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Powering the MSP430 on Launchpad w/Emulator Disconnected


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Hi,

I have a MSP-EXP430G2 Rev. 1.4 with ....G2231 installed.  I have disconnected the on-board emulator supply by removing the J3-VCC jumper.  All info on the web and TI literature that I have found so far state that to power up the MCU, I must provide a 47K pull up from RST to VCC (3.6V).  The schematic for the Launchpad indicates that the pull up is actually on the board already.  Whether I assume that the on-board pull up is really there or connect my own external resistor, the mcu will not power up and begin execution.  The only thing that seems to work is to tie RST directly to VCC.  

 

Can anyone provide insight into my apparent misunderstanding of the circuit operation.

 

Thanks in advance for you help.

 

Gordon

 

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So the reason this happens, is that the RESET and TEST (and RXD, TXD) lines connect through those jumpers to other MCUs that are part of the Emulation layer.

 

If that emulation layer is not powered, i.e. you applied power to the Vcc + GND lines of the target but the USB cable is unplugged, then the emulation-layer IC chips will have a 0V potential on their Vcc lines and seeing a higher-than-0V level on some of their GPIOs (the ones that ultimately lead to your RXD, TXD, TEST, RESET pins in your target) will cause power to flow through the "protection diodes" on the emulation ICs.

 

Your target, in effect, is powering the emulation layer through any means possible ... the RESET pin probably being the only one that has a consistent +Vcc voltage level on it.  But since the only Vcc power applied to RESET is through a fairly high-value resistor, it doesn't take much load to drop that voltage very low to the point that it holds your target in RESET mode.

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