SN8F2280 Series
USB 2.0 Full-Speed 8-Bit Micro-Controller
12.5 P4CON REGISTERS
The Port 4 is shared with ADC input function. Only one pin of port 4 can be configured as ADC input in the same time
by ADM register. The other pins of port 4 are digital I/O pins. Connect an analog signal to COMS digital input pin,
especially, the analog signal level is about 1/2 VDD will cause extra current leakage. In the power down mode, the
above leakage current will be a big problem. Unfortunately, if users connect more than one analog input signal to port 4
will encounter above current leakage situation. P4CON is Port4 Configuration register. Write “1” into P4CON [7:0] will
configure related port 4 pin as pure analog input pin to avoid current leakage.
0B9H
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
P4CON
P4CON7
P4CON6
P4CON5
P4CON4
P4CON3
P4CON2
P4CON1
P4CON0
Read/Write
After reset
W
0
W
0
W
0
W
0
W
0
W
0
W
0
W
0
Bit[4:0]
P4CON[7:0]: P4.n configuration control bits.
0 = P4.n can be an analog input (ADC input) or digital I/O pins.
1 = P4.n is pure analog input, can’t be a digital I/O pin.
Note: When Port 4.n is general I/O port not ADC channel, P4CON.n must set to “0” or the Port 4.n digital
I/O signal would be isolated.
12.6 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. 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. 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 /4)*16 sec
8-bit ADC conversion time = 1/(ADC clock /4)*12 sec
Fcpu = 4MHz ( High clock oscillator frequency (Fosc) is 16MHz and Fcpu = Fosc/4)
ADLEN
ADCKS1 ADCKS0 ADC Clock
ADC conversion time
1/(4MHz/16/4)*12 = 192 us
1/(4MHz/8/4)*12 = 96 us
1/(4MHz/4)*12 = 12 us
0
0
1
1
0
1
0
1
Fcpu/16
Fcpu/8
Fcpu
0 (8-bit)
Fcpu/2
1/(4MHz/2/4)*12 = 24 us
0
0
1
1
0
1
0
1
Fcpu/16
Fcpu/8
Fcpu
1/(4MHz/16/4)*16 = 256 us
1/(4MHz/8/4)*16 = 128 us
1/(4MHz/4)*16 = 16 us
1 (12-bit)
Fcpu/2
1/(4MHz/2/4)*16 = 32 us
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