| ??? 08/09/07 07:59 Read: times |
#142960 - and what Responding to: ???'s previous message |
Richard Erlacher said:
and other parts of that ilk, as they may assert reset, but the MCU potentially will ignore it and run away as Vcc decays. We ARE talking out of specs, but then, as I already said several times, the manufacturers won't put anything into the specs until the customers won't kick and punch them to do it. However, prudent manufacturers make sure that the RESET input is listened upon to as low VCC as possible - and it is not impossible at all to make it work properly the whole range, starting from zero, to VCCmax, as far as the consequences are concerned. Let's just look at the consequences side. Again, I already said, the problem is not the mcu itself - it won't get destroyed (except maybe the crap items), although sctrictly speaking the manufacturers won't guarantee that, either! (look at the formulation of what may happen if you operate outside the 5V+-10% range) - the problem is the peripherals including memories (and including the internal FLASH). Now in CMOS it is quite possible to block the FLASH charge pump once the reset signal comes in; even at low voltages. Also, it is relatively trivial to block write access to internal RAM, once you have a valid reset signal; so that its content can be preserved under battery. Also, it is relatively easy to hold the I/O pins in the default reset state, if a valid reset is present, even if the internal clock is starting to fail (due to the fact that reset is synchronous, at least the 24 clocks are required while above VCCmin, but that's fulfilled except extremely rapid powerdown - I have read about asynchronously resetted I/O pins in some '51 derivatives, but don't remember which). The same is valid for external SRAM and peripherals; of course, it's your (the designer's) responsibility to take care of the chipselect once the reset signal is asserted (there are reset ICs which provide chipselect gating; but it's also enough to get any 74HC gate and hang it out on the battery too). That there ARE crap mcus on the market (pitifully including '51s), that's an another issue. And that the manufacturers don't want us to dig too deep into this issue, is one more sad fact. Nevertheless, these chips DO work, Richard, face it. JW |
