Z89136 datasheet(21/66 Pages) ZILOG

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Z89135/136 (ROMless)

Zilog

Low-Cost DTAD Controller

DS97TAD0300

P R E L I M I N A R Y

1-21

Port 0. (P07-P00). Port 0 is an 8-bit, bidirectional, CMOS-

compatible port. These eight I/O lines are configured un-

der software control as a nibble I/O port, or as an address

port for interfacing external memory. The input buffers are

Schmitt-triggered and the output drivers are push-pull.

Port 0 is placed under handshake control. In this configu-

ration, Port 3, lines P32 and P35 are used as the hand-

shake control /DAV0 and RDY0. Handshake signal direc-

tion is dictated by the I/O direction to Port 0 of the upper

nibble P07-P04. The lower nibble must have the same di-

rection as the upper nibble.

The Auto Latch on Port 0 puts valid CMOS levels on all

CMOS inputs which are not externally driven. Whether this

level is 0 or 1, cannot be determined. A valid CMOS level,

rather than a floating node, reduces excessive supply cur-

rent flow in the input buffer.

For external memory references, Port 0 provides address

bits A11-A8 (lower nibble) or A15-A8 (lower and upper nib-

ble) depending on the required address space. If the ad-

dress range requires 12 bits or less, the upper nibble of

Port 0 can be programmed independently as I/O while the

lower nibble is used for addressing. If one or both nibbles

are needed for I/O operation, they are configured by writ-

ing to the Port 0 mode register.

In ROMless mode, after a hardware reset, Port 0 is config-

ured as address lines A15-A8, and extended timing is set

to accommodate slow memory access. The initialization

routine can include reconfiguration to eliminate this ex-

tended timing mode. (In ROM mode, Port 0 is defined as

input after reset.)

Port 0 is set in the high-impedance mode if selected as an

address output state along with Port 1 and the control sig-

nals /AS, /DS and R//W (Figure 10).

Figure 10. Port 0 Conﬁguration

OEN

Out

PAD

Auto Latch

Port 0

(I/O or A15 - A8)

Handshake Controls

/DAV0 and RDY0

(P32 and P35)

Z89135/136

MCU

R = 500 K

Ω

1.5

2.3V Hysteresis

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