joost@neabbs.UUCP (JOOST BOERHOUT) (09/15/90)
In article 446008@neabbs.UUCP Steve Warren wrote: >In article <444299@neabbs.UUCP> joost@neabbs.UUCP (JOOST BOERHOUT) writes: >>I can't explain why FRANCES needs the warming up except the darn thing >>is working on the edge of the (timing) specs. A heated chip will have >>a shift in the timing specs so this seems to be the case. >Yes, a common failure on higher-speed systems is the "cold" failure. >Some technicians debug a board like this with a can of freon: Hmm, yes I did this 'freeze' test. I went all over LUCAS and FRANCES with the freon nozzle (one chip at the time) but could not find anything. I looked at the power and ground lines of the 74ls245 drivers using a 20 MHz scoop. This showed a lot of high frequency junk going on (peaks of 1 Volt !) which seemed to be related to the /DATABUSEN signal. There is no way I can get rid of these peaks (I use 1uF tant parallel to 100n ceramic for decouple caps). Another thing is the infamous U9 flip-flop (it's more a flop then a flip :-). The only chip which worked for LUCAS is a 74HC74 and I have to solder a jumper from the GND pin to the expansion port GND. I checked several GND points at the two boards with a volt meter (one terminal to the pin, one to the expansion GND). The meter read 2.5mV at most in all cases. It makes no difference if I use a 16MHz or 20MHz oscillator. All 'features' stay on. What I haven't tried yet is to change the PALs on the piggy back board. However, I must say I don't expect much from such a change. The system works, however, after a short warming up time. And when it chrashes it mostly happens with the same programs. BLink for example chrashes 1 in 3 times ! Ideas anyone ?? - joost -
<LEEK@QUCDN.QueensU.CA> (09/16/90)
In article <449417@neabbs.UUCP>, joost@neabbs.UUCP (JOOST BOERHOUT) says: > >I looked at the power and ground lines of the 74ls245 drivers using a >20 MHz scoop. This showed a lot of high frequency junk going on >(peaks of 1 Volt !) which seemed to be related to the /DATABUSEN signal. >There is no way I can get rid of these peaks (I use 1uF tant parallel >to 100n ceramic for decouple caps). > A got about 20mV DC volt difference between the 74LS245's and the power supply ground. I put in pieces of wire (about 22 AWG) to ground the 245's directly to the power supply connector and changed the bypass capacitors to 0.47uF monolithic. I don't have a scope, so I don't know what kind of noise I am dealing with. Right now the 245 GNDs vs power supply GND reads 0.5mV DC on my digital voltmeter, so I would assume the grounding is good. t Hope this works for you. > >- joost - K. C. Lee Elec. Eng. Grad. Student