--- a/main.c	Sun May 06 23:13:14 2012 +0800
+++ b/main.c	Mon May 07 00:14:53 2012 +0800
@@ -4,9 +4,11 @@
  * License: <insert your license reference here>
  */
 
-#include <avr/io.h>
 #include <stdio.h>
 #include <string.h>
+#include <avr/io.h>
+#include <avr/interrupt.h>
+#include <avr/sleep.h>
 #include <util/crc16.h>
 
 // configuration params
@@ -15,6 +17,10 @@
 // - bluetooth params
 // - number of sensors (and range?)
 
+// 1 second. we have 1024 prescaler, 32768 crystal.
+static const uint8_t CNT2_COMPARE = 32;
+static const int SECONDS_WAKE = 60;
+
 static int uart_putchar(char c, FILE *stream);
 static FILE mystdout = FDEV_SETUP_STREAM(uart_putchar, NULL,
         _FDEV_SETUP_WRITE);
@@ -22,9 +28,13 @@
 static uint8_t n_measurements;
 static uint8_t measurements[500];
 
+static uint8_t need_measurement;
+
 static uint8_t readpos;
 static char readbuf[30];
 
+static uint8_t sec_count;
+
 static void 
 uart_init(unsigned int baud)
 {
@@ -54,9 +64,9 @@
     for (i = 0; i < n_measurements; i++)
     {
         printf("%3d : %d\n", i, measurements[i]);
-        crc = _crc_ccitt_updatec(crc, measurements[i]);
+        crc = _crc_ccitt_update(crc, measurements[i]);
     }
-        printf("CRC : %d\n", i, measurements[i]);
+    printf("CRC : %d\n", crc);
 }
 
 static void
@@ -90,7 +100,7 @@
     }
 }
 
-ISR(UART_RX_vect)
+ISR(USART_RXC_vect)
 {
     char c = UDR;
     if (c == '\n')
@@ -110,16 +120,53 @@
     }
 }
 
+ISR(TIMER2_COMP_vect)
+{
+    sec_count ++;
+    if (sec_count == SECONDS_WAKE)
+    {
+        sec_count = 0;
+        need_measurement = 1;
+    }
+}
+
+static void
+deep_sleep()
+{
+    // p119 of manual
+    OCR2 = CNT2_COMPARE;
+    loop_until_bit_is_clear(ASSR, OCR2UB);
+
+    set_sleep_mode(SLEEP_MODE_PWR_SAVE);
+    sleep_mode();
+}
+
+static void
+do_measurement()
+{
+    need_measurement = 0;
+}
+
 int main(void)
 {
     uart_init(9600);
 
     fprintf(&mystdout, "hello %d\n", 12);
 
-    // get current time
+    // set up counter2. 
+    // COM21 COM20 Set OC2 on Compare Match (p116)
+    // WGM21 Clear counter on compare
+    // CS22 CS21 CS20  clk/1024
+    TCCR2 = _BV(COM21) | _BV(COM20) | _BV(WGM21) | _BV(CS22) | _BV(CS21) | _BV(CS20);
+    // set async mode
+    ASSR |= _BV(AS2);
 
     for(;;){
         /* insert your main loop code here */
+        if (need_measurement)
+        {
+            do_measurement();
+        }
     }
     return 0;   /* never reached */
 }