Blast from the past
MikeM sent this photo he took of the guts of the revolutionary-at the-time Sinclair Black watch beside the Bubble clock shield . Mike also noticed odd behaviour of the clock shields (both bubble and OLED ) when powered from the 3.7V LiPo battery (as shown here ). After a while, the RTC returns bogus time. A quick check of the DS1307 datasheet (page 3) confirmed my suspicion: When 5V is applied within normal limits, the device is fully accessible and data can be written and read. When a 3V battery is connected to the device and VCC is below 1.25 x VBAT, reads and writes are inhibited. However, the timekeeping function continues unaffected by the lower input voltage. As VCC falls below VBAT the RAM and timekeeper are switched over to the external power supply (nominal 3.0V DC) at VBAT. Therefore, if one really wants the clock(s) powered from LiPo, a step-up converter must be used. Another solution is to replace the DS1307 with the (almost) pin-to-pin compatible DS1337 . This one does