Applications Information
10-2
MC68307 USER’S MANUAL
MOTOROLA
IRQ7 is used as a non-maskable software abort interrupt source, driven by a debounced
abort button. Whenever the button is pressed, a level 7 interrupt is asserted to the MC68307.
At the next instruction boundary the interrupt request is acknowledged, with the MC68307
generating the corresponding interrupt vector. In fact, the MC68307 always supplies all its
own interrupt vectors, whether the interrupt request is generated internally (by on-chip mod-
ules), or externally (IRQ7, INT8–INT1). Therefore, the interrupt vector numbers should be
set up in the MC68307’s PIVR to point to the appropriate vector location in memory.
DTACK is a bidirectional open drain line used to terminate bus accesses. In this design,
DTACK is generated internally and asserted as an output using the MC68307’s internal chip
select and wait state control. External decode may also assert DTACK as an input.
AS, UDS, LDS, and R/W control signals are all outputs. As the main asynchronous bus con-
trols, each line has a pull-up resistor such that they are in the inactive state when they are
not actively driven. They are not driven during reset, arbitration or CPU stop periods.
Chip select outputs (CS0, CS1, CS2 and CS3) are always driven and do not need pull-up
resistors.
The 8051-compatible control lines (ALE, RD, WR) are all unused outputs.
The JTAG input lines (TDI, TMS, TCK) are not used, so are pulled inactive through resistors.
The TDO output is not connected.
Port A and port B signals default to general purpose inputs after reset. If they are to remain
unconnected, they should be reprogrammed as outputs. Otherwise pull-up resistors are
necessary to ensure known input logic levels. All the port lines except PB2–PB5 remain
unconnected and are reprogrammed as outputs. Port lines PB2–PB5 are used in their ded-
icated UART function. The TXD and RTS output lines use pull-up resistors to ensure logic
levels between booting-up as a general purpose input and reprogramming as a UART out-
put.
A23 uses a pull-up resistor to select normal MC68307 processor mode at power-up. If
instead it is pulled low through a resistor, a slave mode is selected. Do not connect A23 to
a supply rail directly. Always use a pull-up/pull-down resistor.
Address/data/control buses are three-stated when the MC68307 is arbitrated off the bus,
held in reset, or the CPU is stopped (via LPEN bit in SCR). If the buses are to be three-stated
for an extended period of time, pull-up resistors are recommended for these lines. The pull-
up resistor value is typically in the range 20k to 50k ohms, subject to leakage current and
loading requirements.