SI100X FAMILY

# Si100x - General I/O

Article by:
 Date Published: September 15, 2011 Last Modified: September 15, 2011

This page shows how to configure a Si100x microcontroller’s pins to standard I/O functions such as a digital input, digital output, and analog input.

## Configuring A Digital Output

### With Push/Pull

The following code sets channel P2.2 for use as a digital output

P2MDIN |= (1<<2); // Set channel P2.2 as digital I/O
P2MDOUT |= (1<<2); // Set channel P2.2 as push-pull drive
P2SKIP |= (1<<2); // Set channel P2.2 to skip cross-bar assignment


Channel P2.2 is then set high and low by writing the following code

P2 |= (1<<2); // Set channel P2.2 high
P2 %= ~(1<<2); // Set channel P2.2 low


### With Open Drain

Open-drain allows for higher voltages to be present on the pin than with push/pull drive. However it cannot source current, so if you are switching a signal using an open-drain configuration you will notice the rise-time is much slower than the fall time (since the rise-time will be set by external pull-ups, if any). The code for configuring a digital output with push/pull procedure is followed, except that the bit in P2MDOUT is cleared rather than set. So that it now becomes:

P2MDOUT &= ~(1<<2); // Set channel P2.2 as open-drain drive


## Configuring A Digital Input

The following code sets channel P1.7 for use as a digital input

P2SKIP |= (1<<2);  // Set channel P1.7 to skip cross-bar assignment
P1MDIN |= (1<<7);  // Set channel P1.7 as digital I/O
P2MDOUT &= ~(1<<2);  // Set channel P1.7 with open-drain drive
P2 |= (1<<2);   // Set channel 2.2 high (actually disables both drives since open-drain)


Setting a pin to an analog input disables all digital drivers and allows the pin to be used for an analog function such as the ADC. The analog input configuration is the lowest power state for a pin, and it is recommended to set any unused pins to analog inputs when designing a low power circuit. The last two lines of code are not strictly necessary, but are good practice. This code uses channel P2.2 as the example pin.

P2SKIP |= (1<<1);  // Set channel P2.2 to skip cross-bar assignment
P2MDIN &= ~(1<<1);  // Set port 2.2 to analog input
P2MDOUT &= ~(1<<1);    // Set channel 2.2 with open-drain drive
P2 |= (1<<1);   // Set channel 2.2 high (actually disables both drives since open-drain)


## Macro For Specific Pin Addressing

Keil allows a method of writing a function macro so that from that point on you can address a pin directly (as opposed to above where we were modifying the whole byte). The first step is to define a function typically called SBIT (although it could be named anything)

#define SBIT(name, addr, bit)  sbit  name = addr^bit


This function takes a word to represent the pin, the special function register that controls the entire port that the pin is on, and the pin number on that port. The next trick is to associate a word with a pin, and use the sbit function as a macro to replace the word whenever you use it in the code.

SBIT(LED1, SFR_P1, 5); // Channel P1.5 is assigned to the word LED1


Now the pin can be addressed with just the keyword, LED1. For example,

LED1 = 1; // This sets the pin high
LED1 = 0; // This sets the pin low
variable = LED1; // This reads the value of the pin