[EE] Looking for a RTC - proven track record, small, tiny.

[Harrison Cooper]

I have a design going where they now want to add a RTC to the board, and I have very little space. So, any suggestions on a device that:
- smallest package size
- fewest/smallest support (such as the crystal). Built in crystal?
- good track record.
- battery will be off board, so that size isn't an issue.

There is a PIC on the board, 18F series. Pretty sure it does not have a Vbat pin (do any?) so coding up a RTC internally isn't an option. The PIC will be talking to the RTC, probably I2C but again, no hard requirement (could be SPI just as easily)

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[Denny Esterline]

The DS1307 is usually the first place to start. External parts are crystal
and battery. It's I2C bus so you _may_ need pullup resistors for that as
well. Several second-source equivalents exist.

Another interesting alternative is the DS1904 RTC iButton. In a package
that looks a bit like a coin cell battery, it contains the battery and the
crystal and uses a 1-wire protocol. It's an option when you truly want
"minimum support parts."




On Mon, Oct 23, 2017 at 10:41 AM, Harrison Cooper <***@wdc.com>
wrote:

> I have a design going where they now want to add a RTC to the board, and I
> have very little space. So, any suggestions on a device that:
> - smallest package size
> - fewest/smallest support (such as the crystal). Built in crystal?
> - good track record.
> - battery will be off board, so that size isn't an issue.
>
> There is a PIC on the board, 18F series. Pretty sure it does not have a
> Vbat pin (do any?) so coding up a RTC internally isn't an option. The PIC
> will be talking to the RTC, probably I2C but again, no hard requirement
> (could be SPI just as easily)
>
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> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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[Jean-Paul Louis]

Check Austria Micro Systems, they have a small chip with built in crystal


> On Oct 23, 2017, at 1:41 PM, Harrison Cooper <***@wdc.com> wrote:
>
> I have a design going where they now want to add a RTC to the board, and I have very little space. So, any suggestions on a device that:
> - smallest package size
> - fewest/smallest support (such as the crystal). Built in crystal?
> - good track record.
> - battery will be off board, so that size isn't an issue.
>
> There is a PIC on the board, 18F series. Pretty sure it does not have a Vbat pin (do any?) so coding up a RTC internally isn't an option. The PIC will be talking to the RTC, probably I2C but again, no hard requirement (could be SPI just as easily)
>
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> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

Just my $0.02,

Jean-Paul
N1JPL




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[Bob Blick]

DS3231 isn't the smallest chip, SO16-wide, but the crystal is inside. Pretty much all you need is the chip and a battery.
________________________________________
From: piclist-***@mit.edu on behalf of Harrison Cooper
Sent: Monday, October 23, 2017 10:41 AM
To: Microcontroller discussion list - Public.
Subject: [EE] Looking for a RTC - proven track record, small, tiny.

I have a design going where they now want to add a RTC to the board, and I have very little space. So, any suggestions on a device that:
- smallest package size
- fewest/smallest support (such as the crystal). Built in crystal?
- good track record.
- battery will be off board, so that size isn't an issue.

There is a PIC on the board, 18F series. Pretty sure it does not have a Vbat pin (do any?) so coding up a RTC internally isn't an option. The PIC will be talking to the RTC, probably I2C but again, no hard requirement (could be SPI just as easily)

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[Brent Brown]

> There is a PIC on the board, 18F series. Pretty sure it does not
> have a Vbat pin (do any?) so coding up a RTC internally isn't an
> option.

No Vbat pin, but LF versions of PIC's are ideal for low voltage/low power
applications. Requires some attention to how you switch to/from battery power in
hardware. Add external 32kHz crystal connected to secondary oscillator (a lot of
PIC's have support for this) and coding up the RTC internally can become an
attractive low parts count option. A typical method is to put the micro to sleep for 1
or 2 seconds at a time, wake on timer interrupt, increment RTC, go back to sleep.



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[Harold Hallikainen]

I like RTCs with an internal crystal with temperature compensation. Having
the chip manufacturer deal with the crystal SHOULD ensure a good match.
Also, though I generally prefer SPI to I2C, I2C is easier to deal with
when part of the circuit shuts down. Since SPI uses an active low chip
select, powering down the master results in SPI devices enabling MISO into
a dead master.

Another thing to watch for is the clock oscillator frequency changing
between Vcc and battery operation. We found that in one device, and it
caused a lot of hassle.

In the past several years, where the devices I work on have network
connectivity, I've just used a timer in the PIC running on the CPU clock.
On power up, the period register is set for a 1 second period. Every now
and then, an NTP server (pool.ntp.org) is polled to determine the time. If
the time received is 2 seconds or more from my current second count, I
load the new time in my "RTC" and adjust the period register by 1 count in
the right direction. I use 2 seconds as the threshold so I don't end up
dithering back and forth on a 1 second difference.

Harold


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[James Holland]

Date: Mon, 23 Oct 2017 17:41:47 +0000

From: Harrison Cooper <***@wdc.com>
Subject: [EE] Looking for a RTC - proven track record, small, tiny.
To: "Microcontroller discussion list - Public." <***@mit.edu>

I have a design going where they now want to add a RTC to the board, and I have very little space. So, any suggestions on a device that:
- smallest package size
- fewest/smallest support (such as the crystal). Built in crystal?
- good track record.
- battery will be off board, so that size isn't an issue.

There is a PIC on the board, 18F series. Pretty sure it does not have a Vbat pin (do any?) so coding up a RTC internally isn't an option. The PIC will be talking to the RTC, probably I2C but again, no hard requirement (could be SPI just as easily)

------------------------------

There are 16Fdevices designed for use as LINBus nodes that have a built in voltage regulator and a precision oscillator. From memory they are 20 pin packages.
regards
James


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