Vcc question on Target Board

Hi Brock,

How are you applying power to the board?  The board seems to be pretty flexible.

Without JP10 (J7) installed, the voltage on pin 2 of the JTAG connector would be coming from the rail labeled "VCC", assuming R8 is installed.  (JP10 is not involved in that path.)  The VCC rail seems to be driven from J5, "PWR3".  What do you have attached to J5, "PWR3"?  What is the status of R8 / R9?

Also, without JP10 (J7) installed, the only way to power the MSP430 looks to be directly on J1.1 or on J4.64.  To make JTAG work without JP10 installed, you'd have to tell the JTAG tool to supply the same voltage you are supplying on J1.1 or J4.64 since there's no proper sense voltage on pin 2.  That's a weird setup.

Jeff

Hey Jeff, thanks for responding.

That's just it though, I don't have any voltage applied to the J5 pin. R8 is a 0R and it's in place, there's no R9. I have no wires attached directly to my Vcc pins but without the jumper i'm not sure where it's getting that voltage on the pin from...It's really weird b/c when i vary the voltage to my LCD it affects that voltage i'm getting at Vcc pins... This might be one of those things that is staring you right in the face but i'm just not seeing it.

You have no wires attached to which Vcc pins?  On the JTAG connector?  On the MSP430?  And just to clarify, the unexpected voltage you're talking about is appearing on pin 2 of the JTAG connector, right?

If you are powering the MSP430 via J1.1 or J4.64, then the MSP430 could be pulling up the RST line (newer MSP430s I think), which in turn pulls up the rail called "VCC" through the 47K R5.  That voltage would then appear on pin 2 of the JTAG connector.  But I didn't think you were supposed to power an MSP430 with anything above 3.6V, and you said you were seeing 4V.

Jeff

Hi Jeff,

I meant I had no jumper wires of any type attached to the header DVcc/AVcc(1.1, 4.64) pin of the MSP430. The voltage i'm referring to is appearing on the DVcc pin of the MSP430. My demo and code still run fine but I'm just curious as to how my chip is being powered if I don't have the jumper J7 in place...

Oh, sorry.  I had it all wrong. ;-)

My new guess is that the MSP430 is getting power from some input pin.  The input buffers in the MSP430's pin logic have substrate diodes (aka body diodes, protection diodes, or other names) that conduct when the input voltage is about 0.3V higher than DVcc allowing the input voltage actually to drive Vcc.  The diode can usually support as much as 2mA if I read the data sheets correctly.  Theoretically it's enough to power the miserly MSP430.

If the MSP430 has no power applied to DVcc, then when the MSP430 is running (not sleeping), the voltage you should read at DVcc is about 0.3V lower than the input voltage.  When it is in LPM3, the voltage would pretty much match the input voltage since effectively no current is flowing over the diode.

Technically you're risking damage to the MSP430 like this.  The absolute maximum voltage you should apply to any pin of an unpowered MSP430 is 0.3V.  My guess is that you are violating that rule.  Be sure no input pin sees more than about 4.4V or you really could damage the device.

Jeff