Help understand design requirements using 74AC variants

[Suk]

Hi folks,

Apologies if this question is outside the newbies topic.....

I've read that care needs to be taken with designs using 74AC variants due to ringing. Is there any resource I can refer to explaining the specific design considerations for 74AC over 74HC for a 65C02 and 65C816 SBC?

I'm prototyping with 74HC on solder-less breadboard and expect to go wirewrap (for retro fun and because my soldering is awful, see note below). I have everything I need running right now at 2Mhz. I expect to end up with a 2 layer PCB at some point and would like to get >10Mhz clock. I think 74HC is OK to 8Mhz on a well constructed board so going 74AC will become necessary above that? It's probable that I'll adapt a proved design, any suggestions?

Note: I can only use my left hand following a motorcycle crash in '96. Hence my soldering is rather variable in quality. I've thought about trying wirewrap or SMD with solder paste or drag technique. I'd welcome any thoughts or suggestions on that.

Many thanks,

Steve

[Dr Jefyll]

Hi, Steve

You might want to have a look at the thread Techniques for reliable high-speed digital circuits.

Quote:

It's probable that I'll adapt a proved design, any suggestions?

I don't have a suggestion, but perhaps someone else will speak up.

Quote:

I expect to end up with a 2 layer PCB at some point and would like to get >10Mhz clock. I think 74HC is OK to 8Mhz on a well constructed board so going 74AC will become necessary above that?

There's no exact limit for 74HC clock rates. In fact, at some clock rates it may make sense to use mostly HC logic, with AC used for one or two critical areas. And if the chips are in sockets then you can experiment by changing them.

It strikes me that soldering might be a better choice for you than wire-wrap. Let me describe a trick I sometimes use. Maybe it's one you've not already thought of.

Take the spool of solder and unwind about 10 inches. Place the spool on the edge of the workbench, with the loose end extending outward and drooping down. Then, to do the actual solder joint, place the circuit board on your knees so you can move it around horizontally; also up and down somewhat by stretching your toes. In other words, move the connection to where the solder is, rather than vice versa. In effect, your knees function as an extra hand.

Hope this helps! Have fun, and keep us posted,

Jeff

[BillG]

Dr Jefyll wrote:

It strikes me that soldering might be a better choice for you than wire-wrap. Let me describe a trick I sometimes use. Maybe it's one you've not already thought of.

Take the spool of solder and unwind about 10 inches. Place the spool on the edge of the workbench, with the loose end extending outward and drooping down. Then, to do the actual solder joint, place the circuit board on your knees so you can move it around horizontally; also up and down somewhat by stretching your toes. In other words, move the connection to where the solder is, rather than vice versa. In effect, your knees function as an extra hand.

That would be hard on the back. Mine is hurting just thinking about it.

You know, there may be a market for a solder or PCB holder with foot controls to move it around and up and down.

If it cannot be sold cheaply enough for home use, there are makerspaces and hackerspaces around which may buy them.

[Dr Jefyll]

I don't know why it sounds like a problem. Maybe I'm not describing it properly.

I start by pulling the chair a little further away from the workbench. But my posture is hardly any different than usual. Really the only difference is arching my heels off the floor slightly when it's time to actually solder. The amount of motion is less than an inch. Same for the X-Y motion -- you only need enough to fine tune your aim at the bulls-eye. Everything is 99% in place before you begin.

[Chromatix]

Also, consider using 74AHC where available. It's practically as fast as 74AC, but doesn't have the powerful output drivers that could cause difficulty; the rise and fall times are more similar to 74HC.

With that said, on short signal paths you should have no real difficulty. That means you're sending a signal across a few inches of PCB, in a more-or-less straight line and close to a good current return path (such as a ground plane), and both the origin and destination have well-bypassed power supplies. Where you need to really start paying attention is when the signal goes out on a cable, or when your grounding or power supply arrangements are less than ideal.

Seriously consider using a 4-layer PCB, with power and ground planes, if your design is likely to get complex and fast enough to actually need 74AC logic. Otherwise, I would stick to 74HC.

[drogon]

Chromatix wrote:

Seriously consider using a 4-layer PCB, with power and ground planes, if your design is likely to get complex and fast enough to actually need 74AC logic. Otherwise, I would stick to 74HC.

And on the flip side of that both my 6502 and 65816 boards run perfectly well at 16Mhz on double sided boards. However I use a GAL (or 2 on the '816 board) rather than a "raft of TTL" to do all the decoding, '816 address bus latching, etc.

