MT90812 Mitel Networks Corporation, MT90812 Datasheet - Page 44

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

MT90812
14.1.1
The RX circuit performs two functions. As with the TX circuit, it must generate the proper HDLC receive clock
enable signal (RxCEN). This signal has the same characteristics as TxCEN.
The RxCEN signal enables the HDLC receiver in the appropriate channel as specified by the system, via the
MT90812 microport. RxCEN is enabled for one to eight bits per channel per frame, depending upon the
selected baud rate, specified in HRA CTRL Register 1(HC1).
The desired active channel corresponding to a specific D-Channel is determined from either the Dedicated RX
Channel (DRX) bits in HRA CTRL Register 2 (HC2) or the Present Receive Channel (PRX) bits in HRA Status
registers in dedicated and multiplexed modes, respectively.
14.1.2
When the receiver is operated in Dedicated Receive mode, the Auto-hunt circuitry is ignored and the system
provides the channel number. Dedicated Receive mode is selected by setting DDRX bit in HRA CTRL Register
2 (HC2). When DDRX bit is set high it enables dedicated reception from the channel selected by the DRXi bits
in HRA CTRL Register 2 (HC2).
In Dedicated Receive mode RXCHNL and RXACT bits will be held high. The CTSACT bit will be held low,
disabling the CTS generator. These three status bits are in HRA Status 1 (HS1).
A write to the DRX bits will start RxCEN to be enabled for the channel specified as soon as the receiver
becomes inactive.The DRX bits are double buffered which allows the next receive channel to be specified at
any time, without a concern for whether the present receive packet is finished.
14.1.3
The primary function of the RX channel Auto-hunt circuit is to provide the RX circuit with the next receive
channel number. The Auto-hunt circuit has to interface with both the RX and TX circuits as described in Section
13.3.3. The main function of the Auto-hunt circuit is described in the following sections.
14.1.4
The Auto-hunt circuit scans each of the 16 incoming D-Channels on stream STi1 to determine if any one of
them is requesting to send a packet to the system. The Auto-hunt circuit will scan the channels which are
enabled for receive activity. On reset, all 16 channels are locked out, and the system must specify which
channels are to be scanned for request-to-send (RTS) flags. The two HRA Lockout registers described in
Section 22.30 and Section 22.31 are used to specify the channels. The Auto-hunt circuit will stop monitoring a
channel for flags in three other circumstances which are described in Section 14.1.6.
The HDLC flag control character '01111110', (7E
Channel which is not disabled, the Auto-hunt circuit monitors during the appropriate bit times for flags. If a flag
is undetected after 15 consecutive bit times on the currently scanned D-Channel, the Auto-hunt circuit moves
to the next channel. The amount of time required to accumulate 15 “consecutive” bits is dependent on the
selected baud rate; for a 16K baud rate, each non-disabled-channel is monitored for up to eight frames before
being rejected. This hunt sequence continues until a flag is detected, at which point the scanning stops and the
FLAG bit is set high in the HRA Status register.
Scanning for flags, at least until an RTS is actually detected, is independent of RX circuit activity or inactivity.
The Auto-hunt circuitry cycles endlessly through all available (non-disabled) channels, listening for a
peripheral's request to send. The more channels there are with RTS, the faster the Auto-hunt circuit finds the
next receive channel number, thereby keeping the receiver inactive time to a minimum.
At the start of receiving a packet the RX circuit will latch the NRX channel number, clearing the FLAG bit, and
moving the Auto-hunt circuit on to begin scanning the next potential receive channel.
40
Generation of RxCEN
Dedicated Receive Mode
Multiplexed Receive Mode
Auto-hunt Monitoring
HEX
), is used to indicate an RTS to the system. For each D-
Advance Information