The single in line resistor works, but it slows down the fall and rise times, not a problem with "slow" logic like you probably use with the MSP, but imagine / calculate the problem you get with a 100 MHz bus signal. Some CMOS logic families will latch up the output with this type of drive when the high voltage rail comes up before the low voltage rail. (Not fun to find that fault!)
The FET procedure mentioned earlier works fine. "Quality Semiconductor" have a range of level translator transceivers that work on this principle and feature stunning propagation/delay times (much better than the best of the 74 series devoted to level translation.). But, you don't get any extra drive current ... The problem Chris mentions with jacking up the supplies is a real problem. I have seen a (5v) RS232 converter that pushed the 3.3 volt rail to 4.6 volts (on an MSP430), just by pushing the rail up through the protection diodes (on the RS232 I/O pins) Another problem I have seen with the MSP430 is difficulty with Reset. I (now) deliberately load the MSP supply rail with a 1K resistor to drop the supply rail on powerdown, as otherwise it takes several seconds for the supply to drop enough to guarantee that a correct reset occurs. (A 2u2 capacitor takes a long time to discharge through the powering down current of just the MSP, probably nano-amps below 1 volt...) The Data sheet spec for reset to be guaranteed is 0.4 to 0.0 volts, the supply can easily hover in this region for many seconds without some extra load. Cheers Harry [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]]on Behalf Of Chris Liechti Sent: Tuesday, October 08, 2002 8:21 AM To: [email protected] Subject: Re: [Mspgcc-users] Guys, what do you think of this... Am 07.10.2002 17:26:55, schrieb "Ricardo H. Wiggers" <[email protected]>: >Thanks, but I still have a question.. In the case o MSP can I rely on the >internal protection diodes and simply add a resistor between buses (5V and >3V), provided my 5V chip accepts TTL levels (and MSP grants it)? With TI >2407 I know it´s possible, provided I keep the current low enough (and thats >not a problem with MOS...). Lets say a 10k res. between buses (in series). > >----- -------- > | | >MSP |----\/\/\-----| 5VCI > | | >----- -------- that works - somehow... i think the protection diodes can take up to 2mA so that's basicaly no problem to connect 5V peripherals through a 1K or bigger resistor. the problem is, that you inject current to VCC. this means it raises VCC if you don't drain the current somewhere. and that can become a problem, especialy as the MSP features low power modes ;-) so, if you drain the current, you inject over the protection diodes, you can do it. if you don't do it, the supply voltage of the MSP can get up to 5V, beyond the specs... (imagine the rest) chris ------------------------------------------------------- This sf.net email is sponsored by:ThinkGeek Welcome to geek heaven. http://thinkgeek.com/sf _______________________________________________ Mspgcc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/mspgcc-users
