> You asked > How 'constructive' this will be is a matter of > interpretation and the mood of the recipient. I'm never a grumpy ingrate when it comes to circuit advice. 1/2 dozen people have made suggestions and I'm just trying to sort out what is going to be an optimum arrangement. I'm not familiar with the physical intricacies of the I2C buss and I appreciate your patience and efforts, really I do > Is the PCF driving the I2C bus when power failed? No, but I2C is idling. Both SDA and SCL high > If it isn't (why would it, it's a slave) then you have > no reason to disconnect the I2C pullups. The power failed > PIC is the chip needing the 1M pulldowns. In an earlier post I mentioned that, in the conventional simple arrangement of PCF8593+pullups+PIC, if the PIC is turned off the backup current is 400uA. With the pullups disconnected in this condition it drops to 5uA. In the original circuit the PCF was supplied with V from the PIC via a diode and the battery via a diode. The pullups went to the PCF's Vcc. I had assumed that the excess current was going through SDA and SCL into the dead PIC, hence disconnecting them In mitigation I'd offer that I'm very busy with s/w that needs doing for the PIC and could have missed something all too obvious. I'd also like several other charges of circuit-building without due care and attention taken into consideration ;-) > Why does the xtal trimcap go to Vcc rather than ground? That is Philips' advice for the PCF8593 specifically. And some of their RTCs have the crystal case grounded, others to Vcc > What is the width of the /int pulse? 500ms on, 500ms off > Why are you not disconnecting it with one of the 4066 gates? Because it can be turned off within the PCF8593. As far as I know. I'm still playing around with the alarm and timer registers. I'd no plans to use it in this application, but will hook it up to a PIC INT input and investigate the pullup size > NiCads are lousy batteries for backups since their self leakage > is high. A diode connected 3V lithium would give you 10 year > shelf life. Ni-Cads are lucky to give you 3 months (just look at your > old 486 motherboard to see your dead nicad) A NiCd 4-pack is all I had to hand that I could make some measurements with. I know about their self-discharge and would not use them for long-term backup > Why are you not using a 4066 which has a much lower on > resistance? I found a 4016 first, I'll be replacing it with a 4066 as soon as > Why are you disconnecting the pullups when there is no > need to do so (per my repeated comments on this point)? > Why pulldown the I2C inputs when the idle state of the bus > is high? Well, I can only direct you back to the 400uA measurement I made. It could well be that this circuit is over-engineered and needs trimming back. I do and did get your point about pullups not using power if they aren't pulling against something > You certainly do NOT want to load the data line with nF worth > of capacitance I'm not putting any capacitance on the lines > if you really have nF cap on the SDA line It isn't actually installed, but if it were to be it isn't on SDA. It's on the control line to the switches that disconnect the SDA pullup. SDA and SCL have no added capacitance > > With the PIC turned off, consumption is 7uA > > What is the PCF spec? Around 5uA at 5V, 1uA at 1V. 2.5V minimum for I2C operation. This is not a situation where the circuit will or should be powered down for days, weeks or months. The PIC is still supposed to be doing some work after all. Although primarily the backup is for power cuts, battery changing etc, it is possible that the user would intentionally turn off or disconnect the PIC, but that shouldn't be for any great length of time. And if it was the whole thing would have to be reset anyway > > The ?nF can be installed to introduce a delay and so turn > > SDA after SCL. I'll have to read up on that > > You DO NOT NEED OR WANT IT That was pencilled in per a comment by Peter Peres > Delay the 4066 gate pullup to Vcc (enable delay) with that cap > instead, ensuring that the PIC is alive and well before connecting > the PCF to the bus The PIC has several other things to do, like LCD init, check serial comms to another device, and wouldn't look at the I2C buss for a while after power-up > My advice is worth exactly what you paid for it > Should you find that you use it, contact me off list for > royalty info Or you could contact me and see what you can get ?? ;-))) Thanks a lot, I mean that -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads