Post by thread:[EE] Making sure an NE555 gets reset at startup

part 1 1314 bytes content-type:text/plain; charset=ISO-8859-1 (decoded 7bit) I've hooked up an NE555 in monostable mode which is triggered by a PIC. While functioning perfectly, there is one small annoyance that beats me of why it happens and how I can prevent it. The reset-pin on the 555 is hooked to a 10k pull-up and the trigger-pin is connected directly to the PIC (no caps or anything between). The weird thing is this: When powering up, the 555 is triggered, despite the fact that except for the INTOSC-calibration the first thing I'm doing in the PIC is to set TRIS to output and set the ports to '1'. Why this is weird is because the 555 is supposed to trig on negative flanks according to the datasheet, and the PIC-pins go from either 1->1 or 0->1 (depening on their values on startup) but never from 1->0->1. I'm attaching the small code if anyone wants to peek, but I doubt the error is in there. Anyway, a hardware solution for this problem would be to let the reset-pin on the 555 stay low (i.e. ignoring any triggers) for about a second after starting up, but I'm not sure how to implement this. Sure, I can make it non-simple by adding another 555, or a bunch of other IC's, but I'm pretty sure this delay can be achieved by using RC-logic. Although I have yet no clue how to do it. -- - Rikard - http://bos.hack.org/cv/ part 2 1739 bytes content-type:application/zip; name=main.zip (decode) part 3 35 bytes content-type:text/plain; charset="us-ascii" (decoded 7bit)

At 05:24 PM 1/1/2008, you wrote: {Quote hidden}>I've hooked up an NE555 in monostable mode which is triggered by a >PIC. While functioning perfectly, there is one small annoyance that >beats me of why it happens and how I can prevent it. > >The reset-pin on the 555 is hooked to a 10k pull-up and the >trigger-pin is connected directly to the PIC (no caps or anything >between). The weird thing is this: When powering up, the 555 is >triggered, despite the fact that except for the INTOSC-calibration the >first thing I'm doing in the PIC is to set TRIS to output and set the >ports to '1'. Why this is weird is because the 555 is supposed to trig >on negative flanks according to the datasheet, and the PIC-pins go >from either 1->1 or 0->1 (depening on their values on startup) but >never from 1->0->1. > >I'm attaching the small code if anyone wants to peek, but I doubt the >error is in there. > >Anyway, a hardware solution for this problem would be to let the >reset-pin on the 555 stay low (i.e. ignoring any triggers) for about a >second after starting up, but I'm not sure how to implement this. >Sure, I can make it non-simple by adding another 555, or a bunch of >other IC's, but I'm pretty sure this delay can be achieved by using >RC-logic. Although I have yet no clue how to do it. You should set the relevant port bit to '1' *before* you write to TRIS. The power-up state of the port bits is undefined. If the value at power-up happens to be 0, you're generating a nice clean trigger pulse. If absence of false operation at power-up is important, I would also suggest using external /MCLR, an external reset circuit (tied to both /RESET and /MCLR), and a pullup resistor on the trigger output so it gets pulled up towards Vdd during the power-up. Or dump the 555 entirely and do the timing inside the program. >Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

In message <7.0.0.16.2.20080101175644.05f2d710KILLspaminterlog.com>, Spehro Pefhany <.....speffKILLspam.....interlog.com> writes >Or dump the 555 entirely and do the timing inside the program. Or if the OP absolutely has to have an extra 8 pin chip use one of the 10F series. > >>Best regards, > >Spehro Pefhany --"it's the network..." "The Journey is the reward" >EraseMEspeffspam_OUTTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com >Embedded software/hardware/analog Info for designers: http://www.speff.com > > > -- Clint Sharp