On Saturday 10 June 2006 18:37, Peter Brett wrote: > On Saturday 10 June 2006 18:09, Peter Brett wrote: > > On Saturday 10 June 2006 17:58, Hamish wrote: > > > On Saturday 10 June 2006 16:40, Hamish wrote: > > > > In addition, should we be doing anything with the WP and HOLD pins of > > > > the SPI? They don't seem to be defined as an output for the module > > > > like the other SPI input pins... Is hold simply to be tied and WP > > > > jumpered? Or something else? > > > > > > Ah. Found this bit in the RevB schematics... They appear to be tied to > > > Vcc? No write protect then? Is there any advantage to connecting WP_ to > > > the XP6? (Are there any pins free on the XP6 to tie it to? If the XP6 > > > can come up with WP_ low, then we can prevent spurious code from > > > over-writing the SPI by accident (Or at least till something calls a > > > write_enable function first). > > > > *boggle* Perhaps Timothy can help out here... I was under the impression > > that it was a good idea to connect unused pins to rails through > > pull-up/pull-down resistors rather than directly...? > > Okay, I had a closer look at the schematics & the PROM datasheet, and I see > what's going on. I would have tied the !CE pin high with a 4.7K pull-up to > minimize any chance of junk signals being accepted by the PROM during board > power-up, but it doesn't look like Howard's done that. I assume that's by > deliberate choice. :) >
It's been a long long while since I did any of that... But from memory we only used to connect via a pull-up resistor on logic if we wanted to connect an input to an output AND have a... default... (Maybe my words are wrong). But I would have thought, connect WP_ to GND with a resistor and then also direct to a tristate output of XP6... Then if you don't drive XP6 output, the chip is write protected. Otherwise if you drive the XP6 output HIGH (i.e. Vcc) the chip is NOT write-protected. > Peter
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