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ATMEGA640 Datasheet(PDF) 281 Page - Microchip Technology |
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ATMEGA640 Datasheet(HTML) 281 Page - Microchip Technology |
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281 / 436 page  281 ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET] DS40002211A Example: ADMUX = 0xFB (ADC3 - ADC2, 10× gain, 2.56V reference, left adjusted result). Voltage on ADC3 is 300mV, voltage on ADC2 is 500mV. ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270. ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02. 26.8 Register Description 26.8.1 ADMUX – ADC Multiplexer Selection Register • Bit 7:6 – REFS1:0: Reference Selection Bits These bits select the voltage reference for the ADC, as shown in Table 26-3. If these bits are changed during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external reference voltage is being applied to the AREF pin. Note: 1. If 10x or 200x gain is selected, only 2.56V should be used as Internal Voltage Reference. For differential conver- sion, only 1.1V cannot be used as internal voltage reference. Table 26-2. Correlation Between Input Voltage and Output Codes VADCn Read Code Corresponding Decimal Value VADCm + VREF / GAIN 0x1FF 511 VADCm + 0.999 VREF / GAIN 0x1FF 511 VADCm + 0.998 VREF / GAIN 0x1FE 510 ... ... ... VADCm + 0.001 VREF / GAIN 0x001 1 VADCm 0x000 0 VADCm - 0.001 VREF / GAIN 0x3FF -1 ... ... ... VADCm - 0.999 VREF / GAIN 0x201 -511 VADCm - VREF / GAIN 0x200 -512 Bit 7 654 321 0 (0x7C) REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 ADMUX Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 Table 26-3. Voltage Reference Selections for ADC REFS1 REFS0 Voltage Reference Selection(1) 0 0 AREF, Internal VREF turned off 0 1 AVCC with external capacitor at AREF pin 1 0 Internal 1.1V Voltage Reference with external capacitor at AREF pin 1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin |
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