31.3.1. Switch IP Latency
Clean Switch Latency
When you turn off Crash switching, the IP switch occurs cleanly, with the last packet on each output completed legally, as denoted by tlast.
The switch IP latency for clean switches depends on the complexity of the switch made, the configuration of the switch, the timing of the switch command, the timing on the inputs, and any backpressure experienced on the outputs.
The minimum switch latency (Lclean_switch) is the number of clock cycles from the submitting of a new switch configuration via a write to the COMMIT register, to the start of the first image information packet (full variants) or first line (lite variants) produced at the configured outputs.
Lclean_ switch = Tremaining + 8 + (C ? 6 : 3)*I + 8*O
- Tremaining = the number of cycles from the write to COMMIT to the end-of-field packet of the current input field (for full variants) or to the TLAST of the current line (lite variants) or to the next TUSER (lite variants with All inputs are uninterrupted on).
- I = The number of inputs whose state is changing (either consume, enable, disable or destination)
- O = The number of outputs whose state is changing (either enable, disable, or source)
- C is 1 with Autoconsume inputs on.
This equation holds in the absence of backpressure and in a fully synchronized system with all switch inputs receiving fields of the same size at the same time, and common host and main clocks.
Latency in a real system is dominated by the timing of the input fields and Lswitch usually only represents a very small percentage of overall switching time.
The fastest switching configurations are lite variants with All inputs are uninterrupted off, as changes occur at line endings, not field endings.
Crash Switch Latency
When you turn on Crash switching, the IP switches occur faster than for clean switching but broken packets may occur at the switch outputs. Crash switch latency is given by Lcrash_switch and switch latency is unaffected by backpressure.
Lcrash_switch <= (Total number of outputs configured)*4 + 4