Freescale Semiconductor, Inc.
last falling edge of the clock for that bus cycle. For a write cycle, all 16 bits of the data bus
are driven, regardless of the port width or operand size. The MC68340 places the data on
the data bus approximately one-half clock cycle after AS is asserted in a write cycle.
2.4 FUNCTION CODES (FC3–FC0)
These signals are outputs that indicate one of 16 address spaces to which the address
applies. Fifteen of these spaces are designated as either user or supervisor, program or
data, and normal or direct memory access (DMA) spaces. One other address space is
designated as CPU space to allow the CPU32 to acquire specific control information not
normally associated with read or write bus cycles. The function code signals are valid
while AS is asserted. See Table 2-2 for more information.
Table 2-2. Address Space Encoding
Function Code Bits
3
0
0
0
0
0
0
0
0
1
2
0
0
0
0
1
1
1
1
x
1
0
0
1
1
0
0
1
1
x
0
0
1
0
1
0
1
0
1
x
Address Spaces
Reserved (Motorola)
User Data Space
User Program Space
Reserved (User )
Reserved (Motorola)
Supervisor Data Space
Supervisor Program Space
CPU Space
DMA Space
2.5 CHIP SELECTS (CS3–CS0)
These pins can be programmed to be chip select output signals, port B parallel I/O and
autovector input, or additional interrupt request lines. Refer to Section 4 System
Integration Module for more information on these signals.
CS3– CS0
The chip select output signals enable peripherals at programmed addresses. These
signals are inactive high (not high impedance) after reset. CS0 is the chip select for a
boot ROM containing the reset vector and initialization program. It functions as the boot
chip select immediately after reset.
IRQ4, IRQ2, IRQ1
Interrupt request lines are external interrupt lines to the CPU32. These additional
interrupt request lines are selected by the FIRQ bit in the module configuration register.
MOTOROLA
MC68340 USER’S MANUAL
2- 5
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