I2c Bus / Resistors

Started by Stephen Whittle March 10, 2004
Hi All,

I was hoping that someone can help me with what is probably a very simple (and very stupid) question. I'm constructing a circuit to connect the BX-24 to a SRF08 Ultrasonic Ranger and a CMPS01 Compass Via an I2c Bus.
Now I know next to nothing about electronic (being more of a programmer, that is why I have gone for self contained circuit boards), but in the circuit diagram that I have it uses two 1k8 resistors to pull-up?? the I2c Clock and Data lines and the +5V pin on the BX-24. I've looked through my resistor colour code charts and cannot find what the 4-band colour code sequence is for a 1k8 resistor. Am I right in thinking that a 1k8 resistor is in fact the same as a 1800Kohm Resistor (Colour Sequence Brown/Grey/Green/Gold). In addition to this the I2c Bus cable will be no more that 30cm in length, is this the correct resistance to use for this length of cable.
I'm sorry if this seems like a really dumb question, but this is my first true project.

Thanks In Advance....

Steve ;-)

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You were close, the 1K8 would be a 1.8K resistor. brown-gray-red.
>
> From: "Stephen Whittle" <>
> Date: 2004/03/10 Wed AM 05:56:59 EST
> To: "BasicX Yahoo Group \(E-mail\)" <>
> Subject: [BasicX] I2c Bus / Resistors




Thanks very much, will try this tonight....

-----Original Message-----
From: [mailto:]
Sent: 10 March 2004 15:37
To:
Subject: Re: [BasicX] I2c Bus / Resistors You were close, the 1K8 would be a 1.8K resistor. brown-gray-red.
>
> From: "Stephen Whittle" <>
> Date: 2004/03/10 Wed AM 05:56:59 EST
> To: "BasicX Yahoo Group \(E-mail\)" <>
> Subject: [BasicX] I2c Bus / Resistors
_____

> .
This email is intended only for the above named addressee(s). The information contained in this email may contain information which is confidential. The views expressed in this email are personal to the sender and do not in any way reflect the views of the company. If you have received this email and you are not a named addressee, please contact the sender and then delete it from your system.


Stephan

1K8 is equivalent to 1800 ohms. Another way to write it is 1.8 K ohms.
1K8 notation is often used in schematics to avoid problems when
commas or decimal points are missed and is shorter.

The colour code for a standard 1.8K ohm 5 % tolerance resistor is -
Brown/Grey/Red/Gold. Gold is 5% tolerance.

This link might help:
http://members.tripod.com/~schematics/resistor.htm

When not sure of resistors values use a ohmmeter.

Rob

--- In , "Stephen Whittle" <stephenw@t...>
wrote:
> Hi All,
>
> I was hoping that someone can help me with what is probably a very
simple (and very stupid) question. I'm constructing a circuit to
connect the BX-24 to a SRF08 Ultrasonic Ranger and a CMPS01 Compass
Via an I2c Bus.
> Now I know next to nothing about electronic (being more of a
programmer, that is why I have gone for self contained circuit
boards), but in the circuit diagram that I have it uses two 1k8
resistors to pull-up?? the I2c Clock and Data lines and the +5V pin
on the BX-24. I've looked through my resistor colour code charts and
cannot find what the 4-band colour code sequence is for a 1k8
resistor. Am I right in thinking that a 1k8 resistor is in fact the
same as a 1800Kohm Resistor (Colour Sequence Brown/Grey/Green/Gold).
In addition to this the I2c Bus cable will be no more that 30cm in
length, is this the correct resistance to use for this length of
cable.
> I'm sorry if this seems like a really dumb question, but this is my
first true project.
>
> Thanks In Advance....
>
> Steve ;-)
>
> This email is intended only for the above named addressee(s). The
information contained in this email may contain information which is
confidential. The views expressed in this email are personal to the
sender and do not in any way reflect the views of the company. If
you have received this email and you are not a named addressee,
please contact the sender and then delete it from your system.



No; 1k8 is not 1800k it is 1800 ohm.

-----Original Message-----
From:
[mailto:]
Sent: Wednesday, March 10, 2004 5:57 AM
To:
Subject: [BasicX] I2c Bus / Resistors Hi All,

I was hoping that someone can help me with what is probably a very simple
(and very stupid) question. I'm constructing a circuit to connect the BX-24
to a SRF08 Ultrasonic Ranger and a CMPS01 Compass Via an I2c Bus.
Now I know next to nothing about electronic (being more of a programmer,
that is why I have gone for self contained circuit boards), but in the
circuit diagram that I have it uses two 1k8 resistors to pull-up?? the I2c
Clock and Data lines and the +5V pin on the BX-24. I've looked through my
resistor colour code charts and cannot find what the 4-band colour code
sequence is for a 1k8 resistor. Am I right in thinking that a 1k8 resistor
is in fact the same as a 1800Kohm Resistor (Colour Sequence
Brown/Grey/Green/Gold). In addition to this the I2c Bus cable will be no
more that 30cm in length, is this the correct resistance to use for this
length of cable.
I'm sorry if this seems like a really dumb question, but this is my first
true project.

Thanks In Advance....

Steve ;-)

This email is intended only for the above named addressee(s). The
information contained in this email may contain information which is
confidential. The views expressed in this email are personal to the sender
and do not in any way reflect the views of the company. If you have
received this email and you are not a named addressee, please contact the
sender and then delete it from your system.

Yahoo! Groups Links


I too am using the same 2 i2c devices on a project. However, I am
using 74k resistors for my pullups. On my project, the total I2C
bus length between the pull-ups and the compass is about 45cm. This
setup is working great for me. I'm kinda surprised that your diagram
only calls for 1.8k pull-ups.

