AT91SAM7S64 datasheet(25/42 Pages) ATMEL

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6175BS–ATARM–04-Nov-05

AT91SAM7S Series Summary

10.2

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.

10.2.1

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, as a voltage drop on VDDFLASH or any other power supply of

the device cannot affect the Flash.

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) 25 Page - ATMEL Corporation