STR91XF datasheet(18/72 Pages) STMICROELECTRONICS

18 / 72 page

Functional overview

STR91xF

18/72

2.12.1 Independent A/D converter supply and reference voltage

The ADC unit on 128-pin packages has an isolated analog voltage supply input at pin AVDD to

accept a very clean voltage source, independent of the digital voltage supplies. The analog

voltage supply range on pin AVDD is the same range as the digital voltage supply on pin

VDDQ. Additionally, an isolated analog supply ground connection is provided on pin AVSS only

on 128-pin packages for further ADC supply isolation. On 80-pin packages, the analog voltage

supply is shared with the ADC reference voltage pin (as described next), and the analog ground

is shared with the digital ground at a single point in the STR91xF device on pin AVSS_VSSQ.

A separate external analog reference voltage input for the ADC unit is available on 128-pin

packages at the AVREF pin for better accuracy on low voltage inputs, and the voltage on

AVREF can range from 1.0V to VDDQ. For 80-pin packages, the ADC reference voltage is tied

internally to the ADC unit supply voltage at pin AVCC_AVREF, meaning the ADC reference

voltage is fixed to the ADC unit supply voltage.

2.12.2 Battery supply

An optional stand-by voltage from a battery or other source may be connected to pin VBATT to

retain the contents of SRAM in the event of a loss of the VDD supply. The SRAM will

automatically switch its supply from the internal VDD source to the VBATT pin when the voltage

of VDD drops below that of VBATT.

The VBATT pin also supplies power to the RTC unit, allowing the RTC to function even when

the main digital supplies (VDD and VDDQ) are switched off. Using the STR91xF device

configuration software tool, it is possible to select whether or not to power from VBATT only the

RTC unit, or power the RTC unit and the SRAM when the STR91xF device is powered off.

STR91XF 数据表(PDF) 18 Page - STMicroelectronics