72 www.ubicom.com IP2022 Data Sheet 5.7 Analog to Digital Converter (ADC) The on-chip A/D converter has the following features: • 10-bit ADC, ½ LSB accuracy • 8 input channels • 48 kHz maximum sampling rate • One-shot conversion. • Optional external reference voltage • Vmax = AVdd (max 2.7V) • Result returned in the ADCH and ADCL registers Figure 5-18 shows the A/D converter circuitry. The ADC
input pins use alternate functions of the Port G pins. The
result of an ADC sample is the analog value measured on
the selected pin. To correctly read an external voltage, the
pin being sampled must be configured as an input in the
port direction register (i.e. the RGDIR register). If the pin
is configured as an output, then the result will indicate the
voltage level being driven by the output buffer. The RG1
and   RG2   port   pins   are   also   used   as   the   analog
comparator input pins. The result of sampling the RG1 or
RG2 pins will be correct whether or not the comparator is
operating. The RG0 pin is also used as the comparator
output pin. If the comparator is enabled, then sampling the
RG0 pin will indicate the voltage level being driven by the
comparator. The RG3 pin is multiplexed with the external
reference voltage. Figure 5-18  A/D Converter Block Diagram 5.7.1 ADC Reference Voltage The  reference  voltage  (Vref)  can  come  from  either  the
RG3 port pin or from the AVdd supply voltage. If AVdd is
used,  the  RG3  port  pin  may  be  used  as  a  channel  of
analog input or as a general-purpose port pin. Vref defines a voltage level which reads as one increment
of  resolution  below  the  full-scale  voltage.  The  full-scale
voltage  reads  as  0x3FF,  so  the  Vref  voltage  reads  as
0x3FE and the A/D converter resolution is 10 bits. Table
5-12 shows the values reported at the upper and lower
limits of the ADC input voltage range. 5.7.2 ADC Result Justification The 10 bits of the ADC value can be mapped to the 16 bits
of the ADCH/ADCL register pair in three different ways, as
shown in Table 5-13. In this table, the numbers in the cells
represent   bit   positions   in   the   10-bit   ADC   value,   Z
represents  zero  (as  opposed  to  bit  position  0),  and  -9
represents the inversion of bit position 9. 515-016.eps ADCH A/D Vref
Reference
Voltage ADCREF ADC Timer 8 7 0 ADCGO 10 ADCL 8 7 0 AVdd RG7:0 System Clock ADCS2:0 RG3 Data Bus    Data Bus                Table 5-12  ADC Values
Vin Voltage ADC Value 0V 0x000 Vref/0x3FE 0x001 Vref 0x3FE Vref + (Vref/0x3FE) 0x3FF