cx28560 Mindspeed Technologies, cx28560 Datasheet - Page 262

• Manufacturer Part Number: cx28560
• Description: Hdlc Controller
• Manufacturer: Mindspeed Technologies

Buffer Controller FIFO Size Calculation
E.6
E.6.1
E-12
Transmit FIFO Calculation
Service Request Scheme
Reports are sent to the system containing a count of the number of double words that
were delivered to the TSLP since the last request (including CMND dwords). The
CX28560 sends reports for a channel according to the Flexiframe algorithm, as long
as the channel is activated and overflow did not occur. The system should maintain an
empty dword counter for each channel, which will indicate how many empty dwords
the CX28560’s buffer has for that channel. For each request from the CX28560, the
system first updates the empty byte count for this channel and decides if it can deliver
another fragment of data or not, according to the next amount of data it has to deliver
(see Appendix C: FlowConductor).
If a threshold (configurable per channel) has been passed or if a full message is in the
buffer, the CX28560 starts transmitting packets. In case of all packets of 40 bytes or
57 bytes, at the beginning of the transmission the channel does not transmit at full
speed until the buffer has been filled. This occurs each time the buffer becomes
empty, and starts transmitting packets of 40 bytes or 57 bytes at full speed. The slow
down in a channels transmission rate may also occur when the frame is changed at
worst case conditions.
In normal operation there also may be a temporary slowdown in the channel’s rate
when the packet size changes from short to long (due to the time until threshold
dwords are filled to start transmitting the new packet).
The transition from long packets to 57-byte packets (without taking into consideration
the frame change effect) will cause no slowdown of the channel bit rate.
A slowdown in a channel’s rate can also occur when the packets are shorter than 28
bytes (this is the minimum size of fragments that can hold a full rate under the
Flexiframe scheme for this buffer size).
Buffer calculations:
• For a 52-Mbps channel:
• For a 39-Mbps channel: (mid range):
Mindspeed Technologies™
Advance Information
60 B (base) + 28 B (missed request) + 28 B
(frame change) + 5 µs (latency)* 52 Mbps/8=
60 B (base) + 42 B (missed request) + 42 B
(frame change) + 5 µs (latency)*39 Mbps / 8 =
6 B (base) + 42 B (missed request) + 42 B
(frame change) + 25 B (latency) = 169 B->172 B
(to be divided by 4) (193 B with 10 µs latency)
60 B (base) + 28 B (missed request) + 28 B
(frame change) + 33 B (latency) = 149 B ->
152 B (to be divided by 4) (181 B with 10 µs
latency)
CX28560 Data Sheet
28560-DSH-001-B