SN8P2740 Series
ADC, OP-amp, Comparator 8-Bit Micro-Controller
13.4 ADC OPERATION DESCRIPTION AND NOTIC
13.4.1 ADC SIGNAL FORMAT
ADC sampling voltage range is limited by high/low reference voltage. The ADC low reference voltage is Vss and not
changeable. The ADC high reference voltage includes internal Vdd and external reference voltage source from
P4.0/AVREFH pin controlled by AVREFH bit. If AVREFH=0, ADC reference voltage is from internal Vdd (MCU power
voltage). If AVREFH=1, ADC reference voltage is from external voltage source (P4.0/AVREFH). ADC reference voltage
range limitation is “(ADC high reference voltage – low reference voltage) ≧ 2V”. ADC low reference voltage is
Vss = 0V. So ADC high reference voltage range is 2V~Vdd. The range is ADC external high reference voltage
range.
ADC Internal Low Reference Voltage = 0V.
ADC Internal High Reference Voltage = Vdd. (AVREFH=0)
ADC External High Reference Voltage = 2V~Vdd. (AVREFH=1)
ADC sampled input signal voltage must be from ADC low reference voltage to ADC high reference. If the ADC input
signal voltage is over the range, the ADC converting result is error (full scale or zero).
ADC Low Reference Voltage ≦ ADC Sampled Input Voltage ≦ ADC High Reference Voltage
13.4.2 ADC CONVERTING TIME
The ADC converting time is from ADS=1 (Start to ADC convert) to EOC=1 (End of ADC convert). The converting time
duration is depend on ADC resolution and ADC clock rate. 12-bit ADC‟s converting time is 1/(ADC clock /4)*16 sec,
and the 8-bit ADC converting time is 1/(ADC clock /4)*12 sec. ADC clock source is Fcpu and includes Fcpu/1, Fcpu/2,
Fcpu/8 and Fcpu/16 controlled by ADCKS[1:0] bits.
The ADC converting time affects ADC performance. If input high rate analog signal, it is necessary to select a high
ADC converting rate. If the ADC converting time is slower than analog signal variation rate, the ADC result would be
error. So to select a correct ADC clock rate and ADC resolution to decide a right ADC converting rate is very important.
12-bit ADC conversion time = 1/(ADC clock rate/4)*16 sec
Fcpu=4MHz
Fcpu=16MHz
ADCKS1, ADC Clock
ADLEN
ADC Converting
ADC Converting
Rate
ADC Converting
ADC Converting
Rate
ADCKS0
Rate
Fcpu/16
Fcpu/8
Fcpu
time
time
1/(4MHz/16/4)*16
= 256 us
3.906KHz
7.813KHz
62.5KHz
1/(16MHz/16/4)*16
= 64 us
15.625KHz
31.25KHz
250KHz
00
01
10
11
1/(4MHz/8/4)*16
= 128 us
1/(16MHz/8/4)*16
= 32 us
1 (12-bit)
1/(4MHz/4)*16
= 16 us
1/(16MHz/4)*16
= 4 us
1/(4MHz/2/4)*16
= 32 us
31.25KHz
1/(16MHz/2/4)*16
= 8 us
125KHz
Fcpu/2
8-bit ADC conversion time = 1/(ADC clock rate/4)*12 sec
Fcpu=4MHz
Fcpu=16MHz
ADCKS1, ADC Clock
ADLEN
0 (8-bit)
ADC Converting
ADC Converting
Rate
ADC Converting
time
ADC Converting
Rate
ADCKS0
Rate
Fcpu/16
Fcpu/8
Fcpu
time
1/(4MHz/16/4)*12
= 192 us
5.208KHz
10.416KHz
83.333KHz
41.667KHz
1/(16MHz/16/4)*12
= 48 us
20.833KHz
41.667KHz
333.333KHz
166.667KHz
00
01
10
11
1/(4MHz/8/4)*12
= 96 us
1/(16MHz/8/4)*12
= 24 us
1/(4MHz/4)*12
= 12 us
1/(16MHz/4)*12
= 3 us
1/(4MHz/2/4)*12
= 24 us
1/(16MHz/2/4)*12
= 6 us
Fcpu/2
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