Reply by Bill Auerbach●November 27, 20092009-11-27
Jerry, you didn't mention how these 2 HCS12 are connected. If these
are both on the same circuit board, you're ok just connecting
them. But for some reason I suspect you're going through a
cable. If you are, then you need to add pull-ups or you will have
very poor immunity to noise and ESD. Your current readings are
correct since the input is high impedance and there is almost no
load. Although there are internal pull-ups on the port (if enabled)
they are very weak pull-ups.
Bill
> Jerry Fields writes on 10:49 AM 11/27/2009 >I have had the 2 controllers connected with no current
limiting
>resistor, for 3 days now, and everything has been working great!
>With port T set to input on the second controller, and port P set to
>output on the first, I hooked a amp meter inline between the
>connection. The amp meter has a sensitivity as low as 10 micro amps,
>but the way the digital display jumps around due to noise, I
>estimate that I would need a current flow of at least 30 micro amps
>to overcome the noise, and get a study reading on the display.
>
>I could detect no current flow between the connections weather I was
>driving it high or low, so it must be under 30 micro amps. I guess
>the real test would have been to set them both to output, and drive
>one high and one low, but I am sure Tom is correct anyway.
>
>Thanks to Tom and everyone else who replied!
>
>We have lift off! :)
>
>Jerry
>To: 6...
>From: m...@almy.us
>Date: Tue, 24 Nov 2009 06:11:29 -0800
>Subject: Re: [68HC12] HCS12 to HCS12 Port interfacing
>
>It doesn't matter which port(s) you use, you don't need a current
>limiting resistor. All ports default to input (and almost all with
>pull-up enabled) so they can be safely left unconnected or connected
>to either logic voltage level.
>
>Tom Almy
>Tualatin, Oregon USA
>Internet: t...@almy.us
>Website: almy.us
>
>On Nov 23, 2009, at 12:45 PM, Jerry Fields wrote:
>
> >
> > Hello,
> >
> > I have two MC9S12DP512 microcontrollers, and I need to interface
> them. The first mc9s12, has port P set to output, the second mc9s12
> has port T set to input, and I want to connect one line on those
> ports. There are no large amounts of data being sent, as I am just
> holding the line high or low. My question is, do I need a current
> limiting resistor in between the two? I have not yet found the
> documentation that shows me the internal circuitry configurations
> of the ports. If somebody could point me to that, or give me a yea
> or nay on the resistor, that would be great! Or you can just tell
> me not to be lazy, and figure it out myself! lol It would be great
> to hear what a few of you have to say though. Seems like I remember
> seen a open collector configuration on my old HC11. I must have
> been looking at port B (that was output only)!
>
>_________________________________________________________________
>Bing brings you maps, menus, and reviews organized in one place.
>http://www.bing.com/search?q=restaurants&form=MFESRP&publ=WLHMTAG&crea=TEXT_MFESRP_Local_MapsMenu_Resturants_1x1
>
>
>
>
Reply by Jerry Fields●November 27, 20092009-11-27
I have had the 2 controllers connected with no current limiting resistor, for 3
days now, and everything has been working great! With port T set to input on the
second controller, and port P set to output on the first, I hooked a amp meter
inline between the connection. The amp meter has a sensitivity as low as 10
micro amps, but the way the digital display jumps around due to noise, I
estimate that I would need a current flow of at least 30 micro amps to overcome
the noise, and get a study reading on the display.
I could detect no current flow between the connections weather I was driving it
high or low, so it must be under 30 micro amps. I guess the real test would have
been to set them both to output, and drive one high and one low, but I am sure
Tom is correct anyway.
Thanks to Tom and everyone else who replied!
We have lift off! :)
Jerry
To: 6...
From: m...@almy.us
Date: Tue, 24 Nov 2009 06:11:29 -0800
Subject: Re: [68HC12] HCS12 to HCS12 Port interfacing
It doesn't matter which port(s) you use, you don't need a current
limiting resistor. All ports default to input (and almost all with pull-up
enabled) so they can be safely left unconnected or connected to either logic
voltage level.
Tom Almy
Tualatin, Oregon USA
Internet: t...@almy.us
Website: almy.us
On Nov 23, 2009, at 12:45 PM, Jerry Fields wrote:
>
> Hello,
>
> I have two MC9S12DP512 microcontrollers, and I need to interface them. The
first mc9s12, has port P set to output, the second mc9s12 has port T set to
input, and I want to connect one line on those ports. There are no large amounts
of data being sent, as I am just holding the line high or low. My question is,
do I need a current limiting resistor in between the two? I have not yet found
the documentation that shows me the internal circuitry configurations of the
ports. If somebody could point me to that, or give me a yea or nay on the
resistor, that would be great! Or you can just tell me not to be lazy, and
figure it out myself! lol It would be great to hear what a few of you have to
say though. Seems like I remember seen a open collector configuration on my old
HC11. I must have been looking at port B (that was output only)!
It doesn't matter which port(s) you use, you don't need a current
limiting resistor. All ports default to input (and almost all with pull-up
enabled) so they can be safely left unconnected or connected to either logic
voltage level.
Tom Almy
Tualatin, Oregon USA
Internet: t...@almy.us
Website: almy.us
On Nov 23, 2009, at 12:45 PM, Jerry Fields wrote:
>
> Hello,
>
> I have two MC9S12DP512 microcontrollers, and I need to interface them. The
first mc9s12, has port P set to output, the second mc9s12 has port T set to
input, and I want to connect one line on those ports. There are no large amounts
of data being sent, as I am just holding the line high or low. My question is,
do I need a current limiting resistor in between the two? I have not yet found
the documentation that shows me the internal circuitry configurations of the
ports. If somebody could point me to that, or give me a yea or nay on the
resistor, that would be great! Or you can just tell me not to be lazy, and
figure it out myself! lol It would be great to hear what a few of you have to
say though. Seems like I remember seen a open collector configuration on my old
HC11. I must have been looking at port B (that was output only)!
Reply by Michael●November 23, 20092009-11-23
I think the resistor is in case you accidentally configure both ports as output
and drive them at opposite levels, thereby creating a short circuit. A resistor
would limit the current and protect your port pin.
I have connected 2 MCUs many times but I always use the SPI bus. the HC12 has 3
so you have plenty to choose from. It's easy to use and allows you to
exchange 1 byte at a time bi-directionally. It's like a bi-directional
shift register.
Mike
--- In 6..., Jerry Fields wrote: > Hello,
>
> I have two MC9S12DP512 microcontrollers, and I need to interface them. The
first mc9s12, has port P set to output, the second mc9s12 has port T set to
input, and I want to connect one line on those ports. There are no large amounts
of data being sent, as I am just holding the line high or low. My question is,
do I need a current limiting resistor in between the two? I have not yet found
the documentation that shows me the internal circuitry configurations of the
ports. If somebody could point me to that, or give me a yea or nay on the
resistor, that would be great! Or you can just tell me not to be lazy, and
figure it out myself! lol It would be great to hear what a few of you have to
say though. Seems like I remember seen a open collector configuration on my old
HC11. I must have been looking at port B (that was output only)!
>
>
> Thanks in advance everyone! This is a GREAT yahoo group!
>
> Jerry Fields
> _________________________________________________________________
> Hotmail: Trusted email with powerful SPAM protection.
> http://clk.atdmt.com/GBL/go/177141665/direct/01/
>
>
>
Reply by justin lucas●November 23, 20092009-11-23
You could always configure the SCI ports. That would eliminate the concern to
use a resistor.
We did something like this during our lab using the HC12 board and a HC11.
The instructor said to use a resistor if you port to port, he didn't
explain the why or I didn't catch it.
We did the SCI ports which was more of a challenge, but freed up extra ports we
were using on the HC11.
Good luck
Justin
To: 6...
From: e...@hotmail.com
Date: Mon, 23 Nov 2009 15:45:05 -0500
Subject: [68HC12] HCS12 to HCS12 Port interfacing
Hello,
I have two MC9S12DP512 microcontrollers, and I need to interface them. The first
mc9s12, has port P set to output, the second mc9s12 has port T set to input, and
I want to connect one line on those ports. There are no large amounts of data
being sent, as I am just holding the line high or low. My question is, do I need
a current limiting resistor in between the two? I have not yet found the
documentation that shows me the internal circuitry configurations of the ports.
If somebody could point me to that, or give me a yea or nay on the resistor,
that would be great! Or you can just tell me not to be lazy, and figure it out
myself! lol It would be great to hear what a few of you have to say though.
Seems like I remember seen a open collector configuration on my old HC11. I must
have been looking at port B (that was output only)!
Thanks in advance everyone! This is a GREAT yahoo group!
I have two MC9S12DP512 microcontrollers, and I need to interface them. The
first mc9s12, has port P set to output, the second mc9s12 has port T set to
input, and I want to connect one line on those ports. There are no large amounts
of data being sent, as I am just holding the line high or low. My question is,
do I need a current limiting resistor in between the two? I have not yet found
the documentation that shows me the internal circuitry configurations of the
ports. If somebody could point me to that, or give me a yea or nay on the
resistor, that would be great! Or you can just tell me not to be lazy, and
figure it out myself! lol It would be great to hear what a few of you have to
say though. Seems like I remember seen a open collector configuration on my old
HC11. I must have been looking at port B (that was output only)!
Thanks in advance everyone! This is a GREAT yahoo group!