everything it's footprint. are ceramic 0.1uF caps enough for decoupling capacitors? or is there another type i should be using?
I'll just assume every unpolarized capacitor is supposed to be ceramic, and every polarized one electrolytic.

[cjs]

Proxy wrote:

the pinout is not the problem, i already have the symbol (as it can seen on the picture), just not the footprint for the PCB.

Oh, I was talking about footprint: pinouts are footprint, not schematic symbol, to my mind.

I thought about mentioning that the physical dimenions are also standardized by JEDEC, but got lazy and didn't because I figured "DIP is DIP: just copy any existing DIP footprint with the right number of pins and tweak the pinout."

But yes, JEDEC has a specific standard for the physical footprints, as well. A dual-in-line package (DIP) is "A device package configuration that has two parallel rows of pins that are spaced nominally 0.3 inch, 0.4 inch, or 0.6 inch apart with the pins on 0.1-inch centers." The truly gory details of the 0.6 inch devices can be found in MO-015. (That's free to download, but registration with their website is required.) But the one sentence above pretty well sums it up.

And, in case you want them, the specs for byte-wide EPROM pinout standards can be found in EPROM3.4.1.

[Proxy]

cjs wrote:

Proxy wrote:

the pinout is not the problem, i already have the symbol (as it can seen on the picture), just not the footprint for the PCB.

Oh, I was talking about footprint: pinouts are footprint, not schematic symbol, to my mind.

I thought about mentioning that the physical dimenions are also standardized by JEDEC, but got lazy and didn't because I figured "DIP is DIP: just copy any existing DIP footprint with the right number of pins and tweak the pinout."

oh i see. In KiCad Symbols are used for the Schematic, and Footprints for the PCB.

I should now have everything setup... but i'll wait a bit and let some others look over the schematic before i start routing. just in case there are more mistakes. i will update the previous comment that had the "IMAGE" link shortly after this

in the meantime i'll watch some tutorials on routing in KiCad.

[BigDumbDinosaur]

cjs wrote:

There are JEDEC standard pinouts for 8-bit wide static ROM...

Static ROM? What's that?

[cjs]

BigDumbDinosaur wrote:

cjs wrote:

There are JEDEC standard pinouts for 8-bit wide static ROM...

Static ROM? What's that? :D

It's ROM that doesn't need regular refresh cycles in order to retain data! I think it's actually the most common kind. :-)

(Ya, actually I meant to write "ROM (and maybe static RAM)" up there. Most SRAM I've seen has, at first glance, what look like JEDEC pinouts, but I've not actually bothered to dig into their standards to see what JEDEC has to say about this.)

[BigDumbDinosaur]

Proxy wrote:

..."speaking of which i also need a power-up reset circuit" i meant "speaking of reset circuits i also need an additional circuit that brings the reset line low automatically on power up".

The simplest route and also the one that consumes the least amount of board space is using the Maxim DS1813 reset controller. It's available in TO-92 or SMT packaging (SOT-23, which is really tiny). The DS1813 is sensitive to power supply voltage, so it will hold the MPU in reset until 150 milliseconds after the power supply voltage has reached at least 4.0 volts (depends on which model is used). A pushbutton wired from reset to ground can be used to trigger a manual reset—the same 150ms pause will occur. No other components are required. See attached.