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PEB 20321
PEF 20321
Local Bus Interface (LBI)
Data Sheet
159
2001-02-14
LWR and LRD). Bus arbitration is required when the slave also needs to access LBI bus
peripherals.
For this purpose, the external busses of the master and the slave are directly connected
together. However, it must always be assured that at one time only one of them, either
the master or the slave, controls all external bus signals, while the other one drives its
bus pins into an high-impedance state. This arbitration of the external bus is controlled
by the low-level active pins LHOLD (hold request), LHLDA (hold acknowledge), and
LBREQ (bus request) of the two devices.
Note that the definition and function of the bus arbitration signals is different in master
and slave mode. The following table describes these differences.
Table 16
Pin
LHOLD
LBI Bus Arbitration Signals
Direction
Function in Master Mode
Input
While LHOLD is high, the master operates in normal mode.
Upon a high-to-low transition, the master issues a hold
request. The master backs off the bus, activates LHLDA
and goes into hold mode.
A low-to-high transition issues the exit from hold mode. The
master deactivates LHLDA, takes over the bus and enters
normal operation again.
Output
High during normal operation. When the master enters hold
mode, it sets LHLDA to low after releasing the bus. On exit
of hold mode, the master first sets LHLDA to high and then
goes onto the bus again.
Output
High during normal operation. The master activates
LBREQ by setting it to low earliest one TCL after activating
LHLDA if it has to perform an external bus access. If the
master has regained the bus, LBREQ is set to high one TCL
after deactivation of LHLDA.
LHLDA
LBREQ
Pin
LHOLD
Direction
Input
Function in Slave Mode
While both LHOLD and LHLDA are high, the slave is in hold
mode, the bus interface is tristated. When the slave is
released out of hold mode (LHLDA = 0) and has completely
taken control over the external bus, a low level at this pins
requests the slave to go into hold mode again. However, in
any case the slave will perform at least one external bus
cycle before going into hold mode again.