I've been working with MSP430G2452 for some time. Recently, i found this ADC pin problem. I wanted to sense voltage through A1(pin:3) of MSP430G2452. I used a 7V source with a potential divider of 270K &100K across that. I connected the 100K pin to that A1 (ADC pin). So, the voltage across A1 & Gnd should be 1.891V. When the micro controller is powered up the voltage across the ADC pin is 1.891V only. When the micro controller is not powered up the ADC voltage dropped to 0.771V. It seems the ADC pin is drawing current when the MSP is not powered up. Will it affect the MSP?
Thanks in advance for your replies !!!!
When the micro is not powered up, the clamping diode of the analog input is working and the voltage you see is the voltage of the diode. The voltage of the clamping diode is limited to +- 2mA. In your case, with 7V as source and a series resistance of 270K, the current into the pin is of 20-30uA, so it should be OK.
But clamping diodes are there to protect the port if i apply voltage above Vdd. right? If i dont give any supply to the micro the diode will be open only know.
i'm saying this based on PIC adc i/p model.i'm not that much clear about the MSP's internal structure. please correct me if i'm wrong.
muthu revathy a If i dont give any supply to the micro
However, as soon as current begins to flow through the clamp diode, this current has to flow somewhere. And it tries to 'escape' through the MSP, causing a voltage to appear on VCC. As long as the current through the clamp diodes is larger than the current through the MSP (or any other circuitry on VCC), VCC will still raise. But rising VCC reduces the voltage difference and therefore the current through the series resistor. Until equilibrium.There have been people powering their MSP through a port pin without noticing.
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