Well, for all of you that doesn't know, I'm building, at the same time as learning, a SBC as the basis of a portable gaming machine.

I'm using the w65x265s MCU, that includes 576 bytes of RAM and 8K of ROM, some ports, timers and chip selects.

To be able to add external memory and chips, I started to clone the W65C265SXB board, following the WDC's own schematics.

But the thing is that the board is designed to run at 5v, and I need it to run at 3,3v.

And I largely could get the SBC to run... until I got the EEPROM chip there.

The MCU, the RAM chip and the EEPROM chip are 3,3v compatible, but the circuit itself is designed for 5v.

And here is my first wall: how to recalculate the values of the proposed components that the WDC Datasheet has, to be 3,3v compatible?

That's BASIC electronics for many of you, but me being ZERO electronics aware, I'd like to have some guide.

As an example, the datasheet states 3K resistors everywhere they have to limit current, but those are rated for 5v, and I'm sure that new values have to be calculated for 3,3v.

I'd like to know what should I look for in the data sheet to make the calculations needed.

My first guess was that if RESB line has to be tied to VDD with a 3K resistor, and this line counts a HIGH as 0,9xVDD, then the limit should be 2,97V. (3,3v * 0.9 = 2,97).

So the rest of the voltage, to be regulated by the resistor is 0,33v (3,3v-2,97v = 0,33v).

Now I'll have to get the current, and that's where I fail (if I haven't done it before), so I don't know where to look for in the datasheet that tells me the current of that RESB line, so I can finally apply R = V / I and get the proper resistor value.

THANKS ALL!!

I suspect you won't need to change any values at all: the resistors are there for safety and the exact values are not important, so a 2/3 scaling factor is not significant.

Well... I replaced the 3Kohm resistor in the RESB line to a 1.6K one and by some reason the board is stable now. I could even burn a "Hello World!" routine in the EEPROM and it works.

W65C265SXB schematics,
W65C256S datasheet.


In the W65C265SXB schematics, there is a 1uF capacitor between RESB and GND, and a 47k pullup resistor from RESB to +5V.
//There is a diode "in parallel" to the resistor for discharging the capacitor during power_off.
//There is a switch in parallel to the capacitor for "manual RESET".
That's pretty much the "cheapest" way for generating a RESET at power on.

Please add a little schematic drawing of how you generate RESB,
because to me it sounds like your approach of generating RESB seems to be different from what's in the W65C265SXB schematics.

BTW: in your design, is RESB only connected to the W65C256S, or to any other chips, too ?

OUCH!! I meant BE

Ah... now that's something different.

In the W65C265SXB board schematics, BE is tied to +5V with a 3k pullup resistor.
Since nothing ties BE low in the schematics, there is no need to worry about the value of that resistor.

If your schematics are different, and another chip in your computer pulls BE low sometimes,
it would make sense to check the rising edge of BE with an oscilloscope.
//BE rise time from logic low to logic high level, that is.