8bitwizard wrote:Normal TTL data level is like 0.2V low and 2.8V high, which is why 3.3V became a standard. There shouldn't be a problem as long as you can get a voltage converter for the 3.3V power.
It' true, standard TTL use only approx 2,6v as log.1. And WIKI says:
All standardized common TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2.2 V and 5 V[12] (precise logic levels vary slightly between sub-types). TTL outputs are typically restricted to narrower limits of between 0 V and 0.4 V for a "low" and between 2.6 V and 5 V for a "high", providing 0.4V of noise immunity.
But, there are CMOS "TTL". CMOS logic can output almost Vcc at log.1, wich can be more, than 4,5V if Vcc=5V. So, there are strongly reccomended to use level shifter or just limit current by resistor between TTL and LTTL(LVTTL) devices. In practic, 75..100 ohm resistor are best. And, as Shiru says:
Shiru wrote:Some 3V ICs are 5V-tolerant. There are level convertors as well. In one of Flash carts (for SMD) schematics I recall was used 3V Flash with a lot of resistors to convert the levels.
Here is the schematics of the cartridge (bottom part of page 3).
There are 5V signal tolerant 3,3V devices. They don't require any level shifters or converters, but still require 3,3V as power source. For example, MAX3000A CPLD's are 5V tolerant, but use 3,3V as Vcc. So, just RTFM. :3