nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

MCP23017 Pins @ boot (Initial State)

Tue Dec 24, 2013 2:22 am

I have connected a MCP23017 @ 0x20 ... I usually i2cset 0x20 as outputs first (0x00 & 0x01 set to 0x00 respectively ) then I am able to either turn on and off either each bank or single GPIOs (Say LEDs for discussion sake)

Say I reboot, dog pulls out the cord, son trips over it whatever... Once I reboot I have to reset the banks as outputs again etc etc

From what I have read, people say that you can't set the Pi's GPIO pins initial boot state bc the GPU and some other things like the SD need certain things in a certain way( that I won't bore you with) But these pins are running off the i2c which is now always set at boot and are extra pins, not pins the pi needs.

My question is, every time I reboot I have to use terminal and set the pins up again etc. Can I set all of this up at boot?

Thoughts?

ShiftPlusOne
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 6234
Joined: Fri Jul 29, 2011 5:36 pm
Location: The unfashionable end of the western spiral arm of the Galaxy

Re: MCP23017 Pins @ boot (Initial State)

Tue Dec 24, 2013 3:16 am

You can add the line to set the pins as you want them to /etc/rc.local

However, that's not the "right" way to do things. Usually you'd have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time it runs.

Edit: typo

User avatar
Richard-TX
Posts: 1549
Joined: Tue May 28, 2013 3:24 pm
Location: North Texas

Re: MCP23017 Pins @ boot (Initial State)

Tue Dec 24, 2013 6:12 am

ShiftPlusOne wrote: have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time in runs.
That is exactly how I do it. Initializing a MCP 23017 takes just a few lines of code.

Having the raspberry run your program automatically at boot can take a little finesse.

The command nohup is your friend.
Richard
Doing Unix since 1985.
The 9-25-2013 image of Wheezy can be found at:
http://downloads.raspberrypi.org/raspbian/images/raspbian-2013-09-27/2013-09-25-wheezy-raspbian.zip

User avatar
DougieLawson
Posts: 39576
Joined: Sun Jun 16, 2013 11:19 pm
Location: A small cave in deepest darkest Basingstoke, UK
Contact: Website Twitter

Re: MCP23017 Pins @ boot (Initial State)

Tue Dec 24, 2013 8:44 am

Richard-TX wrote:
ShiftPlusOne wrote: have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time in runs.
That is exactly how I do it. Initializing a MCP 23017 takes just a few lines of code.

Having the raspberry run your program automatically at boot can take a little finesse.

The command nohup is your friend.
If you need to start a background process that won't close when you log off then nohup is your tool of choice.

If you want a quick & dirty test of getting something started at boot use /etc/rc.local

If you want a script to start at boot and have an orderly termination at shutdown use an init script.

If you want to run something at boot without giving it root privileges add an @reboot to the users crontab.
Note: Any requirement to use a crystal ball or mind reading will result in me ignoring your question.

Criticising any questions is banned on this forum.

Any DMs sent on Twitter will be answered next month.
All fake doctors are on my foes list.

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Tue Dec 24, 2013 1:37 pm

Hi,

Be aware that at power-on all MCP23O17 I/O pins are set as inputs (what is default also for any bidirectional pin in various other chips). In the circuit where this chip's pin is output and connected to another input, you actually get two inputs wired together after the power on and before your software initialization procedure takes place.. Since you should not leave any input floating, a "pull" resistor is needed to prevent any surprise.

You could even destroy older chips by leaving their inputs floating, but nowadays this remains at least a good design practice (also important noise prevention technique) to add/enable pull-up or pull-down resistor for "hardware-defined" idle state (eg. motor off).


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 2:51 pm

ShiftPlusOne wrote:You can add the line to set the pins as you want them to /etc/rc.local

However, that's not the "right" way to do things. Usually you'd have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time it runs.

Edit: typo
Normally this would be an option, problem is that I have the MCP23017 connected to something that I don't want on unless I say so -post authentication etc... So if all the person would need to do is power cycle the pi... Defeats the purpose.
Rc.local might do the trick.. Wish it was as simple as running a script/prog at boot rather then once my program is initialized. Afteri bot into Raspian to add insult to injury... Leaves my gpio's in the wrong initial state until it's fully booted and I run some commands in terminal
I need something that will set the initial state. - sorry for the delayed response was traveling and not in the office... Thanks all who responded

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 2:53 pm

Richard-TX wrote:
ShiftPlusOne wrote: have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time in runs.
That is exactly how I do it. Initializing a MCP 23017 takes just a few lines of code.

