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External Bus Interface (EBI)
MPC5644A Microcontroller Reference Manual, Rev. 6
324
Freescale Semiconductor
During these accesses, the EBI does not drive the TA signal, leaving it up to an external device (or weak
internal pullup) to drive TA.
For EBI-mastered chip-select accesses, when the SETA bit is 0, the EBI drives TA the entire cycle,
asserting according to internal wait state counters to terminate the cycle. When the SETA bit is 1, the EBI
samples the TA for the entire cycle. During idle periods on the external bus, the EBI drives TA negated as
long as it is granted the bus; when it no longer owns the bus, it lets go of TA.
If no device responds by asserting TA within the programmed timeout period (BMT in EBI_BMCR) after
the EBI initiates the bus cycle, the internal Bus Monitor (if enabled) asserts TEA to terminate the cycle.
An external device may also drive TEA when it detects an error on an external transaction. TEA assertion
causes the cycle to terminate and the processor to enter exception processing for the error condition. To
properly control termination of a bus cycle for a bus error with external circuitry, TEA must be asserted at
the same time or before (external) TA is asserted. TEA must be negated before the second rising edge after
it was sampled asserted in order to avoid the detection of an error for the following bus cycle initiated.
TEA is only driven by the EBI during the cycle where the EBI is asserting TEA and the cycle immediately
following this assertion (for fast negation). During all other cycles, the EBI relies on a weak internal pullup
to hold TEA negated. This allows an external device to assert TEA when it needs to indicate an error.
External devices must follow the same protocol as the EBI, only driving TEA during the assertion cycle
and 1 cycle afterwards for negation.
When TEA is asserted from an external source, the EBI uses a latched version of TEA (1 cycle delayed)
to help make timing at high frequencies. This means that for any accesses where the EBI drives TA
(chip-select accesses with SETA=0), a TEA assertion that occurs 1 cycle before or during the last TA of
the access could be ignored by the EBI, since it will have completed the access internally before it detects
the latched TEA assertion. This means that non-burst chip-select accesses with no wait states (SCY=0)
cannot be reliably terminated by external TEA. If external error termination is required for such a device,
the EBI must be configured for SCY>=1.
NOTE
For the cases discussed above where TEA “could be ignored”, this is not
guaranteed. For some small access cases (which always use chip-select and
internally-driven TA), a TEA that occurs 1 cycle before or during the TA
cycle or for SCY=0 may in fact lead to terminating the cycle with error.
However, proper error termination is not guaranteed for these cases, so TEA
must always be asserted at least 2 cycles before an internally-driven TA
cycle for proper error termination.
External TEA assertion that occurs during the same cycle that TS is asserted by the EBI is always treated
as an error (terminating the access) regardless of SCY.
summarizes how the EBI recognizes the termination signals provided from an external device.
Table 14-18. Termination Signals Protocol
TEA
1
Action
Negated
Negated
No Termination
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