| ??? 08/19/07 15:50 Read: times |
#143404 - Sadly, there's no guarantee it does that! Responding to: ???'s previous message |
Jan Waclawek said:
Erik Malund said:
However, what guarantee do you have that the uC will not emit any signals when 'going dead' ?
Nobody will have any trouble to stop writing on an early power fail interrupt, but, as far as I can see, the REAL problem is "what comes out of the micro when it goes into reset". It is exactly the purpose of reset to prevent anything coming out of micro. If that were true, I'd likely have seen no corrupted BBRAM, methinks! The circuit I used had a supervisor. The MCU had a brownout detector that causes RESET. The BBRAM has its own protection circuitry. The prudent designer won't believe his own brother, though, and double-secures the thing using the chip-select gate... Just how would one do that in light of the fact (a) all the IC's are operating below their specified voltages and (b) they see different Vcc levels due to noise and other factors as power decays? I'd see the NVRAM blocking level below mcu reset level more logical, though. Any decent 5V CMOS mcu should respond to the reset normally down to 3V at least. And, with some moderate design effort from the mcu manufacturer, I think it is not impossible to have perfect reset behavior down to zero. I'd be very interested in seeing where it says what the post-reset behavior in case of decaying Vcc is. JW
It may, in fact, be that the MCU power-fail-detection is set lower than the BBRAM power-fail write-lock threshold. The DS1230Y sets this level at 4.5 volts. The 1230A/B set theirs at 4.75. The DS89C4x0 datasheet says that the MCU power-fail detection is between 4.6 and 4.2 volts. At the 5% trip point, the MAX1232 supervisor detects power-fail at between 4.74 and 4.5 volts, while at the 10% trip-point, it detects it at between 4.49 and 4.25 volts. That looks to me like an invitation for confusion. It gets even more confusing if you consider that the behavior of external logic and of these components isn't well characterized for slowly decaying Vcc. RE |
