At 11:45 PM 2/14/2010, you wrote:
>Well, I thought I was doing well on pin sharing.
>
>Here is a problem I'm running into:
>
>I have a 4x16 LCD display (electrically the same as a 2x16).
>I have a DS1302 clock.
>
>I should be able to share the clock and I/O signals of the clock with
>the data of the LED since the LED has an 'enable' pin and of course
>neither the clock nor the LED data lines are used at the same time.
>
>I know the RST of the clock needs a dedicated line, and it has one.
>
>But, when I redefine the lines of the clock to share the pins with the
>LCD data, the clock does not work.
>
>There is nothing connected between the LCD display data inputs and the
>PIC, and there is nothing connected between the clock and the PIC. Both
>the clock and the LCD should be voltage-sensing pins, and the PIC should
>be the only voltage provider, and I should not be exceeding the rating
>of the PIC pin.
>
>Oh, and the RW pin on the LCD is low (write), so it should not be
>outputting any voltage.
>
>Any ideas why this is not working?
>
>Joe M.

Are you sure the clock line (SCLK) on the DS1302 has reached a valid logic LOW
level well BEFORE asserting the first positive edge on the CE line
(formerly called /RST)?

If the LCD logic leaves the SCLK in a high state or if you switch both
at once (maybe) you'll get one fewer clock edges than necessary and things
could get ugly. The data sheet says a minimum of 1us at 5V and a whopping
4us at 2V (parameter tcc), which could be a lot of PIC instructions.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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