Intel 8XC251SA, 8XC251SP, 8XC251SQ, 8XC251SB, Embedded Microcontroller manual Power-on Reset

MINIMUM HARDWARE SETUP

While the RST pin is high ALE, PSEN#, and the port pins are weakly pulled high. The first ALE occurs 32TOSC after the reset signal goes low. For this reason, other devices can not be synchro- nized to the internal timings of the 8XC251Sx.

NOTE

Externally driving the ALE and/or PSEN# pins to 0 during the reset routine may cause the device to go into an indeterminate state.

Powering up the 8XC251Sx without a reset may improperly initialize the program counter and SFRs and cause the CPU to execute instructions from an undetermined memory location.

11.4.4 Power-on Reset

To automatically generate a reset at power-on, connect the RST pin to the VCC pin through a 1-µF capacitor as shown in Figure 11-1.

When VCC is applied, the RST pin rises to VCC, then decays exponentially as the capacitor charg- es. The time constant must be such that RST remains high (above the turn-off threshold of the Schmitt trigger) long enough for the oscillator to start and stabilize, plus 64TOSC. At power-on, VCC should rise within approximately 10 ms. Oscillator start-up time is a function the crystal fre- quency; typical start-up times are 1 ms for a 10 MHz crystal and 10 ms for a 1 Mhz crystal.

During power-on, the port pins are in a random state until forced to their reset state by the asyn- chronous logic.

Reducing VCC quickly to 0 causes the RST pin voltage to momentarily fall below 0 V. This volt- age is internally limited and does not harm the device.

11-7