C8051F330-GMR Datasheet, PDF (58/210 Pages) Silicon Laboratories – Mixed-Signal ISP Flash MCU

C8051F330/1/2/3/4/5
7. Voltage Reference (C8051F330/2/4 only)
The Voltage reference MUX on the C8051F330/2/4 devices is configurable to use an externally connected
voltage reference, the internal reference voltage generator, or the V_DD power supply voltage (see
Figure 7.1). The REFSL bit in the Reference Control register (REF0CN) selects the reference source. For
an external source or the internal reference, REFSL should be set to ‘0’. To use V_DD as the reference
source, REFSL should be set to ‘1’.
The BIASE bit enables the internal voltage bias generator, which is used by the ADC, Temperature Sensor,
internal oscillators, and Current DAC. This bias is enabled when any of the aforementioned peripherals are
enabled. The bias generator may be enabled manually by writing a ‘1’ to the BIASE bit in register
REF0CN; see SFR Definition 7.1 for REF0CN register details. The electrical specifications for the voltage
reference circuit are given in Table 7.1.
The internal voltage reference circuit consists of a 1.2 V, temperature stable bandgap voltage reference
generator and a gain-of-two output buffer amplifier. The internal voltage reference can be driven out on the
VREF pin by setting the REFBE bit in register REF0CN to a ‘1’ (see SFR Definition 7.1). The maximum
load seen by the VREF pin must be less than 200 µA to GND. When using the internal voltage reference,
bypass capacitors of 0.1 µF and 4.7 µF are recommended from the VREF pin to GND. If the internal refer-
ence is not used, the REFBE bit should be cleared to ‘0’. Electrical specifications for the internal voltage
reference are given in Table 7.1.
Important Note about the VREF Pin: Port pin P0.0 is used as the external VREF input and as an output
for the internal VREF. When using either an external voltage reference or the internal reference circuitry,
P0.0 should be configured as an analog pin, and skipped by the Digital Crossbar. To configure P0.0 as an
analog pin, set to ‘0’ Bit0 in register P0MDIN. To configure the Crossbar to skip P0.0, set Bit 0 in register
P0SKIP to ‘1’. Refer to Section “14. Port Input/Output” on page 123 for complete Port I/O configuration
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 measurements performed on the
sensor result in meaningless data.
REF0CN
VDD
R1
External
Voltage
Reference
Circuit
VREF
GND
+
4.7F
0.1F
Recommended Bypass
Capacitors
IOSCE
N
EN _{Bias Generator}
EN _{Temp Sensor}
0
VDD
1
REFBE
EN
Internal
Reference
To ADC, IDAC,
Internal Oscillators
To Analog Mux
VREF
(to ADC)
Figure 7.1. Voltage Reference Functional Block Diagram
Rev. 1.7
61