Having the raspberry run your program automatically at boot can take a little finesse.

The command nohup is your friend.
How do you do it? In the program you wrote that needs the MCP23017? Or before?

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 2:58 pm

DougieLawson wrote:
Richard-TX wrote:
ShiftPlusOne wrote: have a script or program that does whatever it is you need to do and the pins would be initialized by that program every time in runs.
That is exactly how I do it. Initializing a MCP 23017 takes just a few lines of code.

Having the raspberry run your program automatically at boot can take a little finesse.

The command nohup is your friend.

If you need to start a background process that won't close when you log off then nohup is your tool of choice.

If you want a quick & dirty test of getting something started at boot use /etc/rc.local

If you want a script to start at boot and have an orderly termination at shutdown use an init script.

If you want to run something at boot without giving it root privileges add an @reboot to the users crontab.
Thanks so much.. I need it start it in the bg, hopefully on boot -- I'm not sure what you mean by log off... I don't usually log on and off the pi, just power cycle it from time to time if someone kicks out the power cord etc haha

Nohup vs init script... Vs quick and dirty hmm... I'm gonna test it quick n dirty using rc.local but I'd love to do it a less quick/dirty way and the right way so That is something I need to figure out if this command nohup vs init debate

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:01 pm

FLYFISH TECHNOLOGIES wrote:Hi,

Be aware that at power-on all MCP23O17 I/O pins are set as inputs (what is default also for any bidirectional pin in various other chips). In the circuit where this chip's pin is output and connected to another input, you actually get two inputs wired together after the power on and before your software initialization procedure takes place.. Since you should not leave any input floating, a "pull" resistor is needed to prevent any surprise.

You could even destroy older chips by leaving their inputs floating, but nowadays this remains at least a good design practice (also important noise prevention technique) to add/enable pull-up or pull-down resistor for "hardware-defined" idle state (eg. motor off).


Best wishes, Ivan Zilic.
I'm slightly confused by what you are talking about... Doesn't the pi have some pull-up resistors on the board? Either way I'm trying to initialize them ALL as outputs... I need no inputs - can you extrapolate and elaborate on the message you wrote, I would like to understand more of the point you are trying to convey here... Seems important yet a bit unclear.. Do you mind clarifying ?

User avatar
Richard-TX
Posts: 1549
Joined: Tue May 28, 2013 3:24 pm
Location: North Texas

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:18 pm

He was referring to the MCP23017, not the RPI.
Since you are using the 23017 ports as outputs, the caution does not apply. Furthermore Microchip says nothing about applying external pullups so I believe it is immune from floating inputs. If you want to play it safe, enable the pullups on-board when using the ports as inputs. See section 1.6.7 of the datasheet at http://ww1.microchip.com/downloads/en/D ... 21952b.pdf
Richard
Doing Unix since 1985.
The 9-25-2013 image of Wheezy can be found at:
http://downloads.raspberrypi.org/raspbian/images/raspbian-2013-09-27/2013-09-25-wheezy-raspbian.zip

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:27 pm

Hi,

Are we discussing about RasPi GPIO or MCP23017 ? ;-)
nixem wrote:Doesn't the pi have some pull-up resistors on the board?
Yes, it has... and it has also pull-down resistors.
It is up to you to select (in software) which one is enabled.
nixem wrote:Either way I'm trying to initialize them ALL as outputs...
Before you initialize them as outputs, they are inputs. After the power is applied and before your software is executed, outputs of your system are in unpredictable state.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:30 pm

Hi,
Richard-TX wrote:Since you are using the 23017 ports as outputs, the caution does not apply.
Yes, it does apply...


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:37 pm

FLYFISH TECHNOLOGIES wrote:Hi,
Richard-TX wrote:Since you are using the 23017 ports as outputs, the caution does not apply.
Yes, it does apply...


Best wishes, Ivan Zilic.

Ivan you keep leaving me with MORE questions each time you respond...

Rather then telling me what not to do or that the others are wrong.. It's easier to state what I should do right ...

