LM3S1651 Datasheet, PDF (797/1033 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

Stellaris^® LM3S1651 Microcontroller
Figure 17-3. Comparator Internal Reference Structure
VDDA
8R
R
8R
R
R
•••
EN
VREF
RNG
15 14
•••
1
0
Decoder
internal
reference
VIREF
The internal reference can be programmed in one of two modes (low range or high range) depending
on the RNG bit in the ACREFCTL register. When RNG is clear, the internal reference is in high-range
mode, and when RNG is set the internal reference is in low-range mode.
In each range, the internal reference, V_IREF, has 16 pre-programmed thresholds or step values. The
threshold to be used to compare the external input voltage against is selected using the VREF field
in the ACREFCTL register.
In the high-range mode, the V_IREF threshold voltages start at the ideal high-range starting voltage
of V_DDA/3.875 and increase in ideal constant voltage steps of V_DDA/31.
In the low-range mode, the V_IREF threshold voltages start at:0V and increase in ideal constant voltage
steps of V_DDA/23. The ideal V_IREF step voltages for each mode and their dependence on the RNG
and VREF fields are summarized in Table 17-3 on page 797.
Table 17-3. Internal Reference Voltage and ACREFCTL Field Values
ACREFCTL Register
EN Bit
Value
RNG Bit Value ^{Output Reference Voltage Based on VREF Field Value}
EN=0
RNG=X
0 V (GND) for any value of VREF. It is recommended that RNG=1 and VREF=0 to minimize noise
on the reference ground.
January 21, 2012
797
Texas Instruments-Production Data