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"pull up" of PIC

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kerden

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pic is different of 8051 . such as output, 8051 si TTL,but PIC is ST/TTL. usually,we add a pull up resistance. but who could tell me if i also add a "pull up resistance"with the PIC,what will happen?

thank you!
 

Nothing will happend.

PIC has his internal pull up resistors (they can be switched ON/OFF by software), so in case when pull ups are switched off everything is same as in 8051 case. If they are switched on and you connect additional extern pull you will just boost pull up effect (so everithing is same as before external pull ups).
Internal pull ups are about 30kOhm as I remember well.

Mr.Cube
 

think you !
in my opinion:

NO.1 JPG is the 8051 P1
NO.2 JPG is different ,it is PIC PORTC output .it has tow CMOST Q1 and Q2.because Q1 and Q2 is CMOST, so we could equate it is two "switch" like as NO.3 JPG . when PORTC output 1, Q1(S1) close ,power(vdd) will direct add to LED.
so LED will be very brightness. but LED's "P-N" will fixup the voltage at 0.7V, and the residual voltage(5V-0.7V=4.3V) will be expend by PIC ,so PIC will soon be very hot .

so i suggest that we could add a resistance seriated as NO.4 JPG

are you agree?
 

To be honest I never used connection like on pic1. In this case throught mcu will always be current. in ON state throught CMOS transistor and in OFF state small current via internal pull up (8051).
I always use last schematic for one case or connect resistor to +5v, second pin of resistor I connect to LED's anode, cathode I connect to I/O pin of mcu so on that way you can get inverted logic acording to picture 4.

Mr.Cube
 

you are riht .

but i don't know how i could add a pull up resistance on I2C's SCL and SDA .:

could you help me ?
 

Its very easy. Connect directly SDA and SCL from i2c device to two pins of mcu (I always use pins witout internal pull ups). Simply connect resistor of 4k7 between each of those two pins of mcu to +5V (Vcc).

If you have osciloscope then you can see how are quality of signal on i2c bus. if you want nice egdes you have to connect smaller resistance (e.g. 2k2 or even 1k5).

If you have any problem just ask.


Mr.Cube
 

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