SN8P2740 Series
ADC, OP-amp, Comparator 8-Bit Micro-Controller
SONiX TECHNOLOGY CO., LTD
Page 111
Version 2.0
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
ADLEN
ADCKS1,
ADCKS0
ADC Clock
Rate
Fcpu=4MHz
Fcpu=16MHz
ADC Converting
time
ADC Converting
Rate
ADC Converting
time
ADC Converting
Rate
1 (12-bit)
00
Fcpu/16
1/(4MHz/16/4)*16
= 256 us
3.906KHz
1/(16MHz/16/4)*16
= 64 us
15.625KHz
01
Fcpu/8
1/(4MHz/8/4)*16
= 128 us
7.813KHz
1/(16MHz/8/4)*16
= 32 us
31.25KHz
10
Fcpu
1/(4MHz/4)*16
= 16 us
62.5KHz
1/(16MHz/4)*16
= 4 us
250KHz
11
Fcpu/2
1/(4MHz/2/4)*16
= 32 us
31.25KHz
1/(16MHz/2/4)*16
= 8 us
125KHz
8-bit ADC conversion time = 1/(ADC clock rate/4)*12 sec
ADLEN
ADCKS1,
ADCKS0
ADC Clock
Rate
Fcpu=4MHz
Fcpu=16MHz
ADC Converting
time
ADC Converting
Rate
ADC Converting
time
ADC Converting
Rate
0 (8-bit)
00
Fcpu/16
1/(4MHz/16/4)*12
= 192 us
5.208KHz
1/(16MHz/16/4)*12
= 48 us
20.833KHz
01
Fcpu/8
1/(4MHz/8/4)*12
= 96 us
10.416KHz
1/(16MHz/8/4)*12
= 24 us
41.667KHz
10
Fcpu
1/(4MHz/4)*12
= 12 us
83.333KHz
1/(16MHz/4)*12
= 3 us
333.333KHz
11
Fcpu/2
1/(4MHz/2/4)*12
= 24 us
41.667KHz
1/(16MHz/2/4)*12
= 6 us
166.667KHz
