For the initial version, I plan on driving eight... however, I really=20 expect to end up with a version of this which has 1024 points on it...=20 which would require 32 devices. And, I may end up needing to be able to=20 set each pin's weak 'pull' to either high or low which would require a=20 total of 64 devices. Back when I was doing 74LS00 series TTL (which tells you how long it has=20 been), this was a major issue since the fanouts were so small - in the 2=20 to 10 range. It seems that CMOS isn't nearly as much of an issue. I tend to agree with Russel though - the thing that has me most=20 concerned right now is the capacitive loading issues... According to the=20 mcp23s17 and the PIC18f26J60 datasheet, you can only have 400pF of=20 capacitive loading to meet the timing specs - although this looks like=20 the I2C spec... not necessarily the SPI spec. But assuming it's=20 accurate, and assuming a 7.5pf per input capacitance (which seems common=20 for other CMOS series inputs), this means that we're talking no more=20 than 53 to be in spec. But, there seems to be no firm documentation on it= .. I think the question I'm really asking here is... How do I compute=20 fanout in cmos, seeing as there seems to be little of the data which=20 would actually be required to do so? -forrest On 9/12/2010 5:04 PM, Jan-Erik Soderholm wrote: > Oops, sorry, that is correct. > > Anyhow, driving 8 CMOS inputs from 1 CMOS output > should not be a problem, which was the initial > question, I think. > > Jan-Erik. > > > On 2010-09-13 00:36, Steve Maroney wrote: >> With the MCP23S17, both the SPI and I2C versions have address pins. On t= he SPI version, you can put 8 chips on one CS line. >> >> >> Best Regards, >> Steve Maroney >> >> Business Computer Support, LLC >> Mobile Phone:504-914-4704 >> Office Phone: 504-904-0266 >> Fax: 866-871-7797 >> >> >> >> -----Original Message----- >> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf= Of Jan-Erik Soderholm >> Sent: Sunday, September 12, 2010 5:02 PM >> To: Microcontroller discussion list - Public. >> Subject: Re: [PIC] SPI Fanout with large numbers of MCP23S17's >> >> To let 1 CMOS output drive (only!) 8 CMOS inputs >> should never be any real problem. I saw a circuit >> with at last 100's of driven CMOS inputs when I saw >> your questions at first. You never told us that it >> was only 8 inputs you was looking at, did you ? :-) >> >> Note also that the I2C version has 3 address lines, >> so you can run 8 devices on the same I2C bus without >> separate address och chip-select lines from the PIC. >> >> In the SPI case you need to have a separate CS line >> for each MCP23S17, either as separate I/O pins >> or through some extra address decoder chip. >> >> Jan-Erik. >> >> >> >> >> On 2010-09-12 23:45, Forrest Christian wrote: >>> Unfortunately I need the pins to behave much like a pin on a >>> traditional PIC.. that is, being able to be individually driven high or >>> low or read as an input. >>> >>> Basically I have weak pullups/pulldowns on everything - and then I set >>> everything to a weak high pullup then pull one pin low and see which >>> other pins went with it. Then do the same for a pulldown, except I pul= l >>> one pin high. Repeat for each of the 128 pins... >>> >>> I think I'm just going to try this with the eight I need for this desig= n >>> and see how it works. I realized I actually have two SPI devices on >>> this pic so perhaps I'll be ok... >>> >>> -forrest >>> >>> On 9/12/10 7:10 AM, Isaac Marino Bavaresco wrote: >>>> Em 11/9/2010 22:36, Forrest W Christian escreveu: >>>>> I am building a test apparatus (ok, it's a cable mapper) which = will >>>>> require around 256 bits of I/O on a pic system. Maybe even more. >>>>> >>>> If you don't need the pins to be bi-directional you could use the >>>> 74HC595 (output only) and 74HC597 (input only). They work serially and >>>> can be daisy-chained, you would save all the chip-selects. >>>> >>>> Isaac >>>> >>>> __________________________________________________ >>>> Fale com seus amigos de gra=E7a com o novo Yahoo! Messenger >>>> http://br.messenger.yahoo.com/ --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .