AT91SAM7S64 datasheet(29/50 Pages) ATMEL

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6175GS–ATARM–24-Dec-08

AT91SAM7S Series Summary

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 AT91SAM7S 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 con-

troller 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 deac-

tivation is configured through the GPNVM bit 0 of the Flash.

AT91SAM7S64 Datasheet(PDF) 29 Page - ATMEL Corporation