FWIW, according to the SGS databook (the one closest to my desk...), their
74HC04 has an absolute rating of Vss-.45. So, if the part is wired to +5
and GND, an input of -.5V (borderline for RS-232) is outside of spec.
If you are talking about a 74HC04 to couple 3V devices to 5V devices, I'd
say fine, the CMOS input of the HC04 should typically do the trick.
I'm not sure I follow your compiler tip at all. All the SPI control
registers are accessible from memory registers. There are a few special PIC
fuses that have to be set in the image file, but I'm not sure how any of
them would relate to IO.
Best Regards,
-jjf
-----Original Message-----
From: Geoff Smith [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, July 25, 2000 3:55 PM
To: Palm Developer Forum
Subject: RE: Serial Communications
Going to a Pic through and inveter works - lots of equipment uses 5v TTL
levels. The 74HC04 inverters are the ones I use. What compiler on the Pic?
The CCS one allows you to software invert the line, thereby doing away with
the inverter completely.
Right or wrong - it does work.
Serial is idle high, so the Palm must be switching off the line to conserve
power.
Geoff
At 03:22 PM 07/25/2000 -0700, you wrote:
>I'd expect an inverter to explode. The RS-232 is going to swing negative
>(below VDD), which will kill most CMOS inverters. If you only have a
single
>voltage (like +5), you can use RS-232 level converters from companies like
>Maxim.
>
>One trick is to use a pair of RS-485 transceivers. They are generally
>cheaper than an RS-232 voltage converter and smaller (no CAP required).
>Using two, you can do RS-422, RS-485, (both current-diff.) and RS-232
>(voltage level).
>
>Really down and dirty, you can use caps and transistors and steal the
>required 12V from the handshaking lines from the other end of the
connection
>(ie, the Palm). I would probably avoid this since the Palm itself is using
>a fairly low voltage converter.
>
>-jjf
>
>-----Original Message-----
>From: Ben Hamby [mailto:[EMAIL PROTECTED]]
>Sent: Tuesday, July 25, 2000 12:23 PM
>To: Palm Developer Forum
>Subject: RE: Serial Communications
>
>
>The Tx and Rx in a RS-232 do indeed act much like I/O for a UART, but they
>are generally speaking the opposite (at least from my other
>experience)....ie. the generic UART defaults low, asserts high, the RS-232
>defaults high, asserts low.....essentially my output is the exact opposite
>of what i need/expected from an RS-232, the start bit goes high, instead of
>low, and the 0's are high while the 1's are low....i'm considering just
>throwing an inverter in between, but that could get a little messy for
>now...
>
>thanks,
>ben
>
>At 10:05 AM 7/25/2000 -0700, you wrote:
> >The TX and RX of RS-232 is essentially UART output (UART -> Voltage
> >Converter -> connector, matches standard). Also, remember that RS-232
uses
> >+/- 12V (nominal) levels, so you will need conditioning before connecting
>to
> >a PIC.
> >
> >Good Luck,
> >-jjf
> >
> >-----Original Message-----
> >From: Ben Hamby [mailto:[EMAIL PROTECTED]]
> >Sent: Tuesday, July 25, 2000 9:36 AM
> >To: Palm Developer Forum
> >Subject: RE: Serial Communications
> >
> >
> >I see what you mean now, as far as the differences between the SPI and
the
> >common UART(ie. 16550)...However, isn't the RS232 supposed to have
similar
> >output to the SPI? Ie. defaults high, assertions are low....and I thought
> >the Palm had an RS232...does it have both and defaults to using the 16550
> >protocol? Or am I out in right field without a glove....
> >
> >Thanks,
> >Ben
> >
> >At 03:14 PM 7/24/2000 -0700, you wrote:
> > >The SPI is quite a bit different from a standard async serial port.
You
> >can
> > >implement an async serial port in software on most PICs. I did one a
>long
> > >time ago on a PIC16C50 derivative and it worked pretty well.
> > >
> > >If you go to www.microchip.com and select Application Notes, a note on
> > >implementing an async port will be one of the first ones listed.
> > >
> > >Good Luck,
> > >-jjf
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