MSP430F2272IRHAT VREF +

See the device datasheet for details. It gives you diagrams for voltage drop vs. output current.

Basically, the output port is a totem-pole FET output. So the output impedance is not a fixed value. For lower currents, it behaves as a resistor, at higher currents it turns over to a current source. The turnover point as well as the low-current resistance depends on VCC.

So far for ports. But question 3 and the 'VREF+' in the topic seems to indicate that you're not talking about port pin capabilities at all. You're likely talking about VREF+ output and VeREF+ input, which is analog pin function and not at all related to a 'port' (the P2.4 port functionality is just coincidentally on the same processor pin as the analog VRef functions). You can take a look at the 'applicaiton information' chapter in the datasheet. By setting ADC10AE0.4 bit, the digital circuitry on the processor pin is completely disabled.

For the VREF+ functionality (REFOUT=1), the datasheet tells a maximum output current of +-0-5mA for VCC=2.2V and +-1mA for VCC=3V. And the '-' means yes, it can sink current. Up to the given limit, before the voltage begins to rise.

The output impedance can be calculated from the load regulation response time.

Is the totem pole basically a P & N channel enhancement FET with each gate driven independently?  If not, can you provide more information about the output structure?

If VREF+ is enabled (output = 1.5V) and tied in parallel with an external precision shunt regulator set 1.65V, will VREF+ measure ~1.49V and sink less than 1mA at the same time? (Both the micro-controller and the shunt regulator are tied to the same 3.3V source and ground)

Thank you.