pt7a6525 Pericom Technology Inc, pt7a6525 Datasheet - Page 15

• Manufacturer Part Number: pt7a6525
• Description: Pt7a6525/6525l/6526 Hdlc Controller
• Manufacturer: Pericom Technology Inc

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Procedural Support (Layer-2 Functions)
Full-Duplex LAPB/LAPD Operation
There are two modes of LAPB/LAPD operation, Full-Duplex
and Half-Duplex operations:
In Full-Duplex LAPB/LAPD operation, the combined
(master+slave) station transmits both commands and responses
and may transmit data at any time.
• Reception of Frames
The logic processing of received S frames is performed by the
PT7A6526 without interrupting the microprocessor. The
microprocessor is merely informed via interrupts concerning
status changes in the opposite station (receive ready/receive
not ready) and protocol errors (unacceptable N(R) or S frame
with I field).
I frames are also processed autonomously and checked for
protocol errors. An I frame will not be accepted if an N(S) error
occurs (no interrupt is forwarded to the microprocessor) but it
will be immediately confirmed by an S response. If the
microprocessor sets the device into a ‘Receive Not Ready’
status, an I frame will not be accepted (no interrupt) and an
RNR response is transmitted. U frames are always stored in the
RFIFO and forwarded directly to the microprocessor.
Note: The state variables N(S), N(R) are evaluated within the
window size, i.e., the device checks only the LSB of the receive
and transmit counter regardless of the selected modulo count.
• Transmission of Frames
The device autonomously transmits S commands and S
responses in Auto Mode. Either Transparent or I frames can be
transmitted by the user. The software timer must be in Internal
Timer Mode in order to transmit I frames. After the frame has
been transmitted, the timer is self-started,the XFIFO is inhibited,
and the device waits for the arrival of a positive
acknowledgement. This acknowledgement can be contained in
an S or I frame.
PT0017(12/05)
15
PT7A6525/6525L/6526 HDLC Controller
If no positive acknowledgement is received during time t1, the
device transmits an S command (p=1), which must be followed
by an S response (f=1). If the S response is not received, the S
command process will be performed n1 times before it is
terminated.
Upon the arrival of an acknowledgement, or after the completion
of this polling procedure, the XFIFO is enabled, and an interrupt
is forwarded to the microprocessor. Interrupts may be triggered
by the any of following events:
- message has been acknowledged as positive (XPR interrupt);
- message must be repeated (XMR interrupt);
- response has not been received (TIN interrupt).
Upon arrival of an RNR frame, the software timer is started and
the status of the opposite station is polled periodically after
expiration of t1, until the status “Receive Ready” has been
detected. The microprocessor is informed accordingly via
interrupt. If no response is received after n1 times an interrupt
will be generated (TIN interrupt).
Note: The Internal Timer Mode should be used only in the
Auto Mode.
After transmission of a transparent frame the XFIFO is
immediately enabled, which is confirmed by interrupt (XPR). In
this case, time monitoring can be performed with the timer in
the external timer mode.
The protocol process of transmit/receive frames among the
microprocessor, the local station and the remote station in Auto
Mode are shown in Figure 5.
ransparent frames can be transmitted in all operating modes.
Data Sheet
Ver:8