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Forum Index : Microcontroller and PC projects : Quick question about DS1307 RTC
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CaptainBoing Guru  Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
Afternoon forum... hope you are all well. I know several of you are using the DS1307 based RTC with micromites and I am considering the move for the smaller footprint (than DS3231) plus the 56 bytes of ram is a nice bonus. Going through the datasheet, The ROC states: " MIN TYP MAX VCC 4.5 5.0 5.5 V " and I notice: "Upon power-up, the device switches from battery to VCC when VCC is greater than VBAT +0.2V and recognizes inputs when VCC is greater than 1.25 x VBAT." So my question is: how happy is the 1307 on a VCC of 3.3V? (I am assuming you are all running them at 3.3 not 5V) and how do you get around VCC being less than the startup voltage for the recognition of inputs? <3V backup cell? Maybe I have a datasheet for an older part? It is rev3/15 which suggests 2015(?) cheers Edited 2020-04-28 00:57 by CaptainBoing |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
I haven't tried running a DS1307 off 3.3V, the usual method is 5V The DS3231 is preferred due to it's much better timekeeping accuracy. Jim VK7JH MMedit |
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bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2950 |
Hi Captain Boing, There is a DS3231M(Z) version that is 8pin and really small and certainly retains its data with 3V.. It is also high accuracy. These are smaller than the SSOP version PIC170 at 5mm x 4mm but MUCH easier to solder than the PIC170 SSOP as the pin spacing is pretty wide in comparison to the ‘170 (I think they are 1.27mm pin spacing). These chips are expensive from the usual suppliers at about $15USea but can be bought from ALiExpress for around 25$US for 10.. but they take time, usually 6 weeks but maybe longer these Covid times. As to these being genuine or not, I have no idea but they look genuine and they definitely work, I have not yet had a failure from about 20 of them. If desperate I am happy to send you one/some.. You have my email.. contact me if you want.. As I said as far as soldering one onto a PCB they are pretty easy.. I use one on my SMD-BackPack170 PCB and my new BackPack-64X PCB.. Manuals can be found by clicking in my signature at the bottom of this message or email be and I will send one via email to you. I am happy to offer either or both of these to you if interested in the chips. Kind Regards, Mick Edited 2020-04-28 10:40 by bigmik Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
Thanks for your answers guys. I genuinely appreciate it - I was going mad yesterday with really confusing diagnostics. Is the Maximite a 5V product? (never played) I am sure I have seen people talking about the 1307 on this forum in a 'mite capacity... maybe with palcal(?) The footprint + RAM is a bonus but what is driving this I have found it really difficult to source genuine parts at a reasonable price (I can get 1307 at £1.50ish) where I know categorically they aren't chinesium knock-offs or seconds/rejects being sold-on instead of destroyed. Anything for a buck eh? These practices have really poisoned the market. I have a small batch of DS3231 here that are doing some very odd things: They are exhausting the 3V cell very quickly - I can see the voltage dropping when on backup, only a millivolt or so every 5 seconds but definite drain over a period of time. Then after a minute or so, the voltage drops suddenly to around 1V, then the internal oscillator is stopped, the OSF flag gets set and the registers go static. Kind of defeats the object really. Nothing in the data sheet (that I can see) can explain this... so is it the chip or the cell? Very odd. The cell recovers after I put VCC back on the thing so it is fairly repeatable. It could be dodgy cells... maybe they are old or something. I need to spend a bit more time on them to determine whether its crappy chips or crappy cells and that is the problem - I have been rushed. The cells are tiny so they don't have a great energy density, the draw on them should be a couple of uA max so it shouldn't really be a problem. I plan to put together an ultra stable low current PSU so I can then get down to measuring the current draw accurately and studying that sudden drop. If it doesn't happen then it's the cells, if it does then the chips - but it will break my information deadlock one way or the other. I have a couple of genuine Dallas parts arriving of both the DS3231 and some DS1307 (I missed that DS3231MZ part - thanks for the nod on that), but the smaller footprint and a bit of backup ram is a nice bonus. $15 a piece just blows the budget but that's the price for pedigree parts I guess. Thanks for your kind offer Mick. I need to play a bit more but if I need to, I will take you up on your offer, promise. Let me see what I can determine from pedigree bits and a PSU that can [meaningfully] remove the 3V cell from the equation. I need to determine which bit is playing up - at this point I just don't know. cheers all |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2950 |
G’day captain, There is also the DS3232M(Z) that I believe has extra ram. I may have one lying around here on a SMD backpack pcb as I used to buy those.. Mik Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
I haven't had time to play with this yet but one thing has struck me staring at the battery section of the datasheet EC: If the on-board temperature sensor is set for conversion enabled, the current consumption goes from about 3uA to 500+uA (bottom of page 3) during a conversion, which happens every 64 seconds. After dropping VCC and monitoring vBatt, I was definitely seeing about 2.9V and then after a delay, vBatt would drop suddenly to around a volt... It occurs to me, the temp conversion is providing that delay if it is enabled while in power-down. So I wonder if I have some crappy cells that really can't provide any juice... can just about hold vBatt under normal power-down circumstances but if the power requirement jumps up (e.g. the 0.5mA of a temp conversion), that could put the kibosh on things... It is a stretch that the cell can't provide half a milliamp, but it's all I've got that even gets close to explaining what I am seeing. That said, I don't know the state of the cells. I have had them for a couple of years at least and they were cheapie chinesium ones I think, so maybe they are shot... Voltage is there but no current capacity - all trousers, no balls so-to-speak. I'll get time on this tonight and hopefully track it down. First thing is to read out register 0Eh... hoping to see bit 5 set - that would back up my theory... then rig up a three volt source through a 1M resistor, see if that produces the same symptom... if so then it has to be the cells. I do enjoy a tricky bug that stretches the grey matter a bit, but not as much as I enjoy a bug-free system  Edited 2020-04-29 02:06 by CaptainBoing |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
got an hour to look at this. My suspicions fall immediately on the battery due to the voltage drop. I took the battery in question (2 actually) which dropped to 1v when VCC is removed after a small delay. I expect some drop due to the nature of these very small cells designed for memory retention but that 1V is very near the give up point for this particular chip's needs (1V @3uA !). I strapped a 10K resistor across it and the voltage fell from 3.232 to 0.59... with 10K across it! Quick calculation gives an internal resistance in the region of 45K  mystery solved.Batteries all in the recycling, replaceable cells + holders ordered (so no tags) As an aside, I put a 470K resistor in series and dropped VCC, current draw was measured (V/R) as about 3.5uA which is a tad on the high side according to the datasheet, but then the measurement was only rough and the 470K was +/-5% so let's say it was a good device. Cheers all. Edited 2020-04-29 05:48 by CaptainBoing |
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