MSP430 value-line startup problems - Page 1
Forum section: electronics / general

author: brainwash
author_rank: Frequent Contributor
timestamp: October 21, 2013, 11:32:50 am
content: I'm having consistent troubles with the power-on of an MSP430G2331. If I'm feeding the 3.3V through a current-limiting resistor (15 ohm or 50 ohm) it fails to power up 90% of the time. As soon as the limiting is removed it powers on consistently. I'm measuring the 3.3V (or 3.6V) at the chip pin not at the supply. The "regulation" is achieved via a 3.3V zener.The datasheet mentions that the BOR/POR is "hardcoded" and always on. The Vcc pin has a 100nF cap placed on the chip. There's also a supply cap of ~100uF.I've never had this problem before with PICs so not sure what's causing it but it seems that if the voltage rising slope is not steep enough the chip fails to start.Has any of you come across this problem before? Is there something on the MSP430 that needs to be configured?

author: brainwash
author_rank: Frequent Contributor
timestamp: October 21, 2013, 01:56:24 pm
content: Well, to try an answer my own questions I stumbled upon two articles:http://e2e.ti.com/support/microcontrollers/msp430/f/166/t/215263.aspxhttp://e2e.ti.com/support/microcontrollers/msp430/f/166/t/19070.aspxIt could also be the fact that I'm not using a resistor between Vcc and nRST, haven't tested that theory, maybe the reset circuitry pulls a lot of current under strange conditions or causes the power to oscillate.

author: Stonent
author_rank: Super Contributor
timestamp: October 22, 2013, 07:35:14 am
content: So you're using the chip alone without the Launchpad board?I seem to remember pulling the chip out before and running it by itself, but I don't remember having to do anything special. Hmm

author: Kjelt
author_rank: Super Contributor
timestamp: October 22, 2013, 07:57:47 am
content: Don't know the chip but generally the /reset line of a microcontroller has to be kept low with a capacitor as long as the VCC is not stable.So I don't understand your remark "seems that if the voltage rising slope is not steep enough". Because the VCC should be steady (end of slope) when the reset of the microcontroller goes inactive.That said, check the VCC with a scope at the moment reset goes inactive and the processor starts up to see its behaviour and to check if the decoupling is good enough?

author: nctnico
author_rank: Super Contributor
timestamp: October 22, 2013, 09:24:27 am
content: The problem is probably that the MSP430 start to run at very low voltages (typically 1.35V). If you switch to a frequency >1MHz it won't work. So either use a lower frequency, use an external reset chip for 3.3V or make sure the power supply ramps up quicker (within 1ms).

author: colotron
author_rank: Contributor
timestamp: October 22, 2013, 05:25:19 pm
content: Could you post a schematic of your circuit?.The 50 ohms resistor can cause a voltage drop that exceeds the allowable voltage difference between VCC and high-voltage level in the I/O pins: 0,3V -> 0.3V/50 ohms = 6mA.I'm assuming that you are powering the msp430 from the zener and a series resistor, an the rest of the circuit directly from the zener output (maybe you are trying to measure msp430 current consumption?).But is too much guessing, that's why you should post a circuit schematic to avoid any confusion.

author: brainwash
author_rank: Frequent Contributor
timestamp: October 22, 2013, 06:46:21 pm
content: Thank you for the suggestions so far. Some anwers:- the chip is barebone, just Vcc (decoupled), GND and nRST tied directly to Vcc- voltage is measured at the Vcc pin so voltage drop does not matter. In fact I'm feeding >4V into the zener- the >1MHz comment is probably spot onI have a suspicion that the BOR/POR has a "hardcoded" value that is not dependent upon the oscillator setup. Also, one app note made mention of a required Vcc rising time of >1V/ms, though the datasheet does not say anything about this. I haven't measured the value but I suspect it's lower than that as there's also another big capacitor that needs to be charged.I might get away with adding a diode between Vcc and nRST or feeding the reset pin through a resistor and then through a cap to the ground.I'm running the chip with the internal 4Mhz (or 8?) oscillator so it needs the proper voltage after start-up. I wasn't expecting it to start up so early and fail, I'm more used to PICs which don't seem to behave this way.Oh, if the chip fails to start up properly, raising the voltage will not help. It needs to be drained to <1V and then powered again. After a failed startup there is absolutely no digital activity on the pins and the GPIO will go to a random state: either Vcc, open or half voltage or something strange. I don't know if it can damage the chip but it can certainly trigger unwanted outputs.It's more of a "heads-up" since some other people might be struggling with this.

author: nctnico
author_rank: Super Contributor
timestamp: October 22, 2013, 08:08:33 pm
content: If the power supply can be dodgy then its a good idea to add an external reset (voltage monitor) chip. About a decade ago I made a USB powered gadget around an MSP430 controller. The problem was that on some laptops while in standby mode the 5V was leaking enough to power the MSP430.

author: chibiace
author_rank: Regular Contributor
timestamp: October 23, 2013, 06:44:44 am
content: 47k resistor from reset to vcc. though i had some projects with reset floating that worked fine