Functional Description - Microchip Technology megaAVR 0 Series Manual

11.3

Functional Description

11.3.1 Initialization
The Reset Controller (RSTCTRL) is always enabled, but some of the Reset sources must be enabled
(either by fuses or by software) before they can request a Reset.
After any Reset, the Reset source that caused the Reset is found in the Reset Flag register
(RSTCTRL.RSTFR).
After a Power-on Reset, only the POR flag will be set.
The flags are kept until they are cleared by writing a '1' to them.
After Reset from any source, all registers that are loaded from fuses are reloaded.
11.3.2 Operation
11.3.2.1 Reset Sources
There are two kinds of sources for Resets:
• Power supply Resets, which are caused by changes in the power supply voltage: Power-on Reset
(POR) and Brown-out Detector (BOD).
• User Resets, which are issued by the application, by the debug operation, or by pin changes
(Software Reset, Watchdog Reset, UPDI Reset, and external Reset pin RESET).
Power-On Reset (POR)
A Power-on-Reset (POR) is generated by an on-chip detection circuit. The POR is activated when the
V rises until it reaches the POR threshold voltage. The POR is always enabled and will also detect
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when the V falls below the threshold voltage.
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Brown-Out Detector (BOD) Reset
The on-chip Brown-out Detection circuit will monitor the V
fixed trigger level. The trigger level for the BOD can be selected by fuses. If BOD is unused in the
application it is forced to a minimum level in order to ensure a safe operation during internal Reset and
chip erase.
Software Reset
The software Reset makes it possible to issue a system Reset from software. The Reset is generated by
writing a '1' to the Software Reset Enable bit (SWRE) in the Software Reset register (RSTCTRL.SWRR).
The Reset will take place immediately after the bit is written and the device will be kept in reset until the
Reset sequence is completed.
External Reset
The external Reset is enabled by a fuse, see the RSTPINCFG field in FUSE.SYSCFG0.
When enabled, the external Reset requests a Reset as long as the RESET pin is low. The device will stay
in Reset until RESET is high again.
Watchdog Reset
The Watchdog Timer (WDT) is a system function for monitoring correct program operation. If the WDT is
not reset from software according to the programmed time-out period, a Watchdog Reset will be issued.
See the WDT documentation for further details.
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2018 Microchip Technology Inc.
level during operation by comparing it to a
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Datasheet Preliminary
®
megaAVR 0-Series
Reset Controller (RSTCTRL)
DS40002015A-page 100