TStik ™
Interfacing TStik
to
common logic families
TINI400 in the familiar
SIMM72 module
package
TStik is powered by Dallas Semiconductor TINI

There seems to be quite a bit of confusion about interfacing 3.3V devices such as TStik to 5V TTL and 5V CMOS peripherals. This page is an attempt to clear up that confusion and explain how to interface 3.3V devices to common logic families. Here's part of the bottom line: Most modern processors are 3.3V devices and have TTL-level I/O which is generally 5-volt tolerant. You cannot reliably connect an output from a 3.3V device to a 5V CMOS input. Read on for the rest of the story.

TStik and common logic family voltage levels
TStik I/O pins are 3.3V TTL-level, and are 5V tolerant. They will interface to other standard TTL-level devices, regardless of whether those devices are themselves powered by 3V or 5V. "TTL-level" means that the voltages which will be interpreted as a "low" or "high" level are anything less than 0.8 volts, or more than 2.0 volts, respectively. There are a confusing variety of component families available, mostly due to the technology push to reduce power consumption. Any device with a "T" in its prefix (e.g., 74HCT574, 74ACT574, 75FCT574) is TTL compatible as are all "LS" devices (e.g. 74LS374). "HC" or "AC" devices are CMOS-, not TTL- level devices. It's always good advice (and will save you time in the long run) to carefully read the data sheets of any chips or devices you will connect to TStik. Most I/O devices are TTL level, but occasionally you find some non-conforming ones.
TStik input thresholdsVil is 0.8V max, Vih is 2.0V min.
TStik output levelsVol is 0.4V max and Voh is 2.4V min. These are for an output loaded to its maximum specified load. At lighter loads, Vol will be lower (closer to 0.0) and Voh will be higher (closer to 3.3V).
TTL input thresholdsVil=0.8V max and Vih=2.0V max. These thresholds are valid regardless of the supply voltage if the part is specified as "TTL level compatible"
TTL output levelsVol is 0.4V max and Voh is 2.4V min. These are for an output loaded to its maximum specified load. At lighter loads, Vol will be lower (closer to 0.0) and Voh will be higher (closer to Vcc).
HC input thresholds CMOS devices switch at Vcc/2 typical. For devices powered from 3.3 volts this is 1.65V.
CMOS devices switch at Vcc/2 typical. For devices powered from 5 volts this is 2.5V. At 4.5V Vcc, Vil max is 1.35V and Vih max is 3.15V.
LS input thresholdsSame as TTL: Vil is 0.8V max, Vih is 2.0V max.
HCT input thresholdsSame as TTL: Vil is 0.8V max, Vih is 2.0V max.

Interfacing TStik to common logic families
TStik I/O pins are 3.3V TTL-level, and are 5V tolerant. This simple statement defines how they interface to other logic families. Here's a table of the common possibilities.
Logic family
Interface considerations to TStik
5V TTL devices (74HCT, 74ACT, 74LS, 74S, 74F, etc)Connect directly. Fully compatible with either TStik or 5.0V TTL devices as the input or output.
3V CMOS (newer families such as LCX or LVX)Connect directly. Fully compatible with either TStik or 3.3V CMOS devices as the input or output. Read the data sheet to be sure as some older CMOS devices are vague about exact performance at 3.3V
5V CMOS output to TStik input Compatible. The CMOS output will swing below 1V and above 2.3V and keep the TStik input satisfied.
TStik output to 5V CMOS input Won't work reliably. 5V CMOS needs Vih of as much as 3.15V and TStik provides only 2.4V min. TStik actually will provide closer to 3.3V with a very light load but don't count on that. Use a level shifter or buffer. There are special buffers made for this - for example "4245" family such as Philips74LVC4245A or Fairchild 74LVX4245.

TStik and Serial I/O

The DS80C400 chip alone (with no additional hardware) has dual internal UARTs and can interface to TTL level asynchronous serial I/O devices (you can't call TTL asynch devices "RS-232" since they are TTL level). "Asynchronous" and "RS-232" do not mean the same thing.

TStik Serial0 *does* include RS-232 level shifters. "RS-232" -- if the term is properly applied -- means asynchronous serial I/O with RS-232 voltage levels.
 
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