MT90812 Mitel Networks Corporation, MT90812 Datasheet - Page 42

• Manufacturer Part Number: MT90812
• Description: Integrated Digital Switch (IDX)
• Manufacturer: Mitel Networks Corporation

MT90812
the MT90812 microport. TxCEN output signal is enabled for one to eight bits per channel per frame, depending
upon the selected baud rate.
The desired active channel is selected by the system via a write to the MT90812 device’s Next Transmit
Channel (NTX) bits defined in HRA CTRL Register 3 (HC3). A write to this register will start TxCEN to be
enabled for the channel specified as soon as the transmitter becomes inactive.
The NTX bits are double buffered which allows the system to specify the next transmit packet at any time,
without concern for whether the present transmit packet is finished.
At the start of the transmission of a packet the following three actions are taken:
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Latching the system's NTX request into the PTX (present transmit channel) register redefines the active
transmit channel time. When the status bit TXCHNL is set, this informs the system that its transmit channel
request has been satisfied. When the transmitter-active flag (TXACT) is set, this allows TxCEN to be enabled
during the appropriate bit times.
13.3.2
A transmit packet will be terminated by a transmit end-of-packet (TEOP) strobe from the HDLC controller chip
or by setting the STEOP flag through the HRA CTRL Register 2 (HC2). The HDLC controller chip asserts
TEOP for one bit period during the last bit of the closing flag of the transmit packet. The system may, at any
time, raise its own STEOP flag through the HRA CTRL Register 2 (HC2). When the STEOP flag has been read
by the system, it will be automatically cleared.
In either case, the end-of-packet signal will cause the transmitter to go inactive, thus allowing another packet
transmission to be initiated (if desired). Whenever TXACT is low, the HDLC controller's transmit clock enable
TxCEN is disabled, and idles are transmitted for the unused remainder (if any) of the D-channel time.
13.3.3
Transmit and receive functions are generally independent of each other. But there is a coupling of the two
functions caused by the requirement for a go-ahead handshake. This handshake must be sent by the MT90812
when a peripheral requests to transmit to the system. While the MT90812 is generating and transmitting this
go-ahead on the channel reserved for the specific peripheral, it pre-empts the system's D-channel transmit
time. This interference occurs for that particular peripheral only, for the duration of the clear-to-send (CTS)
nine-bit go-ahead pattern.
If the system transmits a packet to a peripheral which is also currently being sent a go-ahead this would result
in the loss of data from the system transmit packet. To prevent data loss a number of approaches can be taken.
After specifying the NTX channel, and waiting until TXCHNL goes high, the system can read the present
receive channel (PRX). If PRX is the same as NTX, then the system may either:
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In dedicated receive mode, where the generation and transmission of CTS by the MT90812 is inhibited, no
possibility of contention exists between the system and the MT90812. So such restrictions need not apply.
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the NTX bits are latched into the PTX (present transmit channel) bits in HRA Status 4 (HS4) register.
status bit TXCHNL is set,
a transmitter-active flag (TXACT) is set
send the packet anyway and retransmit on request,
assert a software transmit end-of-packet (STEOP) to terminate the system's request for that particular
transmit channel,
wait for a maximum of five frames (at a 16K baud rate) for the CTS to complete,
or monitor CTSACT until it goes low.
End of the Transmission of a Packet
TX and RX Handshaking
Advance Information