Power supply
AN2339
6/20
Figure 2.
Typical layout for VDD/VSS pair
2.2
Analog supply and reference
The ADC unit on 128-pin packages has an independent A/D Converter Supply and
Reference Voltage. This means that it has an isolated analog voltage supply input at pin
AVDD to accept a very clean voltage source. 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 for further ADC supply
isolation. They offer a separate external analog reference voltage input for the ADC unit on
the AVREF pin for better accuracy on low voltage input, and the voltage on AVREF can
range from 1.0V to VDDQ.
On 80-pin packages, the analog supply is shared with the ADC reference voltage pin, and
the analog ground is shared with the digital ground at a single point in the STR91xF device
on pin AVSS_VSSQ. Also the ADC reference voltage is tied internally to the ADC unit
supply voltage on pin AVCC_AVREF, meaning the ADC reference voltage is fixed to the
ADC unit supply voltage.
2.3 Battery
backup
supply
You can optionally connect a battery to the VBATT pin (2.5V to 3.5V) of the STR91x so that
SRAM contents are automatically preserved when the normal operating voltage on the VDD
pin is lost or drops below the 1.4V threshold. Automatic switchover of VBATT power to
SRAM can be disabled by firmware if you want the battery to power only the RTC and not
the SRAM.
Note:
1
You are advised to ground all unused pins on ports 0-9 to reduce noise susceptibility, noise
generation, and minimize power consumption. There are no internal or programmable pull-
up resistors on ports 0-9.
2
All pins on ports 0-9 are 5V tolerant
3
Pins on ports 0,1,2,4,5,7,8,9 have 4 mA drive and 4 mA sink. Ports 3 and 6 have 8 mA drive
and 8 mA sink.
For more details on the power supply, refer to the STR91xF datasheet.
Via to VSS
Via to VDD
Cap.
V
DD
V
SS
STR91x