Methods and devices for synchronizing to a remotely generated time base
Devices and methods for generating timing signals at a rate that matches a rate of remotely generated timing signals are provided. In some embodiments, a host generates timing signals in accordance with a USB specification, such as keep-alives, start-of-frame packets, or ITPs. An upstream facing port transmits the timing signals over a network to a downstream facing port. The downstream facing port generates and transmits timing signals to a USB device at a predetermined rate, and alters the predetermined rate based on an analysis of the rate at which timing signals are received from the upstream facing port.
1. A computing device configurable to provide a downstream facing port (DFP) for allowing a USB device to communicate with a host via an upstream facing port (UFP), wherein the DFP is configured to:
receive a first remote timing signal from the UFP;
transmit local timing signals to the USB device at a predetermined rate; and
adjust the rate of transmission of local timing signals to the USB device based on further timing signals received from the UFP without determining a round-trip latency between the UFP and the DFP;
wherein adjusting the rate of transmission of local timing signals includes:
determining an average duration between reception of remote timing signals from the UFP; and
adjusting the rate of transmission based on the determined average duration.
2. The computing device of claim 1 , wherein the predetermined rate is based on a connection speed negotiated between the host, the UFP, the DFP, and the USB device.
3. The computing device of claim 1 , wherein transmitting local timing signals at a predetermined rate includes:
setting a timing signal count to a predetermined value;
setting a countdown timer based on the timing signal count; and
transmitting one or more local timing signals based on a value of the countdown timer.
4. The computing device of claim 3 , wherein transmitting local timing signals at a predetermined rate further includes:
detecting that the countdown timer has elapsed; and
resetting the countdown timer based on the timing signal count.
5. The computing device of claim 4 , wherein adjusting the rate of transmission based on the determined average duration includes modifying the timing signal count based on the determined average duration.
6. The computing device of claim 5 , wherein modifying the timing signal count based on the determined average duration comprises:
incrementing the timing signal count by a predetermined amount upon determining that the average duration is longer than a duration represented by the timing signal count; and
decrementing the timing signal count by a predetermined amount upon determining that the average duration is shorter than a duration represented by the timing signal count.
7. The computing device of claim 1 , wherein the DFP is further configured to establish a communication link with the UFP using a communication protocol other than a USB protocol.
8. The computing device of claim 1 , wherein the computing device is configured to communicate with the USB device using one or more of a USB 1.0-compatible protocol; a USB 1.1-compatible protocol; a USB 2.0-compatible protocol; a USB 3.0-compatible protocol; or a protocol that is backward-compatible with USB 1.0, USB 1.1, USB 2.0, or USB 3.0.
9. The computing device of claim 1 , wherein the local timing signals include keep-alive signals.
10. The computing device of claim 1 , wherein the local timing signals include start-of-frame packets.
11. The computing device of claim 10 , wherein the local timing signals include microframe start-of-frame packets.
12. The computing device of claim 1 , wherein the local timing signals include isochronous timestamp packets.
13. A method of generating local timing signals, the method comprising:
receiving, by a first computing device configurable to provide a downstream facing port, a first remote timing signal from a second computing device configurable to provide an upstream facing port;
generating local timing signals at a predetermined rate; and
adjusting the rate of transmission of local timing signals based on further received remote timing signals without determining a round-trip latency between the first computing device and the second computing device, wherein adjusting the rate of transmission of local timing signals includes:
determining an average duration between reception of remote timing signals; and
adjusting the rate of transmission based on the determined average duration.
14. The method of claim 13 , wherein generating local timing signals at a predetermined rate includes basing the predetermined rate on a timing signal count.
15. The method of claim 14 , wherein adjusting the rate of transmission based on the determined average duration comprises:
incrementing the timing signal count upon determining that the average duration is longer than a duration represented by the timing signal count; and
decrementing the timing signal count by a predetermined amount upon determining that the average duration is shorter than a duration represented by the timing signal count.
16. The method of claim 14 , wherein basing the predetermined rate on a timing signal count comprises setting a countdown timer to a value of the timing signal count.
17. The method of claim 16 , wherein generating local timing signals at a predetermined rate includes generating local timing signals based on a value of the countdown timer.
18. The method of claim 13 , wherein the local timing signals are one or more of a keep-alive signal, a start-of-frame packet, a microframe start-of-frame packet, and a isochronous timestamp packet.