Intel Microcontroller, 80C196NU, 8XC196NP manual Special Operating Mode Signals and Registers, Once

CHAPTER 12

SPECIAL OPERATING MODES

The 8XC196NP and 80C196NU provide the following power saving modes: idle, standby (80C196NU only), and powerdown. They also provide an on-circuit emulation (ONCE) mode that electrically isolates the device from the other system components. This chapter describes each mode and explains how to enter and exit each. (Refer to Appendix A for descriptions of the instructions discussed in this chapter, to Appendix B for descriptions of signal status during each mode, and to Appendix C for details about the registers.)

12.1 SPECIAL OPERATING MODE SIGNALS AND REGISTERS

Table 12-1 lists the signals and Table 12-2 lists the registers that are mentioned in this chapter.

Table 12-1. Operating Mode Control Signals

Port Pin	Signal	Type	Description
	Name
P2.7	CLKOUT	O	Clock Output
			Output of the internal clock generator. The CLKOUT frequency is ½
			the internal operating frequency (f). CLKOUT has a 50% duty cycle.
			CLKOUT is multiplexed with P2.7.
P3.7	EXTINT3	I	External Interrupts
P3.6	EXTINT2		In normal operating mode, a rising edge on EXTINTx sets the
P2.4	EXTINT1
			EXTINTx interrupt pending bit. EXTINTx is sampled during phase 2
P2.2	EXTINT0
			(CLKOUT high). The minimum high time is one state time.
			In standby and powerdown modes, asserting the EXTINTx signal for
			at least 50 ns causes the device to resume normal operation. The
			interrupt need not be enabled, but the pin must be configured as a
			special-function input (see “Bidirectional Port Pin Configurations” on
			page 7-7). If the EXTINTx interrupt is enabled, the CPU executes the
			interrupt service routine. Otherwise, the CPU executes the instruction
			that immediately follows the command that invoked the power-saving
			mode.
			In idle mode, asserting any enabled interrupt causes the device to
			resume normal operation.
—	ONCE	I	On-circuit Emulation
			Holding ONCE high during the rising edge of RESET# places the
			device into on-circuit emulation (ONCE) mode. This mode puts all
			pins into a high-impedance state, thereby isolating the device from
			other components in the system. The value of ONCE is latched when
			the RESET# pin goes inactive. While the device is in ONCE mode,
			you can debug the system using a clip-on emulator. To exit ONCE
			mode, reset the device by pulling the RESET# signal low. To prevent
			accidental entry into ONCE mode, connect the ONCE pin to VSS.

12-1