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Guest
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| Pull up resistor |
 Posted: Thu Nov 26, 2009 11:12 am |
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I want to know the pins of pic18f250 and 18f2550 requires external pull up resistors
I know RA4 need one, but what about other pins.
Thanks |
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John P
Joined: 17 Sep 2003
Posts: 331
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| Posted: Thu Nov 26, 2009 11:52 am |
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| Huh. Difficult problem. What did you find when you read the manual? |
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Guest
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| Posted: Thu Nov 26, 2009 12:23 pm |
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| r u jocking... |
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bkamen
Joined: 07 Jan 2004
Posts: 1611
Location: Central Illinois, USA
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| Posted: Thu Nov 26, 2009 12:26 pm |
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Probably not.
The Datasheet for any PIC processor (barring doc errors) will tell you everything you need to know.
You want to look in the section for I/O Ports.
Cheers,
-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Nov 26, 2009 12:39 pm |
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To find this information, look in the PIC's data sheet, in the
Device Overview section. Look at the pinout tables in that section:
| Quote: | | TABLE 1-2: PIC18F2455/2550 PINOUT I/O DESCRIPTIONS |
If an i/o pin says "Open Drain" in the description, it means that the pin
can only be set to a low level. The PIC can't drive it high.
If you want the pin to have the ability to go to a high level, then you
must add an external pull-up resistor to it. Then call the output_float()
function and the pull-up resistor will take it to a high level.
A typical description for an Open Drain pin in the Overview section will
look like this:
| Quote: | | Output is open drain type. |
or this
| Quote: | | Open drain when configured as output. |
Notice that the 18F2550 does not have that description for pin RA4. |
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Guest
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| Posted: Thu Nov 26, 2009 3:08 pm |
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| PCM programmer wrote: | To find this information, look in the PIC's data sheet, in the
Device Overview section. Look at the pinout tables in that section:
| Quote: | | TABLE 1-2: PIC18F2455/2550 PINOUT I/O DESCRIPTIONS |
If an i/o pin says "Open Drain" in the description, it means that the pin
can only be set to a low level. The PIC can't drive it high.
If you want the pin to have the ability to go to a high level, then you
must add an external pull-up resistor to it. Then call the output_float()
function and the pull-up resistor will take it to a high level.
A typical description for an Open Drain pin in the Overview section will
look like this:
| Quote: | | Output is open drain type. |
or this
| Quote: | | Open drain when configured as output. |
Notice that the 18F2550 does not have that description for pin RA4. |
Thanks u for your reply |
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Guest
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| Posted: Thu Nov 26, 2009 3:12 pm |
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one more doubt
| Quote: | Selection of any I2C mode with the SSPEN bit set
forces the SCL and SDA pins to be open-drain, |
from data sheet
what it means...
I am not using I2C mode...Is there any need of pull-up resistors .. |
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Ttelmah
Guest
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| Posted: Thu Nov 26, 2009 3:58 pm |
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Basically no, but it depends what you are actually doing with the pin.
So (for instance), if you are using a pin as an input, and want it to float to a 'known' level, then pull-ups are needed.
Similarly, if using I2C, then they are needed.
Similarly, if you want to implement your own 'wire-or' strategy, then pull-ups become needed.
If you need to ensure that a pin when 'high', goes to a higher level than the data sheet warrants a output to drive 'up' to, then pull-ups are needed (common when driving some types of 5v logic from 3.3v chips).
A few pins (A4, on some PICs), do not have an internal high drive ability.
In each case, the data sheet shows the drive capabilities and levels involved.
If you are using a normal pin as a conventional output, then, 'no'.
Best Wishes |
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