A slightly larger with SPI slave peripheral. /Ruben > Why use a low pin count PIC with extra chips and discrete when a slightly > larger PIC would be cheaper and simpler? > On 28 Apr 2014 14:43, "Isaac Marino Bavaresco" > wrote: >=20 > > Em 28/04/2014 09:36, Larry Bradley escreveu: > > > An LCD display connected to a PIC requires a minimum of 6 port pins. > > > With small pin-count PICs, this can be a problem. > > > > > > Many years ago, when the Basic Stamp came out, some clever chap provi= ded a > > > serial LCD display - it used RS232 - to reduce the pin count (this wa= s in > > > the days of the 16F84). > > > > > > I want to do the same thing but using SPI. (I just want it for myself= - I > > > don't plan on producing anything). I can always get a chip with SPI h= ardware > > > to use as the LCD driver, but I though I would try it with a couple o= f chips > > > I have on hand, using software SPI. > > > > > > I've done SPI master in software - no problem. It seems to me that do= ing the > > > slave shouldn't be difficult at all - it is really a very simple syst= em. > > > I've search the Internet to see if there are any software slave > > > implementations out there, but no luck. Rather than re-invent the whe= el, do > > > any of you know of any? > > > > > > The attached circuit (PDF) shows how to implement an SPI-like slave (it > > works with an SPI master) with a 74HT/HCT595 that uses just two pins of > > the PIC. > > > > The trick is that while SCK is clocking, the parallel load input is kep= t > > at logic level zero by the transistor and RC constant (R4*C2). As soon > > the pin SCK stops clocking (inactive state =3D logic level zero), the > > capacitor starts charging until the parallel load pin sees a logic leve= l > > one and transfers the contents of the shift-register to the outputs. > > If you are worried about the slow rising signal at the parallel load > > input, a single-gate Schmitt-Trigger gate can be used to square the sig= nal. > > > > Several 74HC595 can be chained to expand the output pins to any multipl= e > > of 8. > > > > > > Isaac > > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .