clean up diagnostic more and make it more robust

A number of changes:
- don't enable interrupts, since they often cause hangs (not sure why)
- remove GND measurement
- wait 100 us between AMUX setup and start of conversion, to let input
  settle
- reduce delay between line setup and samping from 100 ms to 50 ms,
  for this is already sufficient

fw/accel.c

@@ -17,6 +17,8 @@
 
 #include <avr/io.h>
 #include <avr/interrupt.h>
+#define F_CPU   8000000UL
+#include <util/delay.h>
 
 #include "io.h"
 #include "accel.h"
@@ -50,13 +52,14 @@ static inline void adcsra(bool start)
 }
 
 
-uint16_t measure_ref(bool gnd)
+uint16_t measure_ref(void)
 {
 	while (ADCSRA & (1 << ADSC));
 	adcsra(0);
 	ADMUX =
 	    1 << REFS0 |	/* Vref is AVcc */
-	    (gnd ? 15 : 14);	/* GND (0 V) or Vbg (1.1 V) */
+	    14;			/* Vbg (1.1 V) */
+	_delay_us(100);
 	adcsra(1);
 	while (ADCSRA & (1 << ADSC));
 	return ADC;

fw/accel.h

@@ -21,7 +21,7 @@
 extern void (*sample)(bool x, uint16_t v);
 
 
-uint16_t measure_ref(bool gnd);
+uint16_t measure_ref(void);
 void accel_start(void);
 
 #endif /* !ACCEL_H */

fw/diag.c

@@ -35,24 +35,26 @@ static bool failed;
 
 static void do_diag(void)
 {
-	uint8_t pkg[3+4*DIAG_SAMPLES] = { DIAG_ACK, 4, 0, };
+	uint8_t pkg[3+2*DIAG_SAMPLES] = { DIAG_ACK, 4, 0, };
 	uint8_t *p = pkg+3;
 	uint16_t v;
 	uint8_t i;
 
 	cli();
 	set_line(localize_line(tmp[0], tmp[1]));
-	_delay_ms(100);
+	_delay_ms(50);
 	for (i = 0; i != DIAG_SAMPLES; i++) {
-		v = measure_ref(1);
-		*p++ = v;
-		*p++ = v >> 8;
-		v = measure_ref(0);
+		v = measure_ref();
 		*p++ = v;
 		*p++ = v >> 8;
 	}
 	set_line(localize_line(0, 0));
-	sei();
+	/*
+	 * @@@ for some reason the firmware often hangs if we enable
+	 * interrupts here
+	 *
+	 * sei();
+	 */
 	rf_send(pkg, sizeof(pkg));
 }