rsd@sei.cmu.edu (Richard S D'Ippolito) (01/12/91)
If my memory is serving me correctly, I seem to recall (from 10 or more years back!) that there is a fundamental design flaw with the 555 timer chip, in that there are certain operating conditions and external part values which could cause part of the chip circuitry to latch up (like a thyristor), a condition requiring power removal to reset, making the 555 definitely unsuitable for a watchdog design. My further recollection is that the flaw was publically aired in some of the professional/trade magazines, such as EDN, causing no little excitement for the original manufacturer. Can anyone verify or refute this and point me to the information? Am I thinking of another chip? If true about the 555, does anyone know if this design was corrected or know of a prefered replacement part? Thanks, Rich
amichiel@rodan.acs.syr.edu (Allen J Michielsen) (01/13/91)
In article <10014@as0c.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) >years back!) that there is a fundamental design flaw with the 555 timer >chip, in that there are certain operating conditions and external part >values which could cause part of the chip circuitry to latch up (like a > >Rich I have used the 555 in many a circuit, and will testify that it is a very good chip. It may still have a tendancy to lock up at the ends of the design limit, but usually the chip specs from like ti are quite reasonable and reliable. al -- Al. Michielsen, Mechanical & Aerospace Engineering, Syracuse University InterNet: amichiel@rodan.acs.syr.edu amichiel@sunrise.acs.syr.edu Bitnet: AMICHIEL@SUNRISE
ftpam1@acad3.alaska.edu (MUNTS PHILLIP A) (01/14/91)
In article <1991Jan13.044249.1640@rodan.acs.syr.edu>, amichiel@rodan.acs.syr.edu (Allen J Michielsen) writes... >In article <10014@as0c.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) >>years back!) that there is a fundamental design flaw with the 555 timer >>chip, in that there are certain operating conditions and external part >>values which could cause part of the chip circuitry to latch up (like a >> >>Rich > I have used the 555 in many a circuit, and will testify that it is a very >good chip. It may still have a tendancy to lock up at the ends of the design >limit, but usually the chip specs from like ti are quite reasonable and >reliable. I have noticed in the past that some makes or perhaps lots absolutely need bypass capacitors on pin 8 and pin 4. Philip Munts N7AHL NRA Extremist, etc. University of Alaska, Fairbanks
ftpam1@acad3.alaska.edu (MUNTS PHILLIP A) (01/14/91)
In article <1991Jan14.055718.18614@ims.alaska.edu>, ftpam1@acad3.alaska.edu (MUNTS PHILLIP A) writes... >In article <1991Jan13.044249.1640@rodan.acs.syr.edu>, amichiel@rodan.acs.syr.edu (Allen J Michielsen) writes... >>In article <10014@as0c.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) >>>years back!) that there is a fundamental design flaw with the 555 timer >>>chip, in that there are certain operating conditions and external part >>>values which could cause part of the chip circuitry to latch up (like a >>> >>>Rich >> I have used the 555 in many a circuit, and will testify that it is a very >>good chip. It may still have a tendancy to lock up at the ends of the design >>limit, but usually the chip specs from like ti are quite reasonable and >>reliable. > > I have noticed in the past that some makes or perhaps lots absolutely >need bypass capacitors on pin 8 and pin 4. ^^^^^ Should be pin 5. Sorry. Philip Munts N7AHL NRA Extremist, etc. University of Alaska, Fairbanks
tomb@hplsla.HP.COM (Tom Bruhns) (01/16/91)
rsd@sei.cmu.edu (Richard S D'Ippolito) writes: >If my memory is serving me correctly, I seem to recall (from 10 or more >years back!) that there is a fundamental design flaw with the 555 timer >chip, in that there are certain operating conditions and external part >values which could cause part of the chip circuitry to latch up (like a >thyristor), a condition requiring power removal to reset, making the 555 >definitely unsuitable for a watchdog design. My further recollection is >that the flaw was publically aired in some of the professional/trade >magazines, such as EDN, causing no little excitement for the original >manufacturer. I don't recall that, and I've never had that severe a problem with a 555 myself, but I recall some time ago having a problem with them in the nature of a 'latchup' condition. It had to do with starting a standard 555 oscillator. As I recall, Threshold and Trigger were tied together and to Discharge (though there may have been a resistor to Discharge -- long time ago ;-), and a rather high-value resistor went off to a + voltage from there. Capacitor from Thr/Trig to gnd. Bypass on pins 5 and 8 like a good boy. Problem was that the voltage on the trigger input was low enough to cause trigger to not trigger; if I raised it a few millivolts, it would take off and run. Hope this tangent helps some. Don't recall if it was just one mfgr or several, but it was repeatable pretty well at room temp from one chip to the next. The resistor to the + voltage was small enough to supply appropriate bias currents. (Reflecting on it, there probably _was_ an R off to Discharge from Trig/Thresh...)