4-26
AT90S8414
Preliminary
POWER-ON RESET
A Power-On Reset (POR) circuit ensures that the device is not started until the XTAL oscillator has stabilized. As shown
in Figure 25, an internal timer clocked from the Watchdog timer oscillator prevents the MCU from starting until after a
certain period after VCC has reached the Power-On Threshold voltage - V
POT
, regardless of the VCC rise time (see Figure
26 and Figure 27). The total reset period is the Power-On Reset period - t
POR
+ the Delay Time-out period - t
TOUT
.
As the RESET pin is pulled high by an on-chip resistor, the pin can be left unconnected if no external reset is required.
Connecting RESET to VCC will have the same effect. By holding the RESET pin low for a period after VCC has been
applied, the Power-On Reset period can be extended. Refer to Figure 28 for a timing example on this.
Figure 26: MCU Start-Up, RESET Tied to VCC or Unconnected. Rapidly Rising VCC
Figure 27: MCU Start-Up, RESET Tied to VCC or Unconnected. Slowly Rising VCC
