Philips Semiconductors Product data
P87C51RA2/RB2/RC2/RD2
80C51 8-bit microcontroller family 8KB/16KB/32KB/64KB OTP
with 512B/1KB RAM, low voltage (2.7 to 5.5 V), low power, high
speed (30/33 MHz)
2003 Jan 24
12
LOW POWER MODES
Stop Clock Mode
The static design enables the clock speed to be reduced down to
0 MHz (stopped). When the oscillator is stopped, the RAM and
Special Function Registers retain their values. This mode allows
step-by-step utilization and permits reduced system power
consumption by lowering the clock frequency down to any value. For
lowest power consumption the Power Down mode is suggested.
Idle Mode
In the idle mode (see Table 2), the CPU puts itself to sleep while all
of the on-chip peripherals stay active. The instruction to invoke the
idle mode is the last instruction executed in the normal operating
mode before the idle mode is activated. The CPU contents, the
on-chip RAM, and all of the special function registers remain intact
during this mode. The idle mode can be terminated either by any
enabled interrupt (at which time the process is picked up at the
interrupt service routine and continued), or by a hardware reset
which starts the processor in the same manner as a power-on reset.
Power-Down Mode
To save even more power, a Power Down mode (see Table 2) 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 values down to 2 V and care must be taken to return V
CC
to the
minimum specified operating voltages before the Power Down Mode
is terminated.
Either a hardware reset or external interrupt can be used to exit from
Power Down. Reset redefines all the SFRs but does not change the
on-chip RAM. An external interrupt allows both the SFRs and the
on-chip RAM to retain their values.
To properly terminate Power Down, the reset or external interrupt
should not be executed before V
CC
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 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.
POWER-ON FLAG
The Power-On Flag (POF) is set by on-chip circuitry when the V
CC
level on the P87C51RA2/RB2/RC2/RD2 rises from 0 to 5 V. The
POF bit can be set or cleared by software allowing a user to
determine if the reset is the result of a power-on or a warm start
after powerdown. The V
CC
level must remain above 3 V for the POF
to remain unaffected by the V
CC
level.
Design Consideration
When the idle mode is terminated by a hardware reset, the device
normally resumes program execution, from where it left off, up to
two machine cycles before the internal reset algorithm takes control.
On-chip hardware inhibits access to internal 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 reset, 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 without the device having to be removed 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 circuit remains active. While the device is in this
mode, an emulator or test CPU can be used to drive the circuit.
Normal operation is restored when a normal reset is applied.
Programmable Clock-Out
A 50% duty cycle clock can be programmed to come out on P1.0.
This pin, besides being a regular I/O pin, has two alternate
functions. It can be programmed:
1. to input the external clock for Timer/Counter 2, or
2. to output a 50% duty cycle clock ranging from 61 Hz to 4 MHz at a
16 MHz operating frequency in 12-clock mode (122 Hz to 8 MHz in
6-clock mode).
To configure the Timer/Counter 2 as a clock generator, bit C/T
2 (in
T2CON) must be cleared and bit T20E in T2MOD must be set. Bit
TR2 (T2CON.2) also must be set to start the timer.
The Clock-Out frequency depends on the oscillator frequency and
the reload value of Timer 2 capture registers (RCAP2H, RCAP2L)
as shown in this equation:
Oscillator Frequency
n (65536 * RCAP2H, RCAP2L)
n = 2 in 6-clock mode
4 in 12-clock mode
Where (RCAP2H,RCAP2L) = the content of RCAP2H and RCAP2L
taken as a 16-bit unsigned integer.
In the Clock-Out mode Timer 2 roll-overs will not generate an
interrupt. This is similar to when it is used as a baud-rate generator.
It is possible to use Timer 2 as a baud-rate generator and a clock
generator simultaneously. Note, however, that the baud-rate and the
Clock-Out frequency will be the same.
Table 2. External Pin Status During Idle and Power-Down Mode
MODE PROGRAM MEMORY ALE PSEN PORT 0 PORT 1 PORT 2 PORT 3
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
