M87C51FC

271114 – 5

Figure 4 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 M87C51FC either a hardware reset or an

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 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 and INT1 must be

enabled and configured as level-sensitive Holding

the pin low restarts the oscillator but bringing the pin

back high completes 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

DESIGN CONSIDERATION

by an opaque label Otherwise the DC and AC

characteristics may not be met and the device

may functionally be impaired

reset the device normally resumes program exe-

cution from where it left off up to two machine

cycles before the internal reset algorithm takes

control On-chip hardware inhibits access to inter-

nal RAM in this event but access to the port pins

is not inhibited To eliminate the possibility of an

unexpected write when Idle is terminated by re-

set the instruction following the one that invokes

Idle should not be one that writes to a port pin or

to external memory

ONCE MODE

The ONCE (‘‘On-Circuit Emulation’’) Mode facilitates

testing

and

debugging

of

systems

using

the

M87C51FC without the M87C51FC having to be re-

moved from the circuit The ONCE Mode is invoked

by

1) Pull ALE low while the device is in reset and

PSEN is high

2) Hold ALE low as RST is deactivated

While the device is in ONCE Mode the Port 0 pins

go into a float state and the other port pins and ALE

and PSEN are weakly pulled high The oscillator cir-

cuit remains active While the M87C51FC is in this

mode an emulator or test CPU can be used to drive

the circuit Normal operation is restored when a nor-

mal reset is applied

Table 1 Status of the External Pins during Idle and Power Down

Mode

Program

ALE

PSEN

PORT0

PORT1

PORT2

PORT3

Memory

Idle

Internal

1

1

Data

Data

Data

Data

Idle

External

1

1

Float

Data

Address

Data

Power Down

Internal

0

0

Data

Data

Data

Data

Power Down

External

0

0

Float

Data

Data

Data

NOTE

For more detailed information on the reduced power modes refer to Application Note AP-255 ‘‘Military CHMOS Designing

with the M80C51BH’’

5