AT91SAM7S256 ATMEL Corporation, AT91SAM7S256 Datasheet - Page 18

• Manufacturer Part Number: AT91SAM7S256
• Description: AT91 ARM THUMB-BASED MICROCONTROLLERS
• Manufacturer: ATMEL Corporation

Reset Controller
Brownout Detector and
Power-on Reset
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AT91SAM7S256 Summary Preliminary
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 output. It allows to shape a signal on the NRST line,
guaranteeing that the length of the pulse meets any requirement.
The AT91SAM7S256 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 brown-
outs 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, 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 accu-
racy 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.
6117AS–ATARM–20-Oct-04