ÛZIATECH
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11. SYSTEM REGISTERS
The ZT 8907 uses three ports of the onboard 16C50A Digital I/O ASIC device for
system registers. These registers are mapped to I/O ports E3h through E5h and are
illustrated in the following topics.
•
For more information on how the Digital I/O ASIC works, see Chapter 10, "Parallel
I/O," section heading "
•
Refer to Appendix C, "
Digital I/O ASIC System Setup Considerations
," for tips on
preventing latchup from the CMOS parts in the 16C50A.
•
Note that the Digital I/O ASIC inputs are inverting; thus, a logic high (+5 V) will be
read as a logic low (0 V).
The 16C50A operating instructions are outlined below.
•
The reset state for all bits is a logical 0.
•
Unless specifically noted otherwise, bits dedicated to input operation must remain
programmed with a logical 0 to prevent contention with the input device.
•
Bits dedicated to output operation have readback capabilities.
PROGRAMMABLE REGISTERS
The following topics illustrate System Registers 1, 2, and 3.
System Register 1
Digital I/O ASIC System Register 1
7
6
5
4
3
2
1
0
CNT2 CNT1
0
0
0
0
Register:
Address:
Access:
System Register 1
E3h
Read and Write
Revision Jumpers (Input)
Counter/Timer 0 Gate (Output)
0 Logical 1
1 Logical 0
CNT0
Counter/Timer 1 Gate (Output)
0 Logical 1
1 Logical 0
Counter/Timer 2 Gate (Output)
0 Logical 1
1 Logical 0
0
Reserved for Ziatech
ZT8907
†
Always write these bits as zero.
†
†