Master Bus Operation
3-31
ColdFire2/2M User’s Manual
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA
Clock 2 (C2)
During the first half of C2, the ColdFire2/2M negates
MTSB
. The interrupting device uses
the
MRWB
and
MSIZ
signals to place the vector number on the high-order byte of the
master read data bus (
MRDATA
[31:24]) and assert the master input enable (
MIE
) signal.
Concurrently, the interrupting device asserts the master transfer acknowledge (
MTAB
)
signal. At the end of C2, the ColdFire2/2M samples the level of MTAB and latches the
current value on MRDATA[31:24]. If MTAB is asserted, the transfer terminates. If MTAB is
not asserted, the ColdFire2/2M ignores the data and inserts wait states instead of
terminating the transfer. The ColdFire2/2M continues to sample MTAB on successive rising
edges of
CLK
until it is asserted. The interrupting device negates the MIE and MTAB signals
in the first half of the next CLK cycle.
3.7.2 Spurious Interrupt Acknowledge Bus Cycle
When a device does not respond to an interrupt acknowledge bus cycle with
MTAB
, the
external logic typically returns the transfer error acknowledge signal (
MTEAB
). In this case,
it is the responsibility of the external logic to return the spurious interrupt vector number 24
($18) on
MRDATA
[31:24] and assert
MIE
. The vector number will be latched on the first
rising edge of
CLK
after MTEAB is asserted. Because the spurious interrupt vector number
is returned on MRDATA[31:24] in the same manor as a normal interrupt acknowledge,
MTAB may be asserted instead of MTEAB.
3.8 MASTER BUS EXCEPTION CONTROL CYCLES
The ColdFire2/2M bus architecture requires assertion of
MTAB
from an external device to
signal that a bus cycle is complete. MTAB is not asserted in the following cases:
The external device does not respond to a normal bus cycle.
No interrupt vector is provided during an interrupt acknowledge cycle.
Various other application-dependent errors occur.
External circuitry should assert
MTEAB
when no device asserts
MTAB
within an appropriate
period of time after the ColdFire2/2M begins the bus cycle. This terminates the cycle and
allows the ColdFire2/2M to enter exception processing for the error condition.
To properly control termination of a bus cycle for a access error,
MTAB
and/or
MTEAB
must
be asserted and negated for the same rising edge of
CLK
.
Table 3-9
lists the control signal
combinations and the resulting bus cycle terminations. Note that the access error Exception
taken upon an
MTEAB
assertion cannot be masked and will occur for both reads and writes
that result in
MTEAB
being asserted. access error terminations during burst cycles operate
as described in
Section 3.3.4 Line Read Transfer
and
Section 3.3.5 Line Write
Transfers
.
F
Freescale Semiconductor, Inc.
n
.