Rainbow-electronics Atmega169L Uživatelský manuál stáhnout pdf (Strana 204)

204

ATmega169V/L

2514A–AVR–08/02

Example: ADMUX = 0xFB (ADC3 - ADC2, 1.1V reference, left adjusted result)

Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.

ADCR = 512 * (300 - 500) / 1100 = -93 = 0x3A3.

ADCL will thus read 0xC0, and ADCH will read 0xD8. Writing zero to ADLAR right

adjusts the result: ADCL = 0xA3, ADCH = 0x03.

ADC Multiplexer Selection

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 89. 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.

• Bit 5 – ADLAR: ADC Left Adjust Result

The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data

adjusted. Changing the ADLAR bit will affect the ADC Data Register immediately,

regardless of any ongoing conversions. For a complete description of this bit, see “The

ADC Data Register – ADCL and ADCH” on page 207.

Table 88. Correlation Between Input Voltage and Output Codes

ADCn

Read Code Corresponding Decimal Value

ADCm

REF

0x1FF 511

ADCm

+ 0.999 V

REF

0x1FF 511

ADCm

+ 0.998 V

REF

0x1FE 510

... ... ...

ADCm

+ 0.001 V

REF

0x001 1

ADCm

0x000 0

ADCm

-0.001V

REF

0x3FF -1

... ... ...

ADCm

-0.999V

REF

0x201 -511

ADCm

-V

REF

0x200 -512

Bit 76543210

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

InitialValue00000000

Table 89. Voltage Reference Selections for ADC

REFS1 REFS0 Voltage Reference Selection

0 0 AREF, Internal Vref turned off

0 1 AVCC with external capacitor at AREF pin

10Reserved

1 1 Internal 1.1V Voltage Reference with external capacitor at AREF pin

1 2 ... 199 200 201 202 203 204 205 206 207 208 209 ... 316 317

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