Fix host issue with with turnaround (inter-packet) timing #179
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…after received data from device host: send keep-alive packet instead of SOF on lowspeed bus
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USB 2.0 specs 7.1.18 specify 2 bit time for inter-packet, especially when switching bus direction.
This is essential when we overlock mcu to higher clock such as 240/264 Mhz. We basically send handshake too fast after EOP and give device no time to capture the bus correcly. This also occurs with some slow device such as LS gamepad in adafruit/circuitpython#10243
turnarounddelay is done in cycle which is conveniently = 4 pp->clk_div_/fsls_tx.div_int. Since we also have overhead, I think it is best to do 1.5 bit time for LS and 1 bit time for FS.I also notice that we mistakenly send SOF instead of Keep-alive on the LS bus, therefore fix it as well.
@tannewt @FoamyGuy @ladyada Please try to see if this fixes your issue with gamepad (direct attached) and 264Mhz overclocked with hub + mouse + keyboard on fruit jam