Ryan.
--- In , "raunig2003" <raunig2003@y...> wrote:
> Stephan
>
> 1K8 is equivalent to 1800 ohms. Another way to write it is 1.8 K
ohms.
> 1K8 notation is often used in schematics to avoid problems when
> commas or decimal points are missed and is shorter.
>
> The colour code for a standard 1.8K ohm 5 % tolerance resistor is -
> Brown/Grey/Red/Gold. Gold is 5% tolerance.
>
> This link might help:
> http://members.tripod.com/~schematics/resistor.htm
>
> When not sure of resistors values use a ohmmeter.
>
> Rob
>
> --- In , "Stephen Whittle" <stephenw@t...>
> wrote:
> > Hi All,
> >
> > I was hoping that someone can help me with what is probably a
very
> simple (and very stupid) question. I'm constructing a circuit to
> connect the BX-24 to a SRF08 Ultrasonic Ranger and a CMPS01 Compass
> Via an I2c Bus.
> > Now I know next to nothing about electronic (being more of a
> programmer, that is why I have gone for self contained circuit
> boards), but in the circuit diagram that I have it uses two 1k8
> resistors to pull-up?? the I2c Clock and Data lines and the +5V pin
> on the BX-24. I've looked through my resistor colour code charts
and
> cannot find what the 4-band colour code sequence is for a 1k8
> resistor. Am I right in thinking that a 1k8 resistor is in fact the
> same as a 1800Kohm Resistor (Colour Sequence
Brown/Grey/Green/Gold).
> In addition to this the I2c Bus cable will be no more that 30cm in
> length, is this the correct resistance to use for this length of
> cable.
> > I'm sorry if this seems like a really dumb question, but this is
my
> first true project.
> >
> > Thanks In Advance....
> >
> > Steve ;-)
> >
> > This email is intended only for the above named addressee(s).
The
> information contained in this email may contain information which
is
> confidential. The views expressed in this email are personal to
the
> sender and do not in any way reflect the views of the company. If
> you have received this email and you are not a named addressee,
> please contact the sender and then delete it from your system.
> >
> >
> >



Ryan,

A pullup resistor has a number of functions, one of which is to set
the sink current of the device connected to the bus. This current
ensures proper operation, makes the system less prone to noise and
interference. A 74k resistor on the I2c bus is considered to be
a "weak" pullup and you may experience problems. The recommended
sink current is around 3mA so the 1.8k pullup at 5V comes very close
to this and is regarded as a "strong" pullup.

To say that your are surprised that Stephan's circuit "only" calls
for 1.8k is to get this the wrong way round. I am surprised that
your circuit "only" calls for 74k !.

Naturally there is a tradeoff with overall power consumption.
Stephan's pullup will only draw 60uA at 5V......

The bottom line is use any value above 1.8k but be aware of
potential problems.

Cheers

Pat --- In , "the_enigma_lives"
<ryan.m.clarke@p...> wrote:
> I too am using the same 2 i2c devices on a project. However, I am
> using 74k resistors for my pullups. On my project, the total I2C
> bus length between the pull-ups and the compass is about 45cm.
This
> setup is working great for me. I'm kinda surprised that your
diagram
> only calls for 1.8k pull-ups.
>
> Ryan. >
> --- In , "raunig2003" <raunig2003@y...>
wrote:
> > Stephan
> >
> > 1K8 is equivalent to 1800 ohms. Another way to write it is 1.8 K
> ohms.
> > 1K8 notation is often used in schematics to avoid problems when
> > commas or decimal points are missed and is shorter.
> >
> > The colour code for a standard 1.8K ohm 5 % tolerance resistor
is -
> > Brown/Grey/Red/Gold. Gold is 5% tolerance.
> >
> > This link might help:
> > http://members.tripod.com/~schematics/resistor.htm
> >
> > When not sure of resistors values use a ohmmeter.
> >
> > Rob
> >
> > --- In , "Stephen Whittle" <stephenw@t...>
> > wrote:
> > > Hi All,
> > >
> > > I was hoping that someone can help me with what is probably a
> very
> > simple (and very stupid) question. I'm constructing a circuit to
> > connect the BX-24 to a SRF08 Ultrasonic Ranger and a CMPS01
Compass
> > Via an I2c Bus.
> > > Now I know next to nothing about electronic (being more of a
> > programmer, that is why I have gone for self contained circuit
> > boards), but in the circuit diagram that I have it uses two 1k8
> > resistors to pull-up?? the I2c Clock and Data lines and the +5V
pin
> > on the BX-24. I've looked through my resistor colour code charts
> and
> > cannot find what the 4-band colour code sequence is for a 1k8
> > resistor. Am I right in thinking that a 1k8 resistor is in fact
the
> > same as a 1800Kohm Resistor (Colour Sequence
> Brown/Grey/Green/Gold).
> > In addition to this the I2c Bus cable will be no more that 30cm
in
> > length, is this the correct resistance to use for this length of
> > cable.
> > > I'm sorry if this seems like a really dumb question, but this
is
> my
> > first true project.
> > >
> > > Thanks In Advance....
> > >
> > > Steve ;-)
> > >
> > > This email is intended only for the above named addressee(s).
> The
> > information contained in this email may contain information
which
> is
> > confidential. The views expressed in this email are personal to
> the
> > sender and do not in any way reflect the views of the company.
If
> > you have received this email and you are not a named addressee,
> > please contact the sender and then delete it from your system.
> > >
> > >
> > >