C8051F340-GQR Datasheet, PDF (57/276 Pages) Silicon Laboratories – Full Speed USB Flash MCU Family

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 Section “5.1. Analog Multi-
plexer” on page 42 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.
REF0CN
AD0EN
EN _{ADC Bias}
IOSCEN
To ADC,
Internal Oscillator
VDD
R1
External
Voltage
Reference
Circuit
VREF
GND
EN _{Temp Sensor}
0
VDD
1
CLKMUL
Enable
TEMPE
REFBE
EN ^Reference
Bias
To Analog Mux
VREF
(to ADC)
To Clock Multiplier,
Temp Sensor
EN
Internal
Reference
Figure 6.1. Voltage Reference Functional Block Diagram
Rev. 1.3
57