Nuts and Volts - February 2012

SMILEY’S WORKSHOP ☺

LISTING 1

#include
“c:\avrtoolbox\libavr\source\general\util\util.
h”
#include
“c:\avrtoolbox\libavr\source\driver\external_
hardware\
lcd_hd44780\lcd.h”
#include
“c:\avrtoolbox\libavr\source\driver\external_
hardware\
nav_button\nav.h”
 
int main( void )
{
 uint8_t key = 0;
 
 lcd_init();
 nav_init();
 
 lcd_puts(“NavButt”);
 delay(500);
 lcd_clear();
 lcd_puts(“REV028”);
 delay(1000);
 lcd_clear();

for(;;)
{
 if(nav_available())
 {
 key = nav_get_key();

if(key == CTR)
{
 lcd_home();
 lcd_blank(16);
 lcd_puts(“CTR!”);
}
if(key == LFT)
{
 lcd_home();
 lcd_blank(16);
 lcd_puts(“LFT!”);
}
if(key == RGT)
{
 lcd_home();
 lcd_blank(16);
 lcd_puts(“RGT!”);
}
if(key == UP)
{
 lcd_home();
 lcd_blank(16);
 lcd_puts(“UP!”);
}
if(key == DWN)
{
 lcd_home();
 lcd_blank(16);
 lcd_puts(“DWN!”);
}
delay(100);
lcd_home();
lcd_blank(16);

}
  }
}
buttons
uint8_t nav_available(); // true if a new button
press is available
uint8_t nav_get_button(); // return the next
available button

mapped to each of the five buttons. For example:

#define NAV_LFT_PORTX PORTD
#define NAV_LFT_DDR DDRD
#define NAV_LFT_PINX PIND
#define NAV_LFT_PIN PD6

Listing 1 makes for a very simple tester program. When we press the down button, the LCD shows DWN! as shown in Figure 10.

As usual, this simplicity is hiding some complexity under the hood that we will look at in more detail. When you first look at this — especially if you are a novice — you might correctly ask ‘Where is all the code that notes when a button is pressed?’ Well, that is all set up by the nav_init() routine that sets up an ISR (Interrupt Service Routine) that runs in the background, out of sight of our main loop. A timer peripheral is set to interrupt the main code every 5 mS (milli-seconds or 0.005 seconds) and check the state of the buttons. If a button is low for five consecutive 5 mS intervals ( 25 mS), then we are reasonably sure that the button has been pressed. We will get into a lot of detail about timers and ISRs in another Workshop, but for now I recommend that you look at the nav_init() function to get a feel for what is going on.

There are several other neat tricks hidden under the hood of this code, including the way the port pins are Here we see that we require four separate definitions just for setting up the LFT button in the initiation. If you want to wire the buttons to different ports and pins, you will need to change them in these definitions in nav.h.