PIC (16F627A) inverted inputs

inputpic

I'm having problem with inputs of my PIC.

There is somethink like "default value" on all inputs set to logical 1 so if I want to change the state I need to pass GND.

How can I turn it "otherwise"? This is begining of my code:

#include <htc.h>
#include <pic.h>
#include <pic16f627a.h>

__CONFIG(WDTE_OFF & PWRTE_OFF & FOSC_INTOSCCLK & MCLRE_OFF & BOREN_OFF & LVP_OFF & CPD_OFF & CP_OFF);

int main (void)
{
    unsigned int i, j;

    unsigned short count = 0;

    TRISA = 0;
    TRISB = 0b11110000;
    PORTA = 0;
    PORTB = 0;
    nRBPU = 0;
    CMCONbits.CM = 7;
    ...

I don't understand electronics very much and I'm also kind of new to programming in C.

Thank you for any help.

Best Answer

This phenomenon is known as a pullup resistor. As seen on the wikipedia page, it is widely used in conjunction with a physical switch. When pressed, the switch "pulls" the input low, overcoming the resistor's high impedance.

Also, on some MCUs there is an option to invert the pin's function. For example, on the AVR XMega, you can configure it so that both input and output will be inverted. I am not sure whether this specific PIC supports such feature. You'll have to dig through the datasheet.

Related Solutions

Electronic – Configure a PIC pin for Input and Output

Change the pin configuration every time. It's the TRIS register that controls if it's an input (1) or output (0). And, when you change direction, you have to wait a moment (check the datasheet) like a cycle or two before you use that pin. a couple Nop(); will do fine.

And, make sure you write to the LAT register, and read from the R register.

e.g. for port D, pin 0.

//output
_TRISD0 = 0;
Nop();
Nop();
_LATD0 = 1;
_LATD0 = 0;


//input
_TRISD0 = 1;
Nop();
Nop();
int i = _RD0;

PIC ADC Impedance – How to Manage Low Maximum ADC Input Impedance in PIC

The reason the ADC needs a low source impedance is because it has a switched capacitor input. Basically, whenever the ADC 'samples' the voltage on the pin, a small capacitor is connected, charged up, and then disconnected. If the impedance is too large, charging the capacitor up will draw enough current to create a voltage drop large enough to affect the reading.

If you need to read a high speed signal, the best option is to add an amplifier of some sort to provide a low source impedance to the ADC. However, if you are looking at a relatively slow signal there are a couple of other options.

One solution to this is to increase the sample time - the length of time the capacitor is connected to the pin. The chip usually has a limit on how long this time can be, though.

Alternatively, you can add a decent sized capacitor in parallel with the ADC input pin. This will decrease the amount of droop that occurs when the ADC sampling capacitor charges up as most of its charge will be drawn from the capacitor instead of through the resistor.

Best Answer

Related Solutions

Electronic – Configure a PIC pin for Input and Output

PIC ADC Impedance – How to Manage Low Maximum ADC Input Impedance in PIC

Related Topic