Hydra Real time clock

I just wanted to ask if I have the correct location to attach the crystal for the real time clock on the hydra.

Its the section I have highlighted in red.

I would also have to wire a 1.2 volt battery to the 5th pin on socket 1?


Thanks for the fast reply.


I also need to get the RTC going on the Hydra.

Does the crystal not need a capacitor in parallel with it? I.e. the load capacitance.

Hydra already has Capacitors connected to the RTC Crystal Pins. All that is needed is a 32.768 kHz crystal.

Is there a recommended brand/model crystal for this? I need to stock up and I see about 40 different options from DigiKey… If I’m going to solder it to my Hydra I want one that’s of the highest quality.

This is the RTC that we use: VT200F-6PF20PPM. You can get it from DigiKey.

Will it matter if I am using a 12pF crystal? I just (10 minutes before I saw this post) ordered 10 of the CFS206-32.768KDZF-UB from Citizen (on Mouser).

Should be alright

Thanks, Aron.

I am having trouble getting hold of a 1.2V battery for the RTC power. Would a tagged 1.5V AAA battery work alright?

a 1.2v battery - i would go to a AA or AAA rechargeable NiMH battery as an easy option.

I installed Q1 with a crystal as stated earlier. I connected a battery to RTC_VBAT1.2, and I had the real time clock working when the battery was at 1.1V. However, when it dropped to 1.0V the RTC stopped working.

What is the minimum required voltage at RTC_VBAT1.2 for the RTC to function? I measured this net without the battery, and it is 1.03V. VCCINT = 1.195V.

What is the absolute maximum rating for IC5, pin C5 “VDDBU”? A previous post says “1.5V should be fine”. What about 2V, 3V, 3.3V?

Welcome to the forum!

This can be verified by MCU datasheet.

Here is the link:

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I think absolute maximum is 1.5V for Hydra.

From SAM9R/RL64 Complete:
[em]“VDDBU pin: Powers the Slow Clock oscillator and a part of the System Controller; voltage ranges from 1.08V and 1.32V, 1.2V nominal.”[/em]

This doesn’t explain why my clock runs with the 1.03V supplied by VCCINT through a diode. However, maybe the battery was just below the cutoff and the VCCINT is not. This also refutes what Gus said about 1.5V being ok.

[em]“The AT91SAM9R64/RL64 slow clock can be generated either by an external 32768Hz crystal or the on-chip RC oscillator. The 32768Hz crystal oscillator can be bypassed to accept an externalslow clock on XIN32. Configuration is located in the slow clock control register (SCKCR) located at address 0xFFFFFD50 in the backed up part of the system controller and so is preserved while VDDBU is present.”[/em]

I think this was verified using the RTC initialize command. Once it is initialized it must set the register to use the external oscillator, because it won’t let you initialize it again even after a short power cycle with battery applied.

The DC Characteristics states that (page 851 of the datasheet)

-----------------------------------------MIn Typ Max
VVDDBU DC Supply Backup 1.08 1.2 1.32 V

Absolute Max Rating states that this is 1.5V, so in effect, Gus was correct but to be safe, you should always stay within the DC Characteristics values.

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Still having some trouble. I can get the real time clock to run when power is disconnected for a short period (5 minutes), but when it’s disconnected for an hour it resets. I verified the battery voltage is good.

I am using this crystal. But this crystal is rated for 6pF. The board has 27pF. I should have verified before ordering, but according to GHI this was the crystal they used . They did not specify if they changed the capacitors though. And Gus says 12pF crystal “should be alright”.

I am not sure how NETMF uses the RTC, but I am assuming that it reads this on initial loading of the application and then just updates an internal timer. I assume this as I didn’t see any further calls to my RTC read function I had on a previous board. It is also how the full blown MS Windows works. It does not continually read the RTC hardware clock.

If that is the case, then it may be that your oscillator crystal and caps are not matched. 27pF seems a little too high for the 12pF crystal being used. If you check the datasheet for the RTC, they show much lower values being used.

Update: My RTC on the Hydra is working. I had to change caps C21 and C22 to match my crystal. I could not find any crystals that used 27pF, which is what the board comes populated with. So I am using a AB26T-32.768 with 12pF caps. I’m using schottkey diodes (BAT54HT1G) to apply +1.2V to X1 pin 5 when external power is applied or a battery when the power is not applied.