C8051F321-GMR Datasheet, PDF (55/250 Pages) Silicon Laboratories – Full Speed USB, 16 k ISP FLASH MCU Family

C8051F320/1
6. Voltage Reference
The Voltage reference MUX on C8051F320/1 devices is configurable to use an externally connected volt-
age reference, the internal reference voltage generator, or the power supply voltage VDD (see Figure 6.1).
The REFSL bit in the Reference Control register (REF0CN) selects the reference source. For the internal
reference or an external source, REFSL should be set to ‘0’; For VDD 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 Figure 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 voltage reference
and bias circuits are given in Table 6.1.
Important Note About the VREF Input: Port pin P0.7 is used as the external VREF input. When using an
external voltage reference, P0.7 should be configured as analog input and skipped by the Digital Crossbar.
To configure P0.7 as analog input, set to ‘0’ Bit7 in register P0MDIN. To configure the Crossbar to skip
P0.7, set to ‘1’ Bit7 in register P0SKIP. Refer to Section “14. Port Input/Output” on page 126 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 40 for details). The TEMPE bit in register REF0CN enables/disables the temperature sen-
sor. While disabled, the temperature sensor defaults to a high impedance state and any ADC0
measurements 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. 1.4
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