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:39 pm

Richard-TX wrote:He was referring to the MCP23017, not the RPI.
Since you are using the 23017 ports as outputs, the caution does not apply. Furthermore Microchip says nothing about applying external pullups so I believe it is immune from floating inputs. If you want to play it safe, enable the pullups on-board when using the ports as inputs. See section 1.6.7 of the datasheet at http://ww1.microchip.com/downloads/en/D ... 21952b.pdf

Thanks Richard! So what would you recommend I do to make the input state an output state at boot..

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 3:54 pm

Hi,
nixem wrote:Rather then telling me what not to do or that the others are wrong.. It's easier to state what I should do right...
You have to add "physical" pull resistor to each and every MCP23017 pin which you use. (This is the statement which I already wrote above, on Tue Dec 24, 2013 2:37 pm).

If you write more about your circuit (or post the schematics), I can tell you more. As you can hear the tone from the mentioned message, there are also cases where these resisotrs could be omitted... but you can hardly do a mistake by adding them.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 4:08 pm

FLYFISH TECHNOLOGIES wrote:Hi,
nixem wrote:Rather then telling me what not to do or that the others are wrong.. It's easier to state what I should do right...
You have to add "physical" pull resistor to each and every MCP23017 pin which you use. (This is the statement which I already wrote above, on Tue Dec 24, 2013 2:37 pm).

If you write more about your circuit (or post the schematics), I can tell you more. As you can hear the tone from the mentioned message, there are also cases where these resisotrs could be omitted... but you can hardly do a mistake by adding them.


Best wishes, Ivan Zilic.

Excellent- so better safe then sorry in this case as resistors are cheap?


Currently I have an MCP23017 connected to the Pi - the pins on the MCP go to 8ch sainsmart relays and those to LEDs... Then I boot the issue is they are all initial state wrong so I have to pass 2cset commands to make them inputs..

I'd love some help on making them outputs as soon after boot as possible and as reliably as possible...


Also what resistors do you recommend.. And where along each pin? Directly after the chip?

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 4:10 pm

nixem wrote:
FLYFISH TECHNOLOGIES wrote:Hi,
nixem wrote:Rather then telling me what not to do or that the others are wrong.. It's easier to state what I should do right...
You have to add "physical" pull resistor to each and every MCP23017 pin which you use. (This is the statement which I already wrote above, on Tue Dec 24, 2013 2:37 pm).

If you write more about your circuit (or post the schematics), I can tell you more. As you can hear the tone from the mentioned message, there are also cases where these resisotrs could be omitted... but you can hardly do a mistake by adding them.


Best wishes, Ivan Zilic.

Excellent- so better safe then sorry in this case as resistors are cheap?


Currently I have an MCP23017 connected to the Pi - the pins on the MCP go to 8ch sainsmart relays and those to LEDs... Then I boot the issue is they are all initial state wrong so I have to pass 2cset commands to make them inputs..

I'd love some help on making them outputs as soon after boot as possible and as reliably as possible...


Also what resistors do you recommend.. And where along each pin? Directly after the chip?

Oh just an FYI... I know this is overkill to run some LEDs... Lol... The LEDs are just a placeholder so I know it's on until I pan all this out..

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 4:53 pm

Hi,
nixem wrote:Excellent- so better safe then sorry in this case as resistors are cheap?
Exactly.
And also, let me underline this again: these resistors define your state for the first 5 seconds or so (after you apply power and before RasPi boots & executes your init script).
nixem wrote:Currently I have an MCP23017 connected to the Pi - the pins on the MCP go to 8ch sainsmart relays and those to LEDs... Then I boot the issue is they are all initial state wrong so I have to pass 2cset commands to make them inputs..
Without resistors your wrong state will apply for mentioned 5 seconds (or so) each time after power on... with mentioned resistors this time is decreased to zero.
nixem wrote:I'd love some help on making them outputs as soon after boot as possible and as reliably as possible...
With mentioned pull resistors you're not in a hurry that much anymore... ;)
nixem wrote:Also what resistors do you recommend.. And where along each pin? Directly after the chip?
My default value is 10k. It is ok for digital logic chips, where for some discrete elements this required value could be different (mostly lower).
Location itself is not important... anywhere.

