Battery Backup for SRAM or Microcontroller
This circuit can be used as a low cost SRAM and Microcontroller or Microprocessor Battery Backup. All the diodes are 1N4148, The diodes prevent battery discharge back to power source. D8 gives a one way path to charge Battery thru R13 which limits current. D4 ensures a one way path of supply to chip when power is present. D5 is backup supply on power failure.
The chip a real time clock, RAM or Processor can be put to standby or sleep on power failure. If it is not a smart chip then make sure on power failure all outputs of chip are high impedance or floating. do not use any pullups or resistor dividers to Vbat, which is the supply to chip. There should be no leakage path from Vbat, decoupling cap of chip must be plastic.
If you want to use this circuit for short term retention or for CMOS logic chips then you can use a 4700uF Cap in place of battery. This works for many hours but the cap has big footprint on PCB. For long duration use more battery AH Ampere-Hour. Vcc is 5V DC regulated.
The Vbat and Vcc can be monitored with comparator like LM339, this circuit can generate the reset or low battery signals. The power on reset and power down reset can corrupt data on brown outs or black outs or even spikes and EMI. So back up data on flash. For Rapid writing and reading SRAM is better and if write-read cycles are high SRAM is best. But if you need to store values and refer to them like a look-up table flash is better.
The power fluctuations can hang the chip, so a watchdog chip may be required. The conventional way was the to monitor the keyboard-display scan on a i/o port. If the pulses are coming at the rate you programmed the cpu is alive and kicking and doing its job. If the CPU is taking a nap, then the pulses stop coming and it needs to be reset.