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20.6.4
IOM-2 Mode
The AAI can operate in a special IOM-2 compatible mode to
allow to connect to an external ISDN controller device. In
this IOM-2 mode, the AAI can only operate as a slave, i.e.
the bit clock and frame sync signal is provided by the ISDN
controller. The AAI only supports the B1 and B2 data of the
IOM-2 channel 0, but ignores the other two IOM-2 channels.
The AAI handles the B1 and B2 data as one 16-bit data
word.
Figure 74 shows the connections between an ISDN control-
ler and a CP3BT26 using a standard IOM-2 interface for the
B1/B2 data communication and the external bus interface
(IO Expansion) for controlling the ISDN controller.
SCK
SFS
CP3BT2 x
STD
SRD
A[7:0]
D[7:0]
SELIO
RD
Figure 74. CP3BT26/ISDN Controller Connections
www.national.com
SFS
STD/SRD
B1
B2
M
C
IOM-2 Channel 0
Figure 73. IOM-2 Frame Structure
Bit Clock
Frame Sync
ISDN C ontroller
Data In
Data Out
Address
Data
Chip Select
Output Enable
The IOM-2 interface has the following properties:
Bit clock of 1536 kHz (output from the ISDN controller)
Frame repetition rate of 8 ksps (output from the ISDN
controller)
Double-speed bit clock (one data bit is two bit clocks
wide)
B1 and B2 data use 8-bit log PCM format
Long frame sync pulse
Figure 73 shows the structure of an IOM-2 Frame.
IC1
IC2
M
C
IOM-2 Channel 1
IOM-2 Channel 2
IOM-2 Frame (125 µs)
To connect the AAI to an ISDN controller through an IOM-2
compatible interface, the AAI needs to be configured in this
way:
The AAI must be in IOM-2 Mode (AGCR.IOM2 = 1).
The AAI operates in synchronous mode (AGCR.ASS =
0).
The AAI operates as a slave, therefore the bit clock and
frame sync source selection must be set to external
(ACGR.IEFS = 1, ACGR.IEBC = 1).
The frame sync length must be set to long frame sync
(ACGR.FSS = 1).
The data word length must be set to 16-bit (AGCR.DWL
= 1).
The AAI must be set to normal mode (AGCR.SCS[1:0] =
0).
The internal frame rate must be 8 ksps (ACCR = 00BE).
20.6.5
Loopback Mode
In loopback mode, the STD and SRD pins are internally
connected together, so data shifted out through the ATSR
register will be shifted into the ARSR register. This mode
may be used for development, but it also allows testing the
transmit and receive path without external circuitry, for ex-
ample during Built-In-Self-Test (BIST).
DS241
150
C
DS162
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