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Fig. 3-3 ULPWU UnitSetting the microcontroller to sleep mode is another step in the same direction. However, even when both measures are applied, another problem arises. How to wake the microcontroller and set it to normal mode. It is obviously necessary to have an external signal to change logic state on some of the pins. Thus, the problem still exists. This signal must be generated by additional electronics, which causes higher power consumption of the entire device.
Fig. 3-4 Sleep ModeTheoretically, this is a perfect solution. The problem is that all pins able to cause an interrupt in this way are digital and have relatively large leakage current when their voltage is not close to the limit values Vdd (1) or Vss (0). In this case, the capacitor is discharged for a short time since the current amounts to several hundreds of microamperes. This is why the ULPWU circuit able to register slow voltage drops with ultra low power consumption was designed. Its output generates an interrupt, while its input is connected to one of the microcontroller pins. It is the RA0 pin. Referring to Fig. 3-4 (R=200 ohms, C=1nF), discharge time is approximately 30mS, while the total consumption of the microcontroller is 1000 times lower (several hundreds of nanoamperes).
Fig. 3-10 ICSP ConnectionIt is not complicated at all! It is only necessary to install a 4-pin connector onto the target device so that the necessary programmer voltages may be applied to the microcontroller. In order that these voltages don't interfere with other device electronics, design some sort of circuit breaking into this connection (using resistors or jumpers).