C8051F902-GD View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

MFG CO.

C8051F902-GD

Single/Dual Battery, 0.9–3.6 V, 16–8 kB, SmaRTClock, 12/10-Bit ADC MCU

Silicon Laboratories 

C8051F91x-C8051F90x

5.10. External Voltage References

To use an external voltage reference, REFSL[1:0] should be set to 00 and the internal 1.68 V precision ref-

erence should be disabled by setting REFOE to 0. Bypass capacitors should be added as recommended

by the manufacturer of the external voltage reference.

5.11. Internal Voltage References

For applications requiring the maximum number of port I/O pins, or very short VREF turn-on time, the

1.65 V high-speed reference will be the best internal reference option to choose. The high speed internal

reference is selected by setting REFSL[1:0] to 11. When selected, the high speed internal reference will be

automatically enabled/disabled on an as-needed basis by ADC0.

For applications requiring the highest absolute accuracy, the 1.68 V precision voltage reference will be the

best internal reference option to choose. The 1.68 V precision reference may be enabled and selected by

setting REFOE to 1 and REFSL[1:0] to 00. An external capacitor of at least 0.1 µF is recommended when

using the precision voltage reference.

In applications that leave the precision internal oscillator always running, there is no additional power

required to use the precision voltage reference. In all other applications, using the high speed reference

will result in lower overall power consumption due to its minimal startup time and the fact that it remains in

a low power state when an ADC conversion is not taking place.

Note: When using the precision internal oscillator as the system clock source, the precision voltage

reference should not be enabled from a disabled state. To use the precision oscillator and the precision

voltage reference simultaneously, the precision voltage reference should be enabled first and allowed

to settle to its final value (charging the external capacitor) before the precision oscillator is started and

selected as the system clock.

For applications with a non-varying power supply voltage, using the power supply as the voltage reference

can provide ADC0 with added dynamic range at the cost of reduced power supply noise rejection. To use

the 1.8 to 3.6 V power supply voltage (VDD/DC+) or the 1.8 V regulated digital supply voltage as the

reference source, REFSL[1:0] should be set to 01 or 10, respectively.

5.12. Analog Ground Reference

To prevent ground noise generated by switching digital logic from affecting sensitive analog

measurements, a separate analog ground reference option is available. When enabled, the ground

reference for ADC0 during both the tracking/sampling and the conversion periods is taken from the

P0.1/AGND pin. Any external sensors sampled by ADC0 should be referenced to the P0.1/AGND pin. This

pin should be connected to the ground terminal of any external sensors sampled by ADC0. If an external

voltage reference is used, the P0.1/AGND pin should be connected to the ground of the external reference

and its associated decoupling capacitor. If the 1.68 V precision internal reference is used, then P0.1/AGND

should be connected to the ground terminal of its external decoupling capacitor. The separate analog

ground reference option is enabled by setting REFGND to 1. Note that when sampling the internal

temperature sensor, the internal chip ground is always used for the sampling operation, regardless of the

setting of the REFGND bit. Similarly, whenever the internal 1.65 V high-speed reference is selected, the

internal chip ground is always used during the conversion period, regardless of the setting of the REFGND

bit.

5.13. Temperature Sensor Enable

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. See Section “5.8. Temperature Sensor” on page 80 for details on

temperature sensor characteristics when it is enabled.

84

Rev. 1.0

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