Make of that what you want...

-Gordon

[Suk]

Dr Jefyll wrote:

Hi, Steve

You might want to have a look at the thread Techniques for reliable high-speed digital circuits.

I will give that a read, thank you.

Quote:

There's no exact limit for 74HC clock rates. In fact, at some clock rates it may make sense to use mostly HC logic, with AC used for one or two critical areas. And if the chips are in sockets then you can experiment by changing them.

I did wonder if I might be making things more difficult. Oh everything I do gets socketed call me mr static cmos the destroyer.

Quote:

It strikes me that soldering might be a better choice for you than wire-wrap. Let me describe a trick I sometimes use. Maybe it's one you've not already thought of.

Take the spool of solder and unwind about 10 inches. Place the spool on the edge of the workbench, with the loose end extending outward and drooping down. Then, to do the actual solder joint, place the circuit board on your knees so you can move it around horizontally; also up and down somewhat by stretching your toes. In other words, move the connection to where the solder is, rather than vice versa. In effect, your knees function as an extra hand.

Interesting. I shall have to try that. I’ve tried the solder in teeth (with lead protection) and get in close but my eyes aren’t good enough anymore!

[Suk]

Chromatix wrote:

Also, consider using 74AHC where available. It's practically as fast as 74AC, but doesn't have the powerful output drivers that could cause difficulty; the rise and fall times are more similar to 74HC.

With that said, on short signal paths you should have no real difficulty. That means you're sending a signal across a few inches of PCB, in a more-or-less straight line and close to a good current return path (such as a ground plane), and both the origin and destination have well-bypassed power supplies. Where you need to really start paying attention is when the signal goes out on a cable, or when your grounding or power supply arrangements are less than ideal.

Seriously consider using a 4-layer PCB, with power and ground planes, if your design is likely to get complex and fast enough to actually need 74AC logic. Otherwise, I would stick to 74HC.

I’m not familiar with AHC but I’ll take a look. I may be unnecessarily jumping to the conclusion that I need the speed of AC anyway. I need to look at designs further along the process and use sockets as previously suggested.

I will look proper 4 layer pcb design. It’s not something I’ve touched before. But this is exactly the kind of pointers I need. Thank you.

[drogon]

Suk wrote:

Interesting. I shall have to try that. I’ve tried the solder in teeth (with lead protection) and get in close but my eyes aren’t good enough anymore!

Have you got any arm mobility? I'm thinking you could strap a reel of solder to your wrist and feed it down through e.g. an empty biro taped to the back of your hand, and, well, poke it at the soldering iron...

And these are brilliant:

https://www.amazon.co.uk/Carson-MagniVi ... 007CDJKM2/

(I have some cheaper ones, but they are very good)

-Gordon

[Suk]

Chromatix wrote:

And on the flip side of that both my 6502 and 65816 boards run perfectly well at 16Mhz on double sided boards. However I use a GAL (or 2 on the '815 board) rather than a "raft of TTL" to do all the decoding, '816 address bus latching, etc.

Make of that what you want...

-Gordon

Do you have any designs available in the public domain I can check out please? I have some GALs here and intend to look into that side of things. It’s been 35 years since I studied electronic engineering as I went the software route in my career. I have a lot to catch up on.

[drogon]

Suk wrote:

Chromatix wrote:

And on the flip side of that both my 6502 and 65816 boards run perfectly well at 16Mhz on double sided boards. However I use a GAL (or 2 on the '815 board) rather than a "raft of TTL" to do all the decoding, '816 address bus latching, etc.

Make of that what you want...

-Gordon

Do you have any designs available in the public domain I can check out please? I have some GALs here and intend to look into that side of things. It’s been 35 years since I studied electronic engineering as I went the software route in my career. I have a lot to catch up on.

No designs as such, but it's documented at the overview level here: https://projects.drogon.net/ruby/

-Gordon

[Suk]

drogon wrote:

Have you got any arm mobility? I'm thinking you could strap a reel of solder to your wrist and feed it down through e.g. an empty biro taped to the back of your hand, and, well, poke it at the soldering iron.

Almost zero unless it’s a bear hug.... nothing from wrist down.

I have a decent visor but I find it hard for the solder in teeth approach.

[Suk]

drogon wrote:

No designs as such, but it's documented at the overview level here: https://projects.drogon.net/ruby/

-Gordon

Thanks I will look