Credit-based rate control for high-speed interfaces
In accordance with at least one embodiment, credit-based flow control for high-speed interfaces is provided. Decoupling (separation) of high speed interface reference clocks from the interface data rates is enabled. Such decoupling allows consolidation of reference clocks used for high speed interfaces, thereby enabling one reference clock to be used for multiple interfaces