| ??? 01/14/05 12:53 Read: times |
#84972 - Incompatible? Responding to: ???'s previous message |
Alberto said:
Yes, both are tolerant at 5V input, but with a high risk, and they are not capable to drive TTL inputs, stop it !!! TTL and 3.3V logic are INCOMPATIBLE. Let's have a look at a chip that is 5V tolerant: http://www.fairchildsemi.com/ds/74/74VHC244.pdf When running at 3.0V output voltages are VOHmin = 2.48V @ IOH = -4mA and VOLmax = 0.44V @ IOL = 4mA at -40°C...+85°C. This is very well suited to drive a TTL-input! With 3.3V supply voltage things are even better. What about inputs? At 3.0V supply voltage VIHmin = 2.1V and VILmax = 0.9V at -40°C...+85°C. So, the input is very well suited to be driven by a TTL-output. With 3.3V supply voltage situation looks a bit different: VIHmin = 2.3V and VILmax = 1.0V at -40°C...+85°C. So, VIHmin is farer away from TTL-standard (2.0V). But, there's a simple way to accomplish TTL-compatibility, by the introduce of pull-up resistor! How must this be designed? Stray capacitance from input to ground must driven fast enough to fullfill slew rate limitations of to be driven chip. For the 74VHC244 a maximum rise time of 100nsec/V must be guaranteed. This yields 300nsec for a 3V step and if we assume a time constant of 100nsec then we are safe: A stray capacitance of 50pF would need a 2k pull-up resistor per input, while a 100pF stray capacitance would need a 1k pull-up. If very little stray capacitance is involved, let's say only 10pF, then even 10k pull-up is enough. So, you see that these 5V tolerant chips are very well suited to be directly connected to 5V systems: They can drive 5V systems and can be driven by 5V systems. And by introducing pull-ups noise margins can even be improved. The only danger, namely the latch-up possibility, is anyway eliminated by avoiding any input clamp diode circuitry to Vcc, just by design. Kai |
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| My laptop hard drive | 01/01/70 00:00 | |
| ATA specs | 01/01/70 00:00 | |
| ATA specs | 01/01/70 00:00 | |
| Level converters | 01/01/70 00:00 | |
| That's right | 01/01/70 00:00 | |
| should most often be used | 01/01/70 00:00 | |
| 3.3 Volts logic with 5V input ? | 01/01/70 00:00 | |
| LPC9xx has it !! | 01/01/70 00:00 | |
| OK TTL is now =xxV | 01/01/70 00:00 | |
| LPC9xx and 5V | 01/01/70 00:00 | |
| not beginners but ignorami | 01/01/70 00:00 | |
| Begginers or not | 01/01/70 00:00 | |
| well, look here | 01/01/70 00:00 | |
| latchup | 01/01/70 00:00 | |
| RTFM | 01/01/70 00:00 | |
| Cygnal Has It !! | 01/01/70 00:00 | |
| Cygnal and LPC9xx | 01/01/70 00:00 | |
| Incompatible? | 01/01/70 00:00 | |
| Inconpatible ? | 01/01/70 00:00 | |
| 5V tolerant | 01/01/70 00:00 | |
| 5V tolerant. | 01/01/70 00:00 | |
| Relax! | 01/01/70 00:00 | |
| OK is only for Rigo | 01/01/70 00:00 | |
| No more logic levels | 01/01/70 00:00 | |
| Logic Lines Not Hard Lines | 01/01/70 00:00 | |
| Only one answer possible. | 01/01/70 00:00 | |
this guy is crazy .... | 01/01/70 00:00 |