The Voltage Reference (VREF) outputs a fixed (constant) voltage to external circuitry. The voltage source options are the /2, /3 and /4 dividers of the VDD power supply and ACMP IN- references.
Set a source for ‘Source selector’ that is not None. Then set the Connection Output Pin to Analog Input/Output.
VREF0 and TEMP SENSOR share a connection output; disable the Temp Sensor.
The bandgap sources the voltage reference VREF. The bandgap is auto powered on and off when analog components are enabled or disabled. The ‘bandgap output delay time’ can be bypassed if the bandgap is forced on, improving response time of analog comparators.
There is an additional delay gating the analog comparator output after the bandgap has become valid. This gives time for the charge pump and regulator to settle before obtaining valid comparator data. The VREF and Ring OSC components are NOT gated by this output delay. If Forced bandgap on is Enabled, this delay is bypassed.
• Auto-Delay: The additional delay will be either 100 or 550 us depending on the ‘Power Supply Control’ and VDD.
- If Power Supply Control is ‘Charge Pump always OFF’, then the delay will be 100us.
- If Power Supply Control is ‘Charge Pump always ON’, then the delay will be 550us.
- If Power Supply Control is ‘Charge Pump automatically ON/OFF’, then the delay will be 550us if VDD < 2.7 and 100us if VDD > 2.7.
- If Power Supply Control is ‘VDD directly to analog blocks’, then the delay will be 550us.
• 100us:
• 550us: select if charge pump is needed.
Go to ‘Power Control Settings’ to change the ‘Power Supply Control’
The Vref output can be routed through an op-amp, which has much lower output impedance, before reaching the output pin. This will introduce less than 30mV of offset.
VREF outputs this source voltage to the Connection output pin. For ACMP reference voltages, if that ACMP and the bandgap are powered down, VREF will not output a constant voltage. If that ACMP is powered down but the bandgap is powered on, VREF will output a constant voltage.
• None
• ACMP0 Reference Voltage: ACMP0 IN- source.
• ACMP1 Reference Voltage: ACMP0 IN- source.
• VDD/2
• VDD/3
• VDD/4
Only one output pin is hardwired to each VREF. Set this pin to ‘Analog Input/Output’ to connect VREF to an external circuit.
Table 1.
| VDD | Practical VREF range | Note |
|---|---|---|
| 2.0V-5.5V | 50mV ~1.2V | |
| 1.7V – 2.0V | 50mV ~1.1V | Higher than 1.1V negative input, the comparator may show wrong result |