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Output voltage depends on output current.
See the the port driver as a series resistor to the load. The total voltage over both, transistor and load, is VCC. The higher the current through xxyout load, the higher the current through the transistor and the higher the voltage over teh transistor that gets subtrracted from VCC for your output voltage.

Teh output voltage given in the datasheet are worst case for specific output currents. See the diagram drawign below to get an ide of the voltages for other currents.

can you attach the diagram ??

if i test the same ic by  de-soldering the and placed in my ic socket means the port pins are working properly.. so now i found that the problem is in my pcb.. can you say what kind of the possibilities to reduce the port pin voltage ?? so it would help me to trouble shout my pcb??

You have already been given a description of how the load affects port output voltage.

It also, of course, depends on the supply voltage to the chip.

You need to post your schematic (circuit diagram).

now i found that mistakes if i tie my analog reference gnd that is VREF/Ve REF- pin number 11 in msp430f2618 TO gnd am getting voltage reduction on my port.. but as per data sheet in order to use external analog reference for ADC operation i need to tie that pin to gnd.. so what is the problem here??

karthik raja

so what is the problem here??

Once more: we need to see your circuit schematics.
Normally, connecting V_eREF- to GND (if GND is your negative reference - you can put it to a higher voltage to stretch resolution) is correct and has no side-effects. There must be something wrong with the rest.
I can imagine that you connected your V_eREF+ to Pin 10, but have the DAC output on. So the DAC is driving against your reference, causing a high current draw that lowers VCC internally. Or even increases the current so much that your VCC supply is pulled down.

Also, there might be shortcuts of some kind.

What is your external reference?
Do you have proper decouplong capacitors on it?

hi above its my schematic diagram... check my schematic here while i connect the pin11 to gnd means am getting that problem if i disconnect means its working properly. and kindly say me the correct connections diagram for using the external reference for my ADC and DAC.

Hmmm,

Pin11 is directly connected to

• INCH9 of the ADC12 input multiplexer
• a switch that switches between V_eREF- or V_SS based on SREF2 bit
• to VCC and GND through clamp diodes.

AFAIK there is no other internal connection.

If you don't set SREF2 bit, it shouldn't make any difference whether you connect Pin11 to GND or not. And since GND is (or at least should be) your AVss, there is no benefit from using an exgernal V_eREF-. So I wonder why you did it at all.

The only way I can imagine where V_eREF- has any influence on the output voltage would be if the applied voltage is below GND. Then there will be some current through the clamp diodes, possibly shifting your voltage levels. Or if you didn't properly solder the V_SS and V_CC pins, so there are cross-currents through the internal clamp diodes between the digital and analog supply.
And of course shortcuts on the PCB (caused by dirt, bad soldering or even by hair connections between or cuts on a trace) can do weird things. On a professionally produced PCB (one out of 50) we once had a hair shortcut that was invisible to the eye and pulled a signal line to GND. We had to burn it with a high current of several Amperes (it became visible when it flashed and melted). Before doing so, we suspected the chips on the board and replaced all of them connected with this signal.

However, sometimes there is no way to trace this down.

Once I had a PCB for the C64. One out of a pack of 200, which, then plugged into the C64 stopped one of the two timers. Everything else was working, jsu tthe tiemr stopped counting. And the timer clock, sourced by the 50Hz from the unrectified power supply, was not routed anywhere near the module connector with the PCB. It's still a mystery to me what happened. Well, maybe with now 25 more years of experience I could trace it down if I'd try. :)

Nevertheless, I'd suggest

• do not connect AVSS directly to DV_SS. Make a different AGND or AV_SS signal path and connect it directly at the power supply to the normal GND. This keeps voltage ripples caused by the return currents on the GND traces, away from the analog part.
• Connect AV_CC not directly to DV_CC, but put a small resistor (10 Ohms or so) them. The blocking capacitors for AV_CC (C2 and C3) on the same side as the AV_CC pin.
• Put a 10µF (tantalum) 100nF (ceramic) combo between V_eREF+ and V_eREF- (Or AV_SS)

Hint: when using Eagle, you cannot connect two signals without making them one (and the router routing them with the shortest connections). For this purpose, I have defined some 'connector' parts that are basically two SMD pads with an additional copper bridge ona separate layer (don't forget the PCB manufacturer to tell that this layer contains additional copper).
I use this to provide normally-closed solder jumpers or solder switches with semi-circle pads and such. And dividing signals into two sub-nets like in the case of AV_SS and DV_SS. Things that Eagle cannot handle and will complain about with its ERC if the connection is made on the normal copper layer.

One more thing: be sure to not accidentally setting th eDAC12OPS bit. It will switch the DAC0 output to your V_eREF+ input with desastrous results.

Oh, and in your schematics, it reads MSP430F155, but you said you're using the 2618. The two are mostly but not entirely pin compatible. The 155 has a DAC12 without DAC12OPS bit, and other small differences. The 155 has USART, the 2618 USCI (RX and TX are on the same pins still)

unfortunately, the image is so blurred I cannot really read everything (including your UART circuitry). maybe there' ssomething hiding there which i simply cannot decipher.