RTC via 32.768khz crystal - voltage and pF values - Page 1
Forum section: electronics / beginners

author: Nexxen
author_rank: Regular Contributor
timestamp: May 19, 2023, 01:18:23 pm
content: Hello!I was troubleshooting a motherboard that wouldn't turn on. No power on issue. M/b has Intel's ICH7 southbridge.As I was checking voltages I touched the 32KHz lead TX1 and it came to life. By removing the probe's lead it'll slow down to frozen and going back to full working order if touched again with probe.I thought a dying crystal but after replacement it wouldn't power on anymore.The circuit is the usual 32KHz connecting to southbridge, but what are the V values of such a component in order to work?I saw on a datasheet that they may have a 0.04V but on many boards it can vary wildely; up to 0.8V on both. Usually TX2 is a bit lower.There was a capacitor connecting to one of the OSC leads and it just flew away when desoldering the osc. It was a 12pF if it is the same as the other on the TX1.I put a 22pF, is that a problem? Too high? Is there a max tolerance?The two leads are connected by a Mohm res, why is that?Any help is appreciated.I'm lazy and this is adrawing od the circuitSB -----Res1-----Cap1 (12pF) -----TX1 ---| OSC |SB------Res1-----Cap2 (12pF) -----TX2----| 32.768KHz |Cap2 was replaced with a 22pF (I don't have an exact spare)

author: Benta
author_rank: Super Contributor
timestamp: May 19, 2023, 06:31:38 pm
content: Crystal's fine, your values are fine. Clock may run a bit slow with 22 pF, but it'll work.Your problem is elsewhere, most likely the power for the RTC part of your SB. Battery?

author: srb1954
author_rank: Super Contributor
timestamp: May 19, 2023, 08:15:30 pm
content: Quote from: Nexxen on May 19, 2023, 01:18:23 pmThe crystal oscillator should work with the larger value but the start-up time might be longer and the standby power consumption a little higher. I would replace it with the correct value for optimum performance.The resistor is there to bias the internal inverter to the correct operating voltage so as to be operating in a linear mode and provide the gain necessary for oscillation.

author: Nexxen
author_rank: Regular Contributor
timestamp: May 19, 2023, 10:58:13 pm
content: Quote from: Benta on May 19, 2023, 06:31:38 pmThere is for sure a problem with the RTC part, but as it did work for some time by touching the crystal with the multimeter probe... This is a matter for people that actually know what to look for.I'm learning tons in the process.Battery part, I checked all the components and they work as expected. Even double checked with other motherboards (compared against working ones)Inserting the battery once made the motherboard power on by itself and was stable enough to even load a Linux dsitro via USB.Repair videos make everything look easy when it is absolutely not. LOLThanksQuote from: srb1954 on May 19, 2023, 08:15:30 pm.One lead reads 0.22V, the other one reads 0.04V (X2 - the one with 22pF cap)Ok, I had to look up for "bias".Thanks to you too.Basically there's nothing wrong with the cap values. V on lead n.2 isn't too low though?

author: srb1954
author_rank: Super Contributor
timestamp: May 20, 2023, 02:20:53 am
content: Quote from: Nexxen on May 19, 2023, 10:58:13 pmWhat were you measuring these voltages with?Because RTC circuits are designed for very low power and low currents they can be easily upset by the extra loading, either resistive or capacitive, of a DVM or a scope probe. Generally, connecting such a measuring device will upset the operation of the RTC circuit so much that the voltage readings obtained are mostly meaningless. To get get reliable measurements on RTC circuits you need a high impedance, low capacitance probe such as a FET scope probe.

author: Nexxen
author_rank: Regular Contributor
timestamp: May 20, 2023, 11:32:45 am
content: Quote from: srb1954 on May 20, 2023, 02:20:53 amIt's 25 yo multimeter. In the manual there is no mention of FET.Can a crystal die this way? In theory.I'll solder a new one and see if it does make a change.Thanks!

author: srb1954
author_rank: Super Contributor
timestamp: May 21, 2023, 10:28:21 pm
content: Quote from: Nexxen on May 20, 2023, 11:32:45 amThe capacitive loading of an ordinary DMM would likely temporarily stop the oscillator.There would be no permanent damage from this loading so there is no need to replace the crystal.