Re: [N8VEM-S100:2419] Re: 6502 CPU Board V1 components question

[David Riley <frave...@gmail.com>]

On Feb 17, 2014, at 1:50 PM, Vince Mulhollon <vincemu...@gmail.com> wrote:

> I obtained a copy of the Wilcox book to go with my 68000 board build and this kind of thing is discussed at length (pages and pages of equations) early in the book, from memory less than 500 ohms could hit the overcurrent limit for his design and more than 8500 ohms would undercurrent as a pullup depending how thirsty the inputs were for current.  Lower switches faster but wastes power, higher uses less power but switches slower.

Higher can also be more susceptible to EMI if you're attaching to CMOS inputs (but that doesn't really come into play until you're talking about resistances over 10K).  As you point out, the CMOS input bias current overwhelms the pullup long before that.

As to slow switching: bear in mind that a bigger resistor (higher resistance) will actually switch LOW faster, because there's less for the gate to pull up.  Once the gate closes, though, it will result in a slower rise time, which can actually cause hysteresis-related glitching problems unless you have it feeding into a Schmitt trigger (which you probably don't).  That's why a lot of parts specify maximum rise times.

> I ran the numbers myself using a real live Motorola 74LS682 datasheet and you've got two criteria:
> 
> 1) Too low of a resistance to Vcc and a TTL L output will overcurrent and catch fire.
> So figure the data sheet Vol = 0.5V and Vcc at 5 V, thats 4.5 volts across the resistor.  And the 74LS682 (or most any LS) will catch fire around 24 mA so 4.5V/0.024A = 187 so less than 190 ohms to Vcc could burn out a LS output.  Thats assuming there's no load on the TTL output other than the resistor, which would be pretty pointless.  So I'd keep it WELL above 200 ohms.

This can be a real concern when you're trying to drive LEDs, which is why you probably should avoid doing so with 74LS parts.

> TLDR don't worry about 1500 ohms, on the other hand, I wouldn't go trying 4700 ohm packs..

4.7K is not an uncommon value for CMOS circuits, especially when you want to keep the power dissipated by pullups/pulldowns to a minimum.  I don't think I'd use it for TTL unless there was a good reason.


- Dave