And the correct resistor value is...

[+Larry]

In using "pull-up" resistors to select from multiple OS's placed in single eproms, I've seen a variety of recommended values from a low of something like 330 ohms to a max of 4.7K ohms. I've never seem any explanation of how the resistor value is chosen, although I believe that somewhere I read that you should choose the lowest value that worked reliably.

 

Can someone explain the pro's and con's of "low" versus "high" resistance values in this application? Does it really matter?

 

-Larry

[CPUWIZ]

I always use 1K, never had a problem.

[+batari]

In using "pull-up" resistors to select from multiple OS's placed in single eproms, I've seen a variety of recommended values from a low of something like 330 ohms to a max of 4.7K ohms. I've never seem any explanation of how the resistor value is chosen, although I believe that somewhere I read that you should choose the lowest value that worked reliably.

 

Can someone explain the pro's and con's of "low" versus "high" resistance values in this application? Does it really matter?

 

-Larry

The smaller the resistor value, the more current your logic devices and/or CPU must sink. It's a good idea not to use too small a value as the device may only be rated to sink a few milliamps. In the case of the 6502, it is rated to sink 1.6 ma, which isn't very much. The smallest pullup you'd want to use on any 6502 data or address line is around 3k, but larger would be better. Logic chips are better, as many can sink 20 ma or so, which would be around 250 ohms as the smallest you could use (not that I'd recommend that, though.)

 

I can't imagine why anyone would suggest using the smallest resistor value that works reliably, though. The smaller the resistor, the larger the current drain and larger the load on the outputs. Logic gates will not only switch more slowly when loaded, they will output less than their peak logic levels, and fanout, if applicable, will be limited. I would rather say that one should use several times larger than the smallest value that works reliably.

[+bob1200xl]

You want a resistor value that is low enough to 'pull' the input into a 'high' level under all conditions. Depends on the chip physics as to what that may be. For a switch on the input of a CMOS circuit, you could use a 100K resistor. For a TTL circuit, you need to be much, much lower - perhaps 330 ohms, even. (I'd have to look it up - can't find a TTL book at the moment) When you see a circuit that uses an odd value of resistor, be aware that it may have problems if you change technology.

 

Too high a value may affect the rise-time of the circuit. Too low a value might make it too difficult for the driving circuit to switch levels.

 

No, it doesn't matter, usually. I use whatever value I have the most of... <== design secret!! It's pretty hard to determine reliability in our case. You'll get bad switches, noisy wiring, mis-read color codes,,, bunch of 'gotchas'.

 

I have a whole bunch of 1.1k resistors...

 

 

Bob

 

 

In using "pull-up" resistors to select from multiple OS's placed in single eproms, I've seen a variety of recommended values from a low of something like 330 ohms to a max of 4.7K ohms. I've never seem any explanation of how the resistor value is chosen, although I believe that somewhere I read that you should choose the lowest value that worked reliably.

 

Can someone explain the pro's and con's of "low" versus "high" resistance values in this application? Does it really matter?

 

-Larry

[1050]

6.8k if you will allow Atari to set precedence on their XE supercarts. The 330 ohm resistors you've mentioned are used between the old and new memory data buffers on memory expansions and can't really be called "pull up" resistors. Low values would negatively effect the fan out rating which is a term describing how many OTHER devices can be hooked up to the same address or data line and have it still work. Which flies right in the face of what you remember about how it's best to use the lowest value that proves to be reliable. Do just that and you are almost guaranteed NOT to be able to do any further mods.

 

Using the highest value that proves reliable doesn't seem to have any drawbacks...

[Bryan]

The responses so far are pretty much on target. It all depends on what you're pulling up (a single input vs. a bus of several IC's), the switching speed you require (smaller value resistors pull up faster), and the sinking capacity of whatever will be pulling the line down. The good news that slow stuff like our 8-bits tend to be forgiving of non-optimized values. Selecting an OS involves signals that won't be changing at all so the resistor can be anything that will put the input in a high-state but not overtax the power supply.

 

In critical applications you'd do a bunch of math using values from the datasheets of all the IC's involved and possibly add in some margin for the parasitic effects of your PCB.

Edited December 17, 2009 by Bryan

[jaybird3rd]

Thanks to everybody for their responses in this thread.

 

I have a question about the use of pull-up resistors in one particular project of mine. I'm building a custom multicart--not for the A8 computers--which switches between eight 8K banks inside a 27C512. I've connected the top three address lines to SPDT switches, wired so that all three are always switched either to GND or to VCC (none of them are ever left "floating"). Should I be using pull-up resistors instead? The present design seems to work fine without the resistors in the brief amount of testing I've done so far, but I want to make sure I'm "doing it right."

[re-atari]

Thanks to everybody for their responses in this thread.

 

I have a question about the use of pull-up resistors in one particular project of mine. I'm building a custom multicart--not for the A8 computers--which switches between eight 8K banks inside a 27C512. I've connected the top three address lines to SPDT switches, wired so that all three are always switched either to GND or to VCC (none of them are ever left "floating"). Should I be using pull-up resistors instead? The present design seems to work fine without the resistors in the brief amount of testing I've done so far, but I want to make sure I'm "doing it right."

Using a rotary BCD-encoded switch will make selecting banks a lot easier. Well, that's the way I built my homebrew 32-in-1 OS in a 28F020 on a piece of veroboard. I went the 'legal' route and tied the top 4 adresslines via a pull-up resistor to +5V. BTW, I used 4 4k7 resistors.

 

re-atari

[atariksi]

Thanks to everybody for their responses in this thread.

 

I have a question about the use of pull-up resistors in one particular project of mine. I'm building a custom multicart--not for the A8 computers--which switches between eight 8K banks inside a 27C512. I've connected the top three address lines to SPDT switches, wired so that all three are always switched either to GND or to VCC (none of them are ever left "floating"). Should I be using pull-up resistors instead? The present design seems to work fine without the resistors in the brief amount of testing I've done so far, but I want to make sure I'm "doing it right."

Using a rotary BCD-encoded switch will make selecting banks a lot easier. Well, that's the way I built my homebrew 32-in-1 OS in a 28F020 on a piece of veroboard. I went the 'legal' route and tied the top 4 adresslines via a pull-up resistor to +5V. BTW, I used 4 4k7 resistors.

 

re-atari

 

4k7 = 4.7k I guess (rather than 4000+7 ohms).

 

PC Gameport uses 1Kohm pull-ups for the buttons.