The eight I/O bank control registers determine the number of I/O wait states applied to an external I/O access within the zone controlled by each register even if the associated strobes are not enabled. Note that the /IORD and /IOWR signals reflect these registers as well.

The control over the generation of wait states is independent of whether or not the associ- ated strobe in Port E is enabled. The upper 2 bits of each register determine the number of wait states. The four choices are 1, 3, 7, or 15 wait states. On reset, the bits are cleared, resulting in 15 wait states. There is always at least one external I/O wait state, and thus the minimum external I/O read cycle is three clocks long. The inhibit write function applies to both the Port E write strobes and the /IOWR signal.

These control bits have no effect on the internal I/O space, which does not have wait states associated with read or write access. Internal I/O read or write cycles are two clocks long.

The I/O strobes greatly simplify the interfacing of external devices. On reset, the upper 5 bits of each register are cleared. Parallel Port E will not output these signals unless the data-direction register bits are set for the desired output positions. In addition, the Port E function register must be set to "1" for each position.

Each I/O bank is selected by the three most significant bits of the 16-bit I/O address. Table 10-2shows the relationship between the I/O control register and its corresponding space in the 64K address space.

Table 10-2. External I/O Register Address Range and Pin Mapping

Control Register

Port E

I/O Address

I/O Address

 

Pin

A[15:13]

Range

IB0CR

PE0

000

0x0000–0x1FFF

 

 

 

 

IB1CR

PE1

001

0x2000–0x3FFF

 

 

 

 

IB2CR

PE2

010

0x4000–0x5FFF

 

 

 

 

IB3CR

PE3

011

0x6000–0x7FFF

 

 

 

 

IB4CR

PE4

100

0x8000–0x9FFF

 

 

 

 

IB5CR

PE5

101

0xA000–0xBFFF

 

 

 

 

IB6CR

PE6

110

0xC000–0xDFFF

 

 

 

 

IB7CR

PE7

111

0xE000–0xFFFF

 

 

 

 

User’s Manual

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Page 156
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Jameco Electronics 3000, 2000 manual External I/O Register Address Range and Pin Mapping