Silicon Laboratories C8051F347 User Manual
Page 57
Rev. 1.3
57
C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D
6.
Voltage Reference (C8051F340/1/2/3/4/5/6/7/A/B Only)
The Voltage reference MUX on C8051F34x devices is configurable to use an externally connected voltage
reference, the on-chip reference voltage generator, or the power supply voltage V
DD
(see Figure 6.1). The
REFSL bit in the Reference Control register (REF0CN) selects the reference source. For the internal refer-
ence or an external source, REFSL should be set to ‘0’; For V
DD
as the reference source, REFSL should
be set to ‘1’.
The BIASE bit enables the internal ADC bias generator, which is used by the ADC and Internal Oscillator.
This enable is forced to logic 1 when either of the aforementioned peripherals is enabled. The ADC bias
generator may be enabled manually by writing a ‘1’ to the BIASE bit in register REF0CN; see SFR Defini-
tion 6.1 for REF0CN register details. The Reference bias generator (see Figure 6.1) is used by the Internal
Voltage Reference, Temperature Sensor, and Clock Multiplier. The Reference bias is automatically
enabled when any of the aforementioned peripherals are enabled. The electrical specifications for the volt-
age reference and bias circuits are given in Table 6.1.
Important Note About the VREF Pin: The VREF pin, when not using the on-chip voltage reference or an
external precision reference, can be configured as a GPIO Port pin. When using an external voltage refer-
ence or the on-chip reference, the VREF pin should be configured as analog pin and skipped by the Digital
Crossbar. To configure the VREF pin for analog mode, set the corresponding bit in the PnMDIN register to
‘0’. To configure the Crossbar to skip the VREF pin, set the corresponding bit in register PnSKIP to ‘1’.
Refer to
Section “15. Port Input/Output” on page 142
for complete Port I/O configuration details.
The temperature sensor connects to the ADC0 positive input multiplexer (see
for details). The TEMPE bit in register REF0CN enables/disables the temperature
sensor. While disabled, the temperature sensor defaults to a high impedance state and any ADC0 mea-
surements performed on the sensor result in meaningless data.
Figure 6.1. Voltage Reference Functional Block Diagram
VREF
(to ADC)
To Analog Mux
VDD
VREF
R1
VDD
External
Voltage
Reference
Circuit
GND
Temp Sensor
EN
0
1
REF0CN
RE
F
S
L
TEM
PE
BIASE
REFBE
REFBE
Internal
Reference
EN
Reference
Bias
EN
CLKMUL
Enable
TEMPE
To Clock Multiplier,
Temp Sensor
ADC Bias
To ADC,
Internal Oscillator
EN
IOSCEN
AD0EN