cx28500 Mindspeed Technologies, cx28500 Datasheet - Page 162

• Manufacturer Part Number: cx28500
• Description: Cx28500 Multichannel Synchronous Communications Controller
• Manufacturer: Mindspeed Technologies

The transmit and receive Buffer Descriptors and Buffer Status Descriptors are designed to facilitate a mechanism
known as self-servicing buffers. This mechanism allows the Host to configure CX28500 to fill a table of data buffers
as it receives a complete message through a receive channel, then empty the same list of data buffers through a
transmit channel without any further Host intervention.
The self-servicing buffer mechanism works as follows:
1. Host initializes message descriptor table in shared memory.
2. Host configures receive channel head pointer to point to first message descriptor.
3. Host configures transmit channel head pointer to point to the same message descriptor.
4. The OWNER bit field in the buffer descriptor is set to 0. For the transmitter, this means the buffer is owned by
5. The NP bit field in the transmit buffer descriptor is set to 0. This allows the CX28500 to poll the OWNER bit field
6. Both receive and transmit channels are activated.
7. As the receiver detects an incoming message and begins filling the first data buffer, the transmitter remains idle
8. When the receiver fills the first buffer, it writes the Buffer Status Descriptor (setting the OWNER bit field to 1)
9. During the next poll transaction, the transmit channel detects if the OWNER bit field is set to 1 in the first buffer
10. Upon detecting an End Of Message, the receiver writes the Buffer Status Descriptor, marking the buffer as an
11. When the transmitter reads an End Of Message buffer, it sends the BLEN amount of data out the serial port
12. Steps 6-10 are repeated continuously, until the Host deactivates either the receive or transmit channel.
It is important to note that for self-servicing buffers, the Host does not need to write to any descriptors for receive or
transmit operations. CX28500 writes the Receive Buffer Status Descriptor, which is subsequently used as the
Transmit Buffer Descriptor.
28500-DSH-002-C
the Host. For the receiver, this means the buffer is owned by CX28500.
of the buffer descriptors.
and polls the OWNER bit in the buffer descriptor.
and then moves on to the next message descriptor. Although the data buffer is not going to be returned to the
Host, the Inhibit Receive Buffer Status descriptor option should not be enabled (INHRBSD = 0 in RDMA
channel configuration register). In other words, the CX28500 must be allowed to overwrite the Buffer Descriptor
with the Buffer Status Descriptor.
descriptor and assumes ownership. The transmitter then begins emptying the first data buffer and moving data
to the serial port. The Inhibit Transmit Buffer Status Descriptor option must be disabled for CX28500 to return
ownership to the HOST (i.e., INHTBSD = 0 in TDMA Channel Configuration register). In other words, the
CX28500 must be allowed to overwrite the Buffer Descriptor with the Buffer Status Descriptor.
End Of Message and setting the buffer length (BLEN) to indicate the amount of data contained in this buffer.
and writes the buffer status descriptor (setting the OWNER bit field to 0), then moves onto the next buffer or
enters the idle state (if the next buffer is still owned by the receiver).
9.0 Self-Servicing Buffers
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