DS1302 fault finding - fake?

[jucole]

hi, i've got an issue with one or more ds1302 realtime clock chips and I wondered whether I could have some fake chips, these were all sourced at cheaper than standard prices on Ebay; the fault I have is the rtc will very infrequently freeze many hours after power-up, it will also infrequently freeze when the unit it off - and it is using the backup coin cell.

I have a couple of different batches of the ds1302's bought at different times (ebay sellers) and the suspect ones when measured have a very oddly large? pin capacitance, which between the rtc pins x1 & x2 it's around ~40pF, whereas two other ds1302's from another batch bought on ebay are only ~6pF, the latter value is more like I would expect to see?

I've attached a pic; the top ones are the ones with the 40pf and the the two at the bottom of the pic have the ~6pF between pins.

Any thoughts?

 

[KlausST]

Hi,

I can´t say if it is likely or not that these are fake ones.

Assuming these are good ones, then my first thoughts are: Bad PCB layout, power supply instability, EMC problems.

Klaus

 

[schmitt trigger]

Fake chips are a common scourge. If one is an electronics experimenter, one has been bitten by them at least once.

Remember the saying: if something is too good to be true, then it cannot be true. The low price, the pin capacitance difference, and the fact that it freezes, may mean that you got some parametric rejects that were sold for scrap.
But the buyer decided it could get more money by re-selling them in the black market than recycling the metals.

Nowadays many semiconductor companies are crushing the rejected components before selling them for scrap.

 

[jucole]

thanks, what I will do is order some from a trusted supplier along with some watch crystals and then make some more investigations; I'll try a single board before I swap them all out. I did see something someone published online about reject ds1302's, apparently he wrote a bit of code to set and read a few registers, which worked for him; I think I will try something similar and see what happens.

 

[jucole]

Well, I ordered 3 new ones and some new watch crystals which arrived the other day and I measured the x1 & x2 pins and they also measured ~39pf, so, I guess by measuring between the x1 & x2 pins is inconclusive. The new watch crystals had a static capacitance of ~1pf, which was as per the datasheet, wheras the ebay ones measured ~2.3pf the

So next would be to verify all the registers are working and to leave it running on battery backup for a few days to see how it runs, using a combination of the suspect chips with the ebay crystals and then the new crystals; then try the new rtc's with the ebay crystals, and then the new crystals; if that doesn't work then I'll have to start looking at bad pcb, noise etc.

 

[phini]

If they are rejects, they are probably out of spec in some manner. You may want to vary the supply voltage and temp to see if you can cause them to stop working. Alternatively, you could throw them out and march on... "When you’re up to your neck in alligators, it’s easy to forget that the initial objective was to drain the swamp."