How to monitor the MCU voltage by 908QY4 AD?

[lijiandong]

Now my project using  908QY4 device for low power application, and use two 1.5V AAA batterys for power supply,I want to use 908QY4 AD for sample and monitor the voltage,for example, when the power is below 2.7v ,the alarm will turn on.

Could you please tell me if this is doable and  how to achieve this aim?thanks.

[bigmac]

Hello lijiandong,

With a voltage reference connected to an ADC input, as Peg suggested, you can avoid continuous current draw by applying power to the reference for only a short interval while the ADC measurement takes place - however, this will consume another pin of the MCU.  I assume you would use a 1.2 volt reference with a series resistor connected to an output pin.  The alarm would occur when the ADC reading exceeds a decimal threshold value of about 113.

Another comment about your proposal would be that AAA alkaline cells would normally be considered "flat" at a voltage somewhere between 0.9 - 1.1 volts per cell.  This would mean that, with only two cells, you would get a very short battery life, with only a fraction of battery capacity consumed before the threshold of 2.7 volts was reached.  You might do better to use three AAA cells.

Note that the 9S08QG4 MCU is capable of operation at a lower minimum voltage than the HC08 device, so would be more suitable for two AAA cells.

Regards,
Mac

[peg]

Hi lijiandong,

You need a reference, normally the supply voltage is your reference. If your supply voltage varies then you need an independant reference. If you put a fixed reference voltage onto the ADC input you should be able to work this backwards from normal. As the supply voltage drops the reference voltage will appear to rise (or it actually will with reference to the supply). This way you should be able to implement a "fuel gauge" as well as a low alarm. This will come at the expense of the current required to run the voltage reference.

Regards David