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

20 pF

270961 – 5

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-

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