M68HC16 Z SERIES
SYSTEM INTEGRATION MODULE
USER’S MANUAL
5-69
4. Set the BYTE field to lower byte when using a 16-bit port, as the external vector
for a 16-bit port is fetched from the lower byte. Set the BYTE field to upper byte
when using an 8-bit port.
If an interrupting device does not provide a vector number, an autovector acknowledge
must be generated, either by asserting the AVEC pin or by generating AVEC internally
using the chip-select option register. This terminates the bus cycle.
5.9.4 Chip-Select Reset Operation
The least significant bit of each of the 2-bit chip-select pin assignment fields in
CSPAR0 and CSPAR1 each have a reset value of one. The reset values of the most
significant bits of each field are determined by the states of DATA[7:1] during reset.
There are weak internal pull-up drivers for each of the data lines so that chip-select
operation is selected by default out of reset. However, the internal pull-up drivers can
be overcome by bus loading effects.
To ensure a particular configuration out of reset, use an active device to put the data
lines in a known state during reset. The base address fields in chip-select base ad-
dress registers CSBAR[0:10] and chip-select option registers CSOR[0:10] have the re-
set values shown in
. The BYTE fields of CSOR[0:10] have a reset value of
“disable”, so that a chip-select signal cannot be asserted until the base and option reg-
isters are initialized.
Following reset, the MCU fetches the initial stack pointer and program counter values
from the exception vector table, beginning at $000000 in supervisor program space.
The CSBOOT chip-select signal is used to select an external boot device mapped to
a base address of $000000.
The MSB of the CSBTPA field in CSPAR0 has a reset value of one, so that chip-select
function is selected by default out of reset. The BYTE field in chip-select option register
CSORBT has a reset value of “both bytes” so that the select signal is enabled out of
reset. The LSB of the CSBOOT field, determined by the logic level of DATA0 during
reset, selects the boot ROM port size. When DATA0 is held low during reset, port size
is eight bits. When DATA0 is held high during reset, port size is 16 bits. DATA0 has a
weak internal pull-up driver, so that a 16-bit port is selected by default
Table 5-25 Chip-Select Base and Option Register Reset Values
Fields
Reset Values
Base address
$000000
Block size
2 Kbyte
Async/sync mode
Asynchronous mode
Upper/lower byte
Disabled
Read/write
Disabled
AS/DS
AS
DSACK
No wait states
Address space
CPU space
IPL
Any level
Autovector
External interrupt vector
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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