3. STD Bus Interface
27
RESET
The ZT 8904 is automatically reset with a precision voltage monitoring circuit that
detects when Vcc is below the acceptable operating limit of 4.75 V. Other sources of
reset include watchdog timer stage 2, local pushbutton switch, and the STD bus
pushbutton reset signal, PBRESET* (P48).
The ZT 8904 responds to any of these reset sources by initializing local peripherals and
driving the STD bus system reset, SYSRESET* (P47). The ZT 8904 reset is typically
active for 350 milliseconds.
MULTIPLE MASTER AND INTELLIGENT I/O
Ziatech offers the following two architectures for increasing the number of
microprocessors in a single system:
•
Multiple master
•
Intelligent I/O
Applications can use multiple master, intelligent I/O, or a combination of the two.
Multiple Master
A multiple master architecture requires one permanent master and one or more
temporary masters, as illustrated in the "
Multiple Master Architecture
" figure following.
The ZT 8904 is configured for permanent or temporary master operation through the
installation and removal of resistor packs RP16 and RP17. With both resistor packs
installed, the ZT 8904 functions as a permanent master. With both resistor packs
removed, the ZT 8904 functions as a temporary master. The ZT 8903 does not support
multiple master operation.
In a multiple master architecture, each master has complete access to STD bus
resources and operates at full speed when the local CPU is communicating with local
memory and I/O. It is not until the application software attempts an STD bus access that
arbitration occurs.
The ZT 8904 responds to an STD bus access from the application software by
generating an STD bus request, DREQx* (E16), to an external bus arbiter such as the
ZT 89CT39. The ZT 8904 then suspends all local operation until the bus arbiter returns
an STD bus acknowledge, DAKx* (E15). All arbitration is done in hardware on the
external bus arbiter board and is transparent to the application software. The amount of
time required for this arbitration depends on the amount of time higher priority masters
are in control of STD bus resources. A shared resource locking mechanism is supported
to guarantee exclusive access to STD bus memory or I/O.