Do you have input schematics of this sainsmart relays board ? According to my (untrusted) info, it contains 10k pull-down resistor (and additional 10k resistor in series with input; junction of these two resistor is connected to the base of transistor driving the SSR and LED).
If this is true, then I'd rather modify the board by making its pull-down resistors into pull-ups.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 5:15 pm

FLYFISH TECHNOLOGIES wrote:Hi,
nixem wrote:Excellent- so better safe then sorry in this case as resistors are cheap?
Exactly.
And also, let me underline this again: these resistors define your state for the first 5 seconds or so (after you apply power and before RasPi boots & executes your init script).
nixem wrote:Currently I have an MCP23017 connected to the Pi - the pins on the MCP go to 8ch sainsmart relays and those to LEDs... Then I boot the issue is they are all initial state wrong so I have to pass 2cset commands to make them inputs..
Without resistors your wrong state will apply for mentioned 5 seconds (or so) each time after power on... with mentioned resistors this time is decreased to zero.
nixem wrote:I'd love some help on making them outputs as soon after boot as possible and as reliably as possible...
With mentioned pull resistors you're not in a hurry that much anymore... ;)
nixem wrote:Also what resistors do you recommend.. And where along each pin? Directly after the chip?
My default value is 10k. It is ok for digital logic chips, where for some discrete elements this required value could be different (mostly lower).
Location itself is not important... anywhere.

Do you have input schematics of this sainsmart relays board ? According to my (untrusted) info, it contains 10k pull-down resistor (and additional 10k resistor in series with input; junction of these two resistor is connected to the base of transistor driving the SSR and LED).
If this is true, then I'd rather modify the board by making its pull-down resistors into pull-ups.


Best wishes, Ivan Zilic.
Love it!! So I also noticed that the relay board WAS acting in the opposite of my intended selection.. Meaning it seemed to turn off when the GPIO is set to output and turned on when I was using Webiopi but now since I'm just using terminal and inputting the command directly this removes the opposites problem I was having but this new problem arises with the initial state..

From what I am gathering from what you say.. Can I change then resistors on the sainsmart relay board and then be all set?


Also you states above that the relays with then PROPER resistor will solve my problem for the first few seconds during boot... Will that state change back at some point once a script is run at boot back to input? Or once I change the resistor will my problem be solved?


Thanks so much, everyone's discussions and your help is really enlightening me as well as binging me closer to my desired solution :)

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 5:43 pm

FLYFISH TECHNOLOGIES wrote:Hi,
nixem wrote:Excellent- so better safe then sorry in this case as resistors are cheap?
Exactly.
And also, let me underline this again: these resistors define your state for the first 5 seconds or so (after you apply power and before RasPi boots & executes your init script).
nixem wrote:Currently I have an MCP23017 connected to the Pi - the pins on the MCP go to 8ch sainsmart relays and those to LEDs... Then I boot the issue is they are all initial state wrong so I have to pass 2cset commands to make them inputs..
Without resistors your wrong state will apply for mentioned 5 seconds (or so) each time after power on... with mentioned resistors this time is decreased to zero.
nixem wrote:I'd love some help on making them outputs as soon after boot as possible and as reliably as possible...
With mentioned pull resistors you're not in a hurry that much anymore... ;)
nixem wrote:Also what resistors do you recommend.. And where along each pin? Directly after the chip?
My default value is 10k. It is ok for digital logic chips, where for some discrete elements this required value could be different (mostly lower).
Location itself is not important... anywhere.

Do you have input schematics of this sainsmart relays board ? According to my (untrusted) info, it contains 10k pull-down resistor (and additional 10k resistor in series with input; junction of these two resistor is connected to the base of transistor driving the SSR and LED).
If this is true, then I'd rather modify the board by making its pull-down resistors into pull-ups.


Best wishes, Ivan Zilic.

Attempting to source schematics/data sheet/ eagle file.. Something for the 5v DC relay board... So you can have reference... Can I post files here or should I host them elsewhere and Link?

User avatar
FLYFISH TECHNOLOGIES
Posts: 1750
Joined: Thu Oct 03, 2013 7:48 am
Location: Ljubljana, Slovenia
Contact: Website

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 6:16 pm

Hi,
nixem wrote:Attempting to source schematics/data sheet/ eagle file.. Something for the 5v DC relay board... So you can have reference... Can I post files here or should I host them elsewhere and Link?
You can post files here or post a link.. or even send me an email with attachment (to avoid spiders hits, you need to combine strings "i4rp" and domain "@flyfish-tech.com" to get my email address ;-) ).

I'd prefer Eagle file, which would allow quick modification, then I can post it here.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

User avatar
Richard-TX
Posts: 1549
Joined: Tue May 28, 2013 3:24 pm
Location: North Texas

Re: MCP23017 Pins @ boot (Initial State)

Fri Dec 27, 2013 8:51 pm

nixem wrote:
Richard-TX wrote:He was referring to the MCP23017, not the RPI.
Since you are using the 23017 ports as outputs, the caution does not apply. Furthermore Microchip says nothing about applying external pullups so I believe it is immune from floating inputs. If you want to play it safe, enable the pullups on-board when using the ports as inputs. See section 1.6.7 of the datasheet at http://ww1.microchip.com/downloads/en/D ... 21952b.pdf

Thanks Richard! So what would you recommend I do to make the input state an output state at boot..
It doesn't matter. I have a few dozen of these chips in operation today and regardless of input or output, I don't use any pullups. The onboard pullups work just fine.
Richard
Doing Unix since 1985.
The 9-25-2013 image of Wheezy can be found at:
http://downloads.raspberrypi.org/raspbian/images/raspbian-2013-09-27/2013-09-25-wheezy-raspbian.zip

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Sat Dec 28, 2013 1:45 am

FLYFISH TECHNOLOGIES wrote:Hi,
nixem wrote:Attempting to source schematics/data sheet/ eagle file.. Something for the 5v DC relay board... So you can have reference... Can I post files here or should I host them elsewhere and Link?
You can post files here or post a link.. or even send me an email with attachment (to avoid spiders hits, you need to combine strings "i4rp" and domain "@flyfish-tech.com" to get my email address ;-) ).

I'd prefer Eagle file, which would allow quick modification, then I can post it here.


Best wishes, Ivan Zilic.
These are the relays im using... Although when I download the Rar file at the bottom of the product description (right below the videos) It shows a different relay in the eagle files... weird... skinny relay.

nixem
Posts: 16
Joined: Mon Sep 16, 2013 5:10 am

Re: MCP23017 Pins @ boot (Initial State)

Sat Dec 28, 2013 1:46 am

Richard-TX wrote:
nixem wrote:
Richard-TX wrote:He was referring to the MCP23017, not the RPI.
Since you are using the 23017 ports as outputs, the caution does not apply. Furthermore Microchip says nothing about applying external pullups so I believe it is immune from floating inputs. If you want to play it safe, enable the pullups on-board when using the ports as inputs. See section 1.6.7 of the datasheet at http://ww1.microchip.com/downloads/en/D ... 21952b.pdf

Thanks Richard! So what would you recommend I do to make the input state an output state at boot..
It doesn't matter. I have a few dozen of these chips in operation today and regardless of input or output, I don't use any pullups. The onboard pullups work just fine.
I use them without pullups too. they switch on and off fine. What I think he is saying is not that they won't work. But if I understand correctly I will be able to choose the initial state with the proper configuration of resistors. I was hoping to just take the hardware route but if a few in line resistors can help my cause rather then coding headaches then that is an option too.

User avatar
Richard-TX
Posts: 1549
Joined: Tue May 28, 2013 3:24 pm
Location: North Texas

Re: MCP23017 Pins @ boot (Initial State)

Sat Dec 28, 2013 11:33 am

There are no coding headaches one way or the other. There are no special considerations either. In my experience the chip isn't going to self destruct because of the lack of pull-ups or pull downs. In olden days, it was a consideration with 5 volt TTL logic chips but in my experience not with this chip. If you want to really play it safe, configure the unconnected ports as outputs. That is all. The I2C chips from Microchip are so resilient that bypass caps are rarely needed when experimenting. With discrete 5 volt TTL chips that is not the case.

Go play and don't worry about it.
Richard
Doing Unix since 1985.
The 9-25-2013 image of Wheezy can be found at:
http://downloads.raspberrypi.org/raspbian/images/raspbian-2013-09-27/2013-09-25-wheezy-raspbian.zip

Return to “Interfacing (DSI, CSI, I2C, etc.)”