Signal Description
2-10
MC68307 USER’S MANUAL
MOTOROLA
2.4.2 Power-On Reset (RSTIN)
This active-low input signal causes the MC68307 (processor, SIM, and peripherals) to enter
the reset state (cold reset). The assertion of RSTIN causes RESET to be asserted out to
reset external circuitry; however, note that HALT is not asserted as a result of RSTIN asser-
tion. Internal power-on reset circuitry provides a reset pulse of at least 32768 clocks width
at power-on or when RSTIN is subsequently asserted. This reset pulse length can be
extended by adding an external RC network, to ensure that RSTIN is held low for long
enough to apply the reset pulse for 128 CPU clocks after VCC and clock are stable. Refer to
Section 10.1 MC68307 Minimum Stand-Alone System Hardware for an example circuit.
2.4.3 Halt (HALT)
This active-low bidirectional signal can be asserted with RESET to cause a cold reset as
above. If asserted alone, it causes the processor to stop after completion of the current bus
cycle. As long as HALT is held asserted, all bus control signals go to their inactive state,
except BGACK, and all three-state bus signals (A23–A0, D15–D0 only) are placed in the
high-impedance state. When the processor has stopped executing instructions (e.g., after a
double bus fault), the MC68307 asserts this signal.
2.4.4 Bus Request (BR/PA5)
This input signal indicates to the MC68307 that an external device desires to become the
bus master on the MC68307 external bus. Refer to Section 3 Bus Operationfor details of
the bus arbitration features. When programmed as general-purpose input/output, this signal
functions as bit 5 of port A.
2.4.5 Bus Grant (BG/PA6)
This output signal indicates to all external bus master devices (if any) that the MC68307
releases bus control at the end of the current bus cycle to an external requesting bus master.
During cold reset, BG reflects the value of BR. When programmed as general-purpose input/
output, this signal functions as bit 6 of port A.
2.4.6 Bus Grant Acknowledge (BGACK /PA7)
This input signal indicates that some other device besides the MC68307 has become the
bus master. When programmed as general-purpose input/output, this signal functions as bit
7 of port A.
2.5 CLOCK SIGNALS
The following paragraphs describe the clock signals.
2.5.1 Crystal Oscillator (EXTAL, XTAL)
This input provides two clock generation options (crystal and external clock). EXTAL may
be used with XTAL to connect an external crystal to the on-chip oscillator and clock gener-
ator. If an external clock is used, the clock source should be connected to EXTAL, and XTAL
must be left unconnected. The oscillator uses an internal frequency equal to the external
crystal frequency. The frequency of EXTAL may range from DC to 16.67 MHz, although if a