| ??? 08/10/07 18:48 Read: times |
#143060 - Yes Responding to: ???'s previous message |
Jan Waclawek said:
Richard Erlacher said:
I've done a little experimenting with RESET and with Vcc fall times. If I use a really hefty linear power supply, one which, with the on-board ~2750 uF of capacitance still produces a rise time on Vcc of about 140 microseconds, when switched on, I see no observable flaws in the RESET behavior. However, when I switch off the power, regardless of whether I use a DALLAS, Philips, Signetics, Siemens, or Intel MCU, it frequently (about one time in 6) corrupts my BBRAM, which is mapped into both data and program space. It does not exhibit this behavior on a pushbutton RESET. Can you be please more precise in describing, what exactly you did, what was the circuit (especially the chipselect and power of the BB RAM - is it the type integrated with battery, or a standard SRAM and a battery?), and what were the results? -- Or, do I understand it right, that you DID NOT use a proper reset IC with this experiments? And, do I understand it right, that you encountered NO BBRAM corruption when holding the reset input high by a pusbutton, while playing with VCC? If this is the case, is this not enough evidence for you, that it IS the reset IC (which "holds" the "button" for you upon power failure) which is what you really need to prevent the BBRAM corruption, and NOT any sophisticated precise VCC control? JW I used a MAX1232 which was driven by the "classic" and manufacturer-recommended RC reset, wired such that it could be bypassed. My MCU, in one case, the DS89C4x0, has a bidirectional RESET, so the typical supervisor is really not useable. The other 4 MCU's, Philips, Signetics, Siemens, and Intel, behaved nominally the same. Keep in mind that this was an 8032 circuit using only external memory, since that's most easily observable. THe BBRAMs were of two types. One was a DS1230Y and the other was a 62256 in a Dallas "smart-socket." With or without the MAX1232, the circuit showed no problems I could observe when I pressed the reset button. This suggests that both approaches to RESET seem to work just fine. On power-up, the circuit powered up and ran just as one would expect. When powered-down, with or without the MAX1232, there was BBRAM corruption for the first 100 attempts. After 100 power cycles, I removed 2000 of the 2700+ uF of capacitance from the board and repeated the 100 cycles. I did not measure the rise time of Vcc because it was already lower than 1 ms with >2700 uF on the board. With the large capacitors in place, power was still decaying and well above 2 volts 2 seconds after DC power was switched off. I have not paid much attention to the circuit after that step, as I've had other things to do. RE |
