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The RAM can be used just like the RTC registers. The only difference is that the RAM contents do not get incremented automatically like the RTC registers.
The trickle charge should only be left on if you are using a rechargeable battery. My experience is that you should use a lithium battery and keep the charger off. Lithium cell can keep a DS1302 running for many years.
It can be used to store non-volatile varaibles. The ones you wish should retain their value even in case of power failure. THis is an excellent feature since it provides for a very simple and low cost storage of varaiables in a battery backed RAM. Since this is a RAM, you dont need to worry about cycle delays and memory damage as is the case with EEPROMs.
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The trickle charge should only be left on if you are using a rechargeable battery. My experience is that you should use a lithium battery and keep the charger off. Lithium cell can keep a DS1302 running for many years.
If you try to charge a non-rechargeable battery like the lithium cell (not the litium ion used in mobile phones and cameras), you will not only damage them but also can cause fire or explosion hazards.
Initially when I designed some circuits with DS1302 using ni-cd chargeable batteries and then shifted to lithium cells, I did not want to change the firmware, so I used a 1N4148 diode in series with the Lithium cell. At the low current that DS1302 draws, the diode only cause a very small voltage drop and the DS1302 works fine.
This is a rechargeable lithium so it wont get damaged by charging. But charging lithium battery is not very straight-forward. Also chargeable lithium batteries dont last very long and even if you dont charge-discharge often, you will lose capacity in a few years. I dont think you will get much capacity left after 5 years. Compared to that a simple lithium watch cell is very inexpensive and long lasting. 10 years is common. The amount of current the DS1302 draws, it does not make any sense to use chargeable battery.
a couple of points:
- the ram in the RTC is just general memory. I've used it for a number of things: alarm times, passwords and so on but it's up to your creativity.
- never ever charge a lithium cell in anything other than a charger designed for lithium cells. period. techie's cautions about fires are completely correct. there is a video floating around on the internet that shows a small cell catching fire and burning when plugged into a nicad (I believe) charger.
- at the price of 2.50 pounds, I'd look at a super cap.
- if you want a rechargeable battery, use nimh.
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