ATAR080 datasheet(10/68 Pages) ATMEL

10 / 68 page

ATAR080

4675D–4BMCU–12/04

Master Reset

The master reset forces the CPU into a well-defined condition. It is unmaskable and is

activated independent of the current program state. It can be triggered by either initial

supply power-up, a short collapse of the power supply, the brown-out detection circuitry,

an watchdog time-out, or an external input clock supervisor stage (see Figure 9).

A master reset activation will reset the interrupt enable flag, the interrupt pending regis-

ter and the interrupt active register. During the power-on reset phase, the I/O bus control

signals are set to reset mode thereby initializing all on-chip peripherals. All bi-directional

ports are set to input mode.

Attention: During any reset phase, the BP20/NTE input is driven towards V

DD by an

additional internal strong pull-up transistor. This pin must not be pulled down to V

SS dur-

ing reset by any external circuitry representing a resistor of less than 150 k

Ω.

Releasing the reset results in a short call instruction (opcode C1h) to the ROM address

008h. This activates the initialization routine $RESET which in turn has to initialize all

necessary RAM variables, stack pointers and peripheral configuration registers.

Figure 9. Reset Configuration

Power-on Reset and

Brown-out Detection

The ATAR080 has a fully integrated power-on reset and brown-out detection circuitry.

For reset generation no external components are needed.

These circuits ensure that the core is held in the reset state until the minimum operating

supply voltage has been reached. A reset condition will also be generated should the

supply voltage drop momentarily below the minimum operating level except when a

power-down mode is activated (the core is in SLEEP mode and the peripheral clock is

stopped). In this power-down mode the brown-out detection is disabled.

Two values for the brown-out voltage threshold are programmable via the BOT bit in the

SC register.

Reset

timer

Power-on

reset

Internal

reset

res

CL = SYSCL/4

Brown-out

detection

Watch-

dog

CWD

res

Ext. clock

supervisor

ExIn

Pull-up

NRST

ATAR080 Datasheet(PDF) 10 Page - ATMEL Corporation

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