The DS1307’s official operating Vcc range is 4.5 V to 5.5 V, so it is intended for 5 V systems. It will not keep proper time at 3.3 V Vcc. However, it is possible to interface a 5 V DS1307 with a 3.3 V microcontroller via I²C: you can pull-up the I²C lines to 3.3 V, and often the DS1307 will recognize 3.3 V as logic high (because its V_IH threshold is around 2.2 V). Many have done this successfully – the DS1307 will still be powered at 5 V, but the I²C lines at 3.3 V are accepted. Important: the DS1307 has internal pull-ups on SDA/SCL only when powered by Vcc, I believe, so using 3.3 V pull-ups externally is fine. For a truly 3.3 V RTC, consider alternatives like DS3231 (which is 3.3 V) or others. But if you already have a DS1307, it typically works with 3.3 V microcontrollers logic-wise (just ensure the microcontroller’s I/O are 5 V tolerant or use a level shifter if not).

The DS1307 draws extremely little current in backup mode (less than 500 nA, often around 300 nA typically). If you use a standard CR2032 coin cell (~220 mAh capacity), theoretically the backup could last several years. For example, at 300 nA draw, a 220 mAh battery could last ~ >25 years (in ideal conditions). In practice, self-discharge and other factors might make it last on the order of 5–10 years or more, which usually outlives the coin cell’s shelf life anyway. So generally, you can expect multi-year backup operation. Just ensure the battery is good quality and the board design doesn’t leak current (no LED or anything on the battery line). Also note the DS1307 does not itself alert you when the battery is low – you’d typically notice the time resets if battery failed during a power loss.

A few possibilities: - Ensure you have a 32.768 kHz crystal connected properly to X1 and X2 pins. The crystal should be a standard tuning fork type (12.5 pF load typical). No additional capacitors to ground should be used (DS1307 has internal caps). If the crystal isn’t connected or is faulty, the clock won’t run. - Check the Oscillator Enable bit in the DS1307’s control register. Upon first power-up, the DS1307’s oscillator is usually disabled until you clear the CH (clock halt) bit in the seconds register. The most significant bit of the seconds register is CH – it should be 0 to run. If it’s 1, the oscillator is stopped. So writing the time initially, make sure to set CH=0. - If the crystal is connected and CH=0 but it still doesn’t tick, it could be an issue with crystal load or physical placement. The crystal should be very close to the DS1307 pins, and the traces short to avoid stray capacitance or noise. Also, do not route high-speed signals under the crystal. - Another issue could be that the OSF (Oscillator Stop Flag) got set, indicating the clock was stopped at some point. Clearing that flag and re-checking if it sets again can indicate oscillator failures. In summary, most of the time, if a DS1307 appears “dead,” it’s the crystal oscillator not running – often due to CH bit not cleared or incorrect crystal usage.

Out of the box, the DS1307’s accuracy depends on the crystal. Typically, expect around ±20 ppm at room temp (which is about ±1.7 seconds per day). Over a range of temperatures, it could drift more – maybe ±5 seconds or more per day in worst cases. The DS1307 itself does not have a built-in calibration register or digital trim. So calibration can only be done in a system sense by: - Choosing a higher accuracy crystal (some are ±5 ppm). - Adding tiny capacitors or adjusting the load to fine-tune frequency (but this is advanced and often not done). - Or more commonly, by periodically syncing the clock to a reference (like daily syncing to an internet time or GPS or user setting). For many hobby or even commercial uses, a few seconds drift per day is acceptable. If not, the DS1307 might not be the right choice – devices like DS3231 (with internal TCXO) can achieve ±2 ppm (about ±0.17 sec/day) without calibration. But those are more expensive. Some people have tried to calibrate DS1307 by slightly modifying the square wave output frequency and counting pulses, but that’s not a built-in feature – any adjustment has to be external.

The base functionality is the same. DS1307N usually denotes the industrial temperature range version (-40°C to +85°C). DS1307 (no letter) was commercial 0°C to 70°C. The DS1307+ is just indicating RoHS compliance (the “+” in Maxim parts often means lead-free/RoHS). So DS1307+ is the current lead-free device. Essentially, there’s no functional difference aside from temperature spec. When buying, most will get DS1307+ which covers standard temp, or DS1307N+ for industrial. Just ensure you get a genuine source because counterfeit or remarked chips do exist sometimes (especially on modules from uncertain sources).

The RAM is accessed via the same I²C interface and address space as the time registers. After the time and control registers (which go from address 0x00 to 0x07), addresses 0x08 through 0x3F are general-purpose RAM. You can write and read these bytes just by setting the address pointer and doing I²C read/write. They retain their data as long as the battery is present. A common use is to store configuration or the last known good time stamp or other small logs there. Keep in mind it’s not huge – 56 bytes. Also, if the battery fails or is removed, that RAM will reset. There’s no wear-out concern as with EEPROM, since it’s static RAM – you can write unlimited times (only limited by I²C wear on the bus, but practically that’s fine). This RAM is a bonus for small NVRAM needs.