mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 175

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

5.9.2 Chip-Select Operation
5.9.3 Using Chip-Select Signals for Interrupt Acknowledge
MC68HC16Y3/916Y3
USER’S MANUAL
When the MCU makes an access, enabled chip-select circuits compare the following
items:
When a match occurs, the chip-select signal is asserted. Assertion occurs at the same
time as AS or DS assertion in asynchronous mode. Assertion is synchronized with
ECLK in synchronous mode. In asynchronous mode, the value of the DSACK field de-
termines whether DSACK is generated internally. DSACK[3:0] also determines the
number of wait states inserted before internal DSACK assertion.
The speed of an external device determines whether internal wait states are needed.
Normally, wait states are inserted into the bus cycle during S3 until a peripheral as-
serts DSACK. If a peripheral does not generate DSACK, internal DSACK generation
must be selected and a predetermined number of wait states can be programmed into
the chip-select option register. Refer to the SCIM Reference Manual (SCIMRM/AD) for
further information.
Ordinary bus cycles use supervisor or user space access, but interrupt acknowledge
bus cycles use CPU space access. Refer to 5.6.4 CPU Space Cycles and 5.8 Inter-
rupts for more information. There are no differences in flow for chip selects in each
type of space, but base and option registers must be properly programmed for each
type of external bus cycle.
During a CPU space cycle, bits [15:3] of the appropriate base register must be config-
ured to match ADDR[23:11], as the address is compared to an address generated by
the CPU. ADDR[23:20] follow the state of ADDR19 in this MCU. The states of base
register bits [15:12] must match that of bit 11.
Figure 5-22 shows CPU space encoding for an interrupt acknowledge cycle. FC[2:0]
are set to %111, designating CPU space access. ADDR[3:1] indicate interrupt priority,
and the space type field (ADDR[19:16]) is set to %1111, the interrupt acknowledge
code. The rest of the address lines are set to one.
• Function codes to SPACE fields, and to the IP mask if the SPACE field encoding
• Appropriate address bus bits to base address fields.
• Read/write status to R/W fields.
• ADDR0 and/or SIZ[1:0] bits to BYTE field (16-bit ports only).
• Priority of the interrupt being acknowledged (ADDR[3:1]) to IPL fields (when the
is not for CPU space.
access is an interrupt acknowledge cycle).
Freescale Semiconductor, Inc.
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MOTOROLA
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