LED Clock: It’s the Voltage, Man (DS1302)

Well, the DS1302 now seems to be working off my 5V supply–dropped down to < 4V.

The Discovery

Earlier I had realized that I didn’t know whether the read failures were associated with having a 5V supply or having supply from VCC2 (primary). Tonight I swapped the power supply connections, so the lithium cell was connected to VCC2 (primary) and the bench supply was connected to VCC1 (backup). The read failures still occurred when the chip was running off of 5V; so the problem had to be associated with voltage, not supply pin.

The Cure

The datasheet has this to say about the supply voltage:

Supply Voltage VCC1, VCC2 2.0 3.3 5.5 V

5V is within the recommended operating conditions–but 3.3V is preferred. So I added small-signal diodes between the 5V supply and VCC2 until I got down to 3.4V, at which point everything worked.

Curiously, it took four diodes to drop 1.6V, or about .4V each–not the .7V that I’d expect from silicon diodes. Granted, I can’t find a datasheet on them (1N9148); but I really doubt they’re some new exotic that I just happen to have lying around in my parts bin.

My best guess is that the 1302′s operating current is so low (in the .5 – 1mA range) that the diodes are operating below the knee. (To paraphrase Captain Barbosa, we like to think of .7V as more of a guideline.) The knee of the curve is the point at which slight increases in voltage across the diode result in rapidly increasing current. Conversely, if the diode current is externally limited below the level of the knee, the voltage drop across it can be less than .7V. I seem to recall doing a lab on this at PSU.

I’m disinclined to acquiesce to putting four small-signal diodes on the power supply of my timekeeping chip, so I guess I’ll look for a Zener in the 3.5V range. It needs to be more than 3.2V (the lithium cell’s 3.0V plus a .2V margin) to keep the chip from switching to secondary power; and it needs to be less than 4V (but I don’t know how much less), because that’s the last reading I took while it was still malfunctioning before adding two more diodes and dropping VCC2 down to 3.4V.

Best of All,

The burst write now works, so I can set the date and time with a single function call. I can only speculate that whatever was screwing up the reads was also screwing up burst writes.

That is, everything now works on the DS1302. I think I’m supposed to be elated at this point, but I’m strangely not. Maybe I’m waiting for it to fail again. Because at this point, given that it doesn’t work even when I’m operating it within specifications, I can’t figure out what changed between when it did work and when it started not working; or alternately, I can’t figure out why it ever worked.

2 Responses to “LED Clock: It’s the Voltage, Man (DS1302)”

  1. cde says:

    I know this is 4 years too late, but just wanted to mention that the write failures could be due to input voltage tolerances on the data pins, in regards to supply voltage.

    If, when v+ for the rtc is at 5, data input registers a high at 0.7*V+ and higher, which is 3.5v, and the pic is signaling high at its 3 or 3.3v level, and that is above the registering a low voltage, the ic would be getting a floating voltage issue.

    Same thing the other way around, with the rtc signaling at its typical volatge level for high, and that being above the pic’s input voltage.

  2. Keith Neufeld says:

    cde, thanks for the note! I welcome good ideas, even four years later, because it’s never too late to learn.

    In this case, however, it was a 5V PIC (and actually running at 5V), so I don’t think it was a problem with logic-level interfacing on the PIC side.

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