CD2481 Datasheet, PDF (51/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

Programmable Four-Channel Communications Controller — CD2481
6. When the CD2481 completes the transmission, it clears the Tbusy bit. Then it sets the EOB bit
and clears the 2481OWN bit in the ATBSTS. This notifies the host that the transmission is
complete, and it returns ownership of the buffer back to the host.
7. The CD2481 optionally interrupts the host, with EOF and EOB in the TISR both set to indicate
that the transmission is complete, and there was no chaining.
Example 2
Transmit out of channel 0, and chain three buffers into one frame. The frame is 240 bytes long, and
the maximum buffer size is 100.
1. The host checks the Ntbuf bit in the DMABSTS register for channel 0 to determine which
buffer is next. In this example, Ntbuf is set to ‘1’ indicating that buffer B is used next.
2. The host sets up the buffer data, the starting address — BTBADR, and the buffer byte count —
BTBCNT, for the first ‘link’ of the chain to be transmitted. For this example, BTBCNT is set
to 100.
3. The host then sets up the BTBSTS (‘B’ Buffer Status) register. The EOF bit is clear to indicate
that this buffer is the first link in a chain. The 2481OWN bit is set to give ownership to the
CD2481. By setting 2481OWN, the host commands the CD2481 to start transmission. Thus,
everything must be ready (starting address, buffer, data count) prior to setting 2481OWN.
4. At this point, the host has enough time to transmit 100 bytes to set up the next buffer link. If
the host fails to do this in time, there is a transmitter underrun, and the frame is aborted in
HDLC or bisynchronous.
5. The CD2481 starts transmitting buffer B from channel 0. When this is started, the Ntbuf bit is
cleared to ‘0’ to indicate that buffer A is next. This helps the host keep track of which buffer is
next. As transmission progresses, the current buffer pointer, TCBADR, is updated by the
CD2481. During this or prior, the host has made buffer A ready. For buffer A, the EOF bit in
the ATBSTS register is cleared by the host, indicating that the buffer is not the end of the
chain.
6. At the end of transmission of this buffer, the CD2481 does not add any CRCs nor end of frame
delimiters because there is more data for the current frame.
7. After the CD2481 has completed transmission of the first link out of buffer B, the CD2481 sets
the EOB bit and clears the 2481OWN bit in the BTBSTS. This notifies the host that the
transmission is complete, and returns ownership of the buffer back to the host.
8. The CD2481 optionally interrupts the host with EOF clear and EOB set in the TISR to indicate
that the transmission is completed, and that there was chaining.
9. The CD2481 now sees from the ATBSTS register that it has ownership of buffer A for
transmission of the next ‘link’. It also sees that the EOF is clear so that this link is not the last
link in the transmitted chain.
10. The CD2481 continues transmission of the current frame, but now transmission is from buffer
A. This is the second link, and is 100 bytes long. During this time, the host must set up a new
buffer B for the third and final link. The BTBCNT for the last link is set to 40 bytes.
11. After the CD2481 has completed transmission of the second link out of buffer A, the CD2481
sets the EOB bit and clears the 2481OWN bit in the ATBSTS. This notifies the host that the
transmission is completed, and returns ownership of the buffer back to the host. As with the
first link, the CD2481 does not add CRCs nor ending frame delimiters to this link.
12. The CD2481 optionally interrupts the host with EOF clear and EOB set in the TISR to indicate
that the transmission is completed, and there was chaining.
Datasheet
51