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mylibs/841vcc.h

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+/*
+  Macros for easier usage of vcc measurements.
+  For attiny8441 only.
+
+  We use REFS=0 to select VCC as Reference. i.e.  ADMUXB = 0 << REFS0
+  We then use channel #13 to read the BandGap.  i.e. ADMUXA = 13 << MUX0
+
+  And calculate the value of VCC from the reading.  i.e. ADCW / 1024 = 1.1 / VCC
+  Rearrange i.e. VCC = 1.1 * 1024 / ADCW
+
+  Thanks for the help of the guys at AVR-Freaks:
+  https://www.avrfreaks.net/forum/measuring-vcc-attiny841-using-adc-and-internal-bandgap
+*/
+
+
+#include <util/delay.h>
+
+#ifndef VCC_8441_H
+#define VCC_8441_H
+
+// disable ADC globally
+#define adc_disable() ADCSRA &= ~(1<<ADEN)
+
+// enable
+#define adc_enable()  ADCSRA  =  (1<<ADEN);  _delay_ms(10);
+
+// use supply voltage VCC as reference
+// measure internal bandgap 1v1 voltage reference
+// via channel 13, see table 16-3
+// and do not left shift result
+#define adc_setup_vcc_measurement() \
+  ADMUXB = (0<<REFS0);              \
+  ADMUXA = (13<<MUX0);              \
+  ADCSRB = (0<<ADLAR);
+
+// start measurement
+// and wait till complete
+#define adc_start()      \
+  ADCSRA |= _BV( ADSC ); \
+  while( ( (ADCSRA & (1<<ADSC)) != 0 ) );
+
+// return measured voltage
+// >> 1.1 * 1024 / 225 #=> 5.006222222222223
+// >> 1.1 * 1024 / 341 #=> 3.3032258064516133
+#define adc_get_vcc() 1.1 * 1024 / ADCW;
+
+#endif