C8051F320/1
40 Rev. 1.4
5.1. Analog Multiplexer
AMUX0 selects the positive and negative inputs to the ADC. Any of the following may be selected as the
positive input: P1.0-P3.0, the on-chip temperature sensor, or the positive power supply (V
DD
). Any of the
following may be selected as the negative input: P1.0-P3.0, VREF, or GND. When GND is selected as
the negative input, ADC0 operates in Single-ended Mode; all other times, ADC0 operates in Differ-
ential Mode.
The ADC0 input channels are selected in the AMX0P and AMX0N registers as described in
Figure 5.2 and Figure 5.2.
The conversion code format differs between Single-en
ded and Differential modes. The registers ADC0H
and ADC0L contain the high and low bytes of the output conversion code from the ADC at the completion
of each conversion. Data can be right-justified or left-justified, depending on the setting of the AD0LJST bit
(ADC0CN.0). When in Single-ended Mode, conversion codes are represented as 10-bit unsigned integers.
Inputs are measured from ‘0’ to VREF x 1023/1024. Example codes are shown below for both right-justi-
fied and left-justified data. Unused bits in the ADC0H and ADC0L registers are set to ‘0’.
When in Differential Mode, conversion codes are represented as 10-bit signed 2’s complement numbers.
Inp
uts are measured from –VREF to VREF x 511/512. Example codes are shown below for both right-jus-
tified and left-justified data. For right-justified data, the
unused MSBs of ADC0H are a sign-extension of the
data word. For left-justified data, the unused LSBs in the ADC0L register are set to ‘0’.
Important Note About ADC0 Input Configuration: Por
t pins selected as ADC0 inputs should be config-
ured as analog inputs, and should be skipped by the Dig
ital Crossbar. To configure a Port pin for analog
input, set to ‘0’ the corresponding bit in register PnMDIN (for n = 0,1,2,3). To force the Crossbar to skip a
Port pin, set to ‘1’ the corresponding bit in register PnSKIP (for n = 0,1,2). See Section “14. Port Input/Out-
put” on page 126 for more Port I/O configuration details.
Input Voltage
(
Single-Ended)
Right-Justified ADC0H:ADC0L
(AD0LJST = 0)
Left-Justified ADC0H:ADC0L
(AD0LJST = 1)
VREF x 1023/1024 0x03FF 0xFFC0
VREF x 512/1024 0x0200 0x8000
VREF x 256/1024 0x0100 0x4000
0 0x0000 0x0000
Input Voltage
(Differential)
Right-Justified ADC0H:ADC0L
(AD0LJST = 0)
Left-Justified ADC0H:ADC0L
(AD0LJST = 1)
VREF x 511/512 0x01FF 0x7FC0
VREF x 256/512 0x0100 0x4000
0 0x0000 0x0000
–VREF x 256/512 0xFF00 0xC000
–VREF 0xFE00 0x8000
