STR910FAM32X6 datasheet(18/78 Pages) STMICROELECTRONICS

18 / 78 page

Functional overview

STR91xFA

18/78

2.11.3 Sleep mode

In this mode all clock circuits except the RTC are turned off and main oscillator input pins

X1_CPU and X2_CPU are disabled. The RTC clock is required for the CPU to exit Sleep Mode.

The entire chip is quiescent (except for RTC and wake-up circuitry). There are three means to

exit Sleep Mode and re-start the system:

●

Some resets (external reset pin, low-voltage, power-up, JTAG debug command)

●

RTC alarm

●

Input from wake-up unit

2.12

Voltage supplies

The STR91xFA requires two separate operating voltage supplies. The CPU and memories

operate from a 1.65V to 2.0V on the VDD pins, and the I/O ring operates at 2.7V to 3.6V on the

VDDQ pins.

2.12.1 Independent A/D converter supply and reference voltage

The ADC unit on 128-pin and 144-ball packages has an isolated analog voltage supply input at

pin AVDD to accept a very clean voltage source, independent of the digital voltage supplies.

Additionally, an isolated analog supply ground connection is provided on pin AVSS only on 128-

pin and 144-ball 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 STR91xFA device on pin

AVSS_VSSQ.

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

144-ball packages at the AVREF pin for better accuracy on low voltage inputs. For 80-pin

packages, the ADC reference voltage is tied internally to the ADC unit supply voltage at pin

AVREF_AVDD, meaning the ADC reference voltage is fixed to the ADC unit supply voltage.

See Table 7: Operating conditions, for restrictions to the relative voltage levels of VDDQ, AVDD,

AVREF, and AVREF_AVDD.

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. By configuring the RTC register, 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 STR91xFA device is powered off.

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