A long time ago in pre-F15x days, I experienced lots of brownout problems. I have not seen that myself since F15x implemented BOR circuit. But recently, I keep hearing from other users reporting problems that seem to be related to brownout for F2xx chips that do have BOR circuit.
I do not know if those reported problems are caused by inadequacies in the BOR. I start to experiment with what I have. Here is a report of what I learned so far. And I am still working on this.
I know that fast changes in Vcc can cause problems. But those problems are hard to investigate and easy to avoid. Thus I only tried with very slowly changing Vcc. Specifically; I use the ADC12 of a F169 to produce a voltage anywhere from 0 to 2.5V in increments of 0.61mV. This ADC12 output is used as the Vcc of other MSP430 chips under investigation.
The first thing I tried is a F135 (which does not have BOR). When I ramp up Vcc slowly, I found that the F135 starts to run at 1.09V. The DCO frequency is very slow (~1Hz) and erratic at that Vcc. It should be about 800kHz and used for MCLK and SMCLK by default. The CPU seems to be able to execute at least a few instructions that are stored in Flash. But I am not sure if it might crash or not.
Next, I tried with a F2471 (which does have BOR). It starts to run at 1.53V instead. DCO frequency is about 1.1MHz (very close to what it should be). And the CPU seems to be able to execute at least a few instructions that are stored in Flash.
BTW, a lot of the code examples and Applications published by TI raise the clock rate right out of reset. If I do that, quite understandably, both F135 and F2471 crash after a few instructions.
So, based on my experiments of a single F135 and a single F2471, the BOR seems to help. But, a few questions emerged:
(1) The data-sheet says I need min 1.8V to run at 4.15MHz. But it does not say what min Vcc voltage is needed to run at 1.1MHz. Is my F2471 safe when Vcc is between 1.53V and 1.8V with MCLK=SMCLK=~1.1MHz?
(2) The data-sheet says at max 1.71V+210mV=1.92V, the BOR will release the reset. There is no min or tpy value. My F2471 did it at 1.52V. Will other chips with BOR circuit release reset at even lower voltage? (Because there is no min value specified.) If so, is it safe at that even lower voltage?
(3) The data-sheet says below 0.7*1.71V=1.20V the BOR circuit may not do anything. It does not say what the DCO and CPU will do at such voltage. My F135 does funny things at 1.09V Will other chips with BOR circuit start to do funny things before BOR circuit start to take control?