ISO-CMOS
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
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.
derived from the two
MT8981D
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.
Mode
Control
Bits
(unused)
Memory
Select
Bits
(unused)
Stream
Address
Bits
7
6
5
4
3
2
1
0
BIT
7
NAME
Split
Memory
DESCRIPTION
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.
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.
6
Message
Mode
(unused)
5
4-3
Memory
0-0 - Not to be used
Select Bits
0-1 - Data Memory (read only from the microprocessor port)
1-0 - Connection Memory Low
1-1 - Connection Memory High
(unused)
Stream
Address
Bits
Must be a 0.
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.
Figure 4 - Control Register Bits
2-21
2
1-0
MITEL [ MITEL NETWORKS CORPORATION ]
