
CD1865
—
Intelligent Eight-Channel Communications Controller
108
Datasheet
9.3
Indexed Indirect Registers
Certain registers are specially designed to facilitate service-request handling. These registers do not
exist as distinct registers, and can be thought of as pointers. These registers provide functions that
are valid only during service-request service routines, and they must not be accessed at other times.
Three of the registers are actually pointers to the Transmit and Receive FIFOs, that is, when
referenced they cause the appropriate FIFO to be accessed. These registers are: Receive Data
register, Receive Character Status register, and Transmit Data register.
The CD1865 maintains all channel-specific information. During data transfer between the host and
the CD1865, the CD1865 uses a context-switching technique to switch the proper channel-specific
information into the Global registers for use by the host. This reduces the processing burden on the
host by eliminating the need to calculate address offsets.
9.3.1
Receive Data Count Register
Bit
Description
Bits 7:4
Reserved, must be a
‘
0
’
.
Bit 3
Internally, to the CD1865, this is Address bit 7. This bit completes the external to internal CD1865 register
address mapping, but it is only to be used for test purposes. In normal operation, this bit should always be a
‘
0
’
.
Bits 2:0
Channel number
C2
C1
C0
Channel Number
0
0
0
Channel 0
0
0
1
Channel 1
0
1
0
Channel 2
0
1
1
Channel 3
1
0
0
Channel 4
1
0
1
Channel 5
1
1
0
Channel 6
1
1
1
Channel 7
Register Name:
Register Description: Receive Data Count Register
Default Value: 0
Access: Read Only
Bit 7
Bit 6
8-Bit Hex Address: $07
Intel Hex Address: $0E
Motorola Hex Address: $0F
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0
0
0
0
CT3
CT2
CT1
CT0