WirelessUSB™ Protocol 2.2
2.2.9Dynamic Data Rate and Dynamic PA
Dynamic data rate and dynamic PA provide the ability to improve the immunity to interference and reduce power consumption. Dynamic data rate is device behavior based and two data modes, GFSK and 8DR, are used for the data transmission. Depending on the retry number of prior packets, the protocol determines whether to stay with the current data mode or change to another data mode. The dynamic PA relies on both the behavior and the bridge signal strength to the device.
HID
Dynamic Data Rate
If the HID fails to transmit either the application or protocol data to the bridge after DATA_PACKET_RETRIES of retransmissions, it will toggle data modes and transmit again. If the HID still fails after DATA_PACKET_RETRIES of retransmissions, it assumes the channel has become unavailable due to excessive interference and moves to reconnect mode.
If the HID transmits application data successfully, the retries for PKT_NUM packets are summed up in ‘total_retry’. If the total_retry exceeds the threshold ‘total_retry_threshold’, the HID changes to another data mode.
The total_retry_threshold is an adaptive number that has a minimum value of TOTAL_RETRY_THRESHOLD_LOW. The following rules are applied to it:
■When the data mode is toggled, the total_retry for the previous data mode will be used as current total_retry_threshold.
■If the total_retry is zero, the total_retry_threshold will be decreased one until
TOTAL_RETRY_THRESHOLD_LOW.
Dynamic PA
If the total_retry is zero and the RSSI reading for the bridge AutoACK packet is above PA_RSSI_RX_THRESHOLD, the PA is decreased by one until DATA_MODE_PA_MIN.
When one of the following cases occurs, the PA will be set back to its maximum value of DATA_MODE_PA_MAX:
■The data mode is toggled.
■The RSSI reading for the bridge AutoACK packet is equal to or below PA_RSSI_RX_THRESHOLD.
■The retries for any packet exceeds PA_RETRY_THRESHOLD.
Bridge
The bridge does not need to implement the dynamic PA because it is bus powered. Its PA is always set to the maximum value DATA_MODE_PA_MAX in order to get the highest transmission power.
The dynamic data rate is driven by the devices. When the bridge receives the packet, it sets the transmission data mode to the data mode of received packet. As a result, when it sends the back channel data, it uses the same data mode as the device.
22 | CY4672 Reference Design Guide, Document # |
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