Applications Information
MOTOROLA
MC68307 USER’S MANUAL
10-9
The wake-up latency is increased slightly to 24 system clocks when a STOP instruction is
used. Note that the MC68307 does not execute the LPSTOP instruction used by some other
Motorola microprocessors (for example, MC68060, MC68340, and MC68360).
All enabled interrupts wake-up the CPU. If the interrupt mask in the CPU is set to a level
greater than the active interrupt, this interrupt will not be processed immediately by the CPU.
This technique gives the absolute minimum wake-up latency, since time-critical code can be
placed immediately after the instruction that stopped the CPU clock, but before the interrupt
mask is lowered to process the interrupt which caused the wake-up.
The MC68307 does not require or generate a reset on wake-up. If the MC68307 is reset the
CPU clock and all the other internal clocks are restarted, however the wake-up latency will
be large since reset exception processing is required.
10.2.4 Low-Power Sleep Mode
When all the internal clocks are stopped, including the CPU, the MC68307 is said to be in
low-power sleep mode.
In low-power sleep mode the external clock, either crystal oscillator or square wave input, is
still toggling. The only logic internal to the MC68307 that is getting clocked is the reset circuit
and interrupt controller. The device can wake-up from low-power sleep mode by an external
interrupt on IRQ7 or any of the INTx inputs. In addition the UART clock can automatically
restart when a falling edge is detected on RxD, and subsequently wake-up the CPU when
an interrupt occurs.
The CPU is arbitrated off the system bus when it enters low-power sleep mode. The address
and data buses are three-stated. For a system that has the MC68307 in this mode for pro-
longed periods of time, the system address and data buses should be pulled up using high
value resistors (e.g., 22–100Kohms depending upon signal loading) to prevent these buses
floating to mid-range voltage levels and hence generating large currents in input buffers.
When a crystal is connected across EXTAL and XTAL pins on the MC68307 the contribution
of the oscillator cell to the sleep-mode current is large. Depending on the type of crystal
used, this bias current can be up to several milliamps. Careful choice of crystal and board
layout can help minimize the effect of the bias current, but it will always make a significant
contribution to sleep current. If an external oscillator is used to generate a square wave input
to EXTAL, the bias current is much less. When an external oscillator is used, the XTAL pin
should be left completely unconnected.
Low-power sleep mode consumes about 4% to 6% of the fully operational power depending
on the configuration of the oscillator pins.
10.2.5 Low-Power Stop Mode
The power consumption of the MC68307 can be minimized by stopping the clock to the
EXTAL pin externally. This is referred to as low-power stop mode. External hardware is
required to control the system clock circuit. The current consumption of the MC68307 is at
an absolute minimum in this mode.