SAM7S256_14 Datasheet, PDF (28/775 Pages) ATMEL Corporation – ARM-based Flash MCU

9.1 Reset Controller
The Reset Controller is based on a power-on reset cell and one brownout detector. It gives the status of the last reset,
indicating whether it is a power-up reset, a software reset, a user reset, a watchdog reset or a brownout reset. In addition,
it controls the internal resets and the NRST pin open-drain output. It allows to shape a signal on the NRST line,
guaranteeing that the length of the pulse meets any requirement.
Note that if NRST is used as a reset output signal for external devices during power-off, the brownout detector must be
activated.
9.1.1
Brownout Detector and Power-on Reset
The SAM7S Series embeds a brownout detection circuit and a power-on reset cell. Both are supplied with and monitor
VDDCORE. Both signals are provided to the Flash to prevent any code corruption during power-up or power-down
sequences or if brownouts occur on the VDDCORE power supply.
The power-on reset cell has a limited-accuracy threshold at around 1.5V. Its output remains low during power-up until
VDDCORE goes over this voltage level. This signal goes to the reset controller and allows a full re-initialization of the
device.
The brownout detector monitors the VDDCORE level during operation by comparing it to a fixed trigger level. It secures
system operations in the most difficult environments and prevents code corruption in case of brownout on the
VDDCORE.
Only VDDCORE is monitored.
When the brownout detector is enabled and VDDCORE decreases to a value below the trigger level (Vbot-, defined as
Vbot - hyst/2), the brownout output is immediately activated.
When VDDCORE increases above the trigger level (Vbot+, defined as Vbot + hyst/2), the reset is released. The
brownout detector only detects a drop if the voltage on VDDCORE stays below the threshold voltage for longer than
about 1µs.
The threshold voltage has a hysteresis of about 50 mV, to ensure spike free brownout detection. The typical value of the
brownout detector threshold is 1.68V with an accuracy of ± 2% and is factory calibrated.
The brownout detector is low-power, as it consumes less than 20 µA static current. However, it can be deactivated to
save its static current. In this case, it consumes less than 1µA. The deactivation is configured through the GPNVM bit 0 of
the Flash.
SAM7S Series [DATASHEET] 28
6175M–ATARM–26-Oct-12