Re: Code stuck waiting for SPI transmission but only after power-up
2010-03-13 by Andrew Mathison
I'll try and answer your queries:- >The changes were made because we weren't sure what a PC power supply outputs at power-up when the load on the 5V is only in the tens-hundreds milli-amps. Using a scope to determine that is >irrelevant, because another PSU might do something else. Still, I did check it. As you neglected to post EXACTLY which of the many chips of that family, its a bit difficult to answer with certainty, but I will do my best for you:- Are you using a 5 volt chip? If yes, take out the linear regulator, I still feel its a headroom problöem, especially if say your PC powerly puts out 4.99 volts and your reg wants to regulate to 5 volts....... I have also read that putting a Linear reg after a SMPS has other problems, but I have forgotten where I read this or what the problems were (if not what I have just written) sorry.... The chip you are using needs careful calculation of the value for the input capacitor and it must be placed very close to the chip - standard procedures.....read the data sheet. Also, you appear not to have read Note 12 on the data sheet, which says that the Vin must be carefully calculated and must be at least 2.5 volts or greater......it shows that Vin must be greater than Vout, but as you have not told us what chip you have, impossible for me to calculated, but believe me, a 5 volt reg on a 5 volt supply is GOING to give problems.....you have exactly that. >You would be right about the headroom if I wasn't using an LP3966 "3A Fast Ultra Low Dropout Linear Regulators" that drops only 80mV at 300mA. >Plenty headroom :-) Not 100% certain, as I said because I don't know what chip you have, but read Note 12 carefully.....Also the dropout voltage varies with the current taken, up to a nominal 800mV at 3 amps., which could even go to 1 volt on some versions......if your input cap is of the wrong value or too far from the chip, then you will need a greater Vin. I would recommend at least 1.1 volts MORE than you want out......assuming 5 volts out..... >I'm no yank, but I don't get either "Belt & Braces" nor the "suspenders" expression... what's it supposed to mean? Most men only use a belt OR braces to hold their trousers up. Some of them use both together, looks awful!!! But gives more security!!! In this case, your extra security may bite you in the posterior!!! I hope this helps......tell me if I (or anyone else for that matter) can do more for you.....just ask (but do tell us exactly which chip you are using first..... :) Greetings from Andy Mathison [Non-text portions of this message have been removed]