ISO-CMOS

MT8981D

2-21

Control Register, which may be written to or read

from via the Control Interface. The lower order bits

come from the address lines directly.

The Control Register also allows the chip to

broadcast messages on all ST-BUS outputs (i.e., to

put every channel into Message Mode), or to split the

memory so that reads are from the Data Memory

and writes are to the Connection Memory Low. The

Connection

Memory

High

determines

whether

individual output channels are in Message Mode,

and allows individual output channels to go into a

high-impedance state, which enables arrays of

MT8981s to be constructed.

It also controls the

CSTo pin.

All

ST-BUS

timing is

derived from the two

signals C4i and F0i.

Software Control

The address lines on the Control Interface give

access to the Control Register directly or, depending

on the contents of the Control Register, to the High

or Low sections of the Connection Memory or to the

Data Memory.

If address line A5 is low, then the Control Register is

addressed regardless of the other address lines (see

Fig. 3). If A5 is high, then the address lines A4-A0

select the memory location corresponding to channel

0-31 for the memory and stream selected in the

Control Register.

The data in the Control Register consists of mode

control bits, memory select bits, and stream address

bits (see Fig. 4). The memory select bits allow the

Connection Memory High or Low or the Data

Memory to be chosen, and the stream address bits

define one of the ST-BUS input or output streams.

Bit 7 of the Control Register allows split memory

operation - reads are from the Data Memory and

writes are to the Connection Memory Low.

The other mode control bit, bit 6, puts every output

channel on every output stream into active Message

Mode; i.e., the contents of the Connection Memory

Low are output on the ST-BUS output streams once

every frame unless the ODE pin is low. In this mode

the chip behaves as if bits 2 and 0 of every

Connection

Memory

High

location

were

1,

regardless of the actual values.

Figure 4 - Control Register Bits

BIT

NAME

DESCRIPTION

7

Split

Memory

When 1, all subsequent reads are from the Data Memory and writes are to the Connection

Memory Low, except when the Control Register is accessed again. When 0, the Memory

Select bits specify the memory for subsequent operations. In either case, the Stream

Address Bits select the subsection of the memory which is made available.

6

Message

Mode

When 1, the contents of the Connection Memory Low are output on the Serial Output

streams except when the ODE pin is low. When 0, the Connection Memory bits for each

channel determine what is output.

5

(unused)

4-3

Memory

Select Bits

0-0 - Not to be used

0-1 - Data Memory (read only from the microprocessor port)

1-0 - Connection Memory Low

1-1 - Connection Memory High

2

(unused)

Must be a 0.

1-0

Stream

Address

Bits

The number expressed in binary notation on these bits refers to the input or output ST-

BUS stream which corresponds to the subsection of memory made accessible for

subsequent operations.

7

6

5

432

1

0

Mode

Control

Bits

(unused)

Memory

Select

Bits

Stream

Address

Bits

(unused)