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BN87C51FB Datasheet(PDF) 6 Page - Intel Corporation |
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BN87C51FB Datasheet(HTML) 6 Page - Intel Corporation |
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6 / 18 page  AUTOMOTIVE 87C51FAFBFCFC-20 To drive the device from an external clock source XTAL1 should be driven while XTAL2 floats as shown in Figure 5 There are no requirements on the duty cycle of the external clock signal since the in- put to the internal clocking circuitry is through a di- vide-by-two flip-flop but minimum and maximum high and low times specified on the data sheet must be observed An external oscillator may encounter as much as 100 pF load at XTAL1 when it starts up This is due to interaction between the amplifier and its feedback capacitance Once the external signal meets VIL and VIH specifications the capacitance will not exceed 20 pF 270961 – 5 C1 C2 e 30 pF g10 pF for Crystals For Ceramic Resonators contact resonator manufac- turer Figure 4 Oscillator Connections 270961 – 6 Figure 5 External Clock Drive Configuration IDLE MODE The user’s software can invoke the Idle Mode When the microcontroller is in this mode power consump- tion is reduced The Special Function Registers and the onboard RAM retain their values during Idle but the processor stops executing instructions Idle Mode will be exited if the chip is reset or if an en- abled interrupt occurs The PCA timercounter can optionally be left running or paused during Idle Mode POWER DOWN MODE To save even more power a Power Down mode can be invoked by software In this mode the oscillator is stopped and the instruction that invoked Power Down is the last instruction executed The on-chip RAM and Special Function Registers retain their val- ues until the Power Down mode is terminated On the 87C51FAFBFC either a hardware reset or external interrupt can cause an exit from Power Down Reset redefines all the SFRs but does not change the on-chip RAM An external interrupt al- lows both the SFRs and the on-chip RAM to retain their values To properly terminate Power Down the reset or ex- ternal interrupt should not be executed before VCC is restored to its normal operating level and must be held active long enough for the oscillator to restart and stabilize (normally less than 10 ms) With an external interrupt INT0 or INT1 must be en- abled and configured as level-sensitive Holding the pin low restarts the oscillator (the oscillator must be allowed time to stabilize after start up before this pin is released high) but bringing the pin back high com- pletes the exit Once the interrupt is serviced the next instruction to be executed after RETI will be the one following the instruction that put the device into Power Down 6 |
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