Intel 8XC251SQ 사용자 설명서

11-7

While the RST pin is high ALE, PSEN#, and the port pins are weakly pulled high. The first ALE
occurs 32T

OSC

 after the reset signal goes low. For this reason, other devices can not be synchro-

nized to the internal timings of the 8XC251Sx.

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.

To automatically generate a reset at power-on, connect the RST pin to the V

CC

 pin through a 1-µF

capacitor as shown in Figure 11-1. 

When V

CC

 is applied, the RST pin rises to V

CC

, 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 64T

OSC

. At power-on,

V

CC

 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 V

CC

 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.