MC68HC05L28 Datasheet, PDF (103/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

10
RESETS AND INTERRUPTS
10.1
Resets
The MC68HC05L28 can be reset in three ways: by the initial power-on reset function, by an active
low input to the RESET pin and by a COP watchdog timer reset, if the watchdog timer is enabled.
10.1.1 Power-on reset
A power-on reset occurs when a positive transition is detected on VDD. The power-on reset
function is strictly for power turn-on conditions and should not be used to detect drops in the power
supply voltage. The power-on circuitry provides a stabilization delay (t_PORL) from when the
oscillator becomes active. If the external RESET pin is low at the end of this delay then the
processor remains in the reset state until RESET goes high. The user must ensure that the voltage
on VDD has risen to a point where the MCU can operate properly by the time t_PORL has elapsed.
If there is doubt, the external RESET pin should remain low until the voltage on VDD has reached
the specified minimum operating voltage. This may be accomplished by connecting an external
RC-circuit to this pin to generate a power-on reset (POR). In this case, the time constant must be
great enough (at least 100ms) to allow the oscillator circuit to stabilise.
10
10.1.2 RESET pin
When the oscillator is running in a stable state, the MCU is reset when a logic zero is applied to
the RESET input for a minimum period of 1.5 machine cycles (t_CYC). This pin contains an internal
Schmitt trigger as part of its input to improve noise immunity.
MC68HC05L28
RESETS AND INTERRUPTS
TPG
MOTOROLA
10-1