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 2.0 V and care must be taken to return VCC 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 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 10ms).
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
•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.
ONCETM 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 61Hz to 4MHz at a16MHz operating frequency.
To configure the Timer/Counter 2 as a clock generator, bit C/T2 (inT2CON) must be cleared and bit T20E in T2MOD must be set. BitTR2 (T2CON.2) also must be set to start the timer.
The Clock-Out frequency depends on the oscillator frequency andthe reload value of Timer 2 capture registers (RCAP2H, RCAP2L)as shown in this equation:
Oscillator Frequency
4 (65536RCAP2H,RCAP2L)
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.
TIMER 2 OPERATION
Timer 2
Timer 2 is a 16-bit Timer/Counter which can operate as either an event timer or an event counter, as selected by C/T2* in the special function register T2CON (see Figure 1). Timer 2 has three operating modes: Capture, Auto-reload (up or down counting), and Baud Rate Generator. 51单片机英文文献和中文翻译(2):http://www.751com.cn/fanyi/lunwen_51946.html