C8051F388-B Datasheet, PDF (58/285 Pages) Silicon Laboratories – Flash MCU Family

C8051F388/9/A/B
7. Voltage Reference Options
The Voltage reference multiplexer for the ADC is configurable to use an externally connected voltage refer-
ence, the on-chip reference voltage generator routed to the VREF pin, the unregulated power supply volt-
age (V_DD), or the regulated 1.8 V internal supply (see Figure 7.1). The REFSL bit in the Reference Control
register (REF0CN, SFR Definition 7.1) selects the reference source for the ADC. For an external source or
the on-chip reference, REFSL should be set to 0 to select the VREF pin. To use V_DD as the reference
source, REFSL should be set to 1. To override this selection and use the internal regulator as the reference
source, the REGOVR bit can be set to 1.
The BIASE bit enables the internal voltage bias generator, which is used by many of the analog peripherals
on the device. This bias is automatically enabled when any peripheral which requires it is enabled, and it
does not need to be enabled manually. The bias generator may be enabled manually by writing a 1 to the
BIASE bit in register REF0CN. The electrical specifications for the voltage reference circuit are given in
Table 5.12.
The C8051F388/9/A/B devices also include an on-chip voltage reference circuit which consists of a 1.2 V,
temperature stable bandgap voltage reference generator and a selectable-gain output buffer amplifier. The
buffer is configured for 1x or 2x gain using the REFBGS bit in register REF0CN. On the 1x gain setting the
output voltage is nominally 1.2 V, and on the 2x gain setting the output voltage is nominally 2.4 V. The on-
chip voltage reference can be driven on the VREF pin by setting the REFBE bit in register REF0CN to a 1.
The maximum load seen by the VREF pin must be less than 200 μA to GND. Bypass capacitors of 0.1 μF
and 4.7 μF are recommended from the VREF pin to GND, and a minimum of 0.1uF is required. If the on-
chip reference is not used, the REFBE bit should be cleared to 0. Electrical specifications for the on-chip
voltage reference are given in Table 5.12.
Important Note about the VREF Pin: When using either an external voltage reference or the on-chip ref-
erence circuitry, the VREF pin should be configured as an analog pin and skipped by the Digital Crossbar.
Refer to Section “20. Port Input/Output” on page 147 for the location of the VREF pin, as well as details of
how to configure the pin in analog mode and to be skipped by the crossbar.
Figure 7.1. Voltage Reference Functional Block Diagram
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Rev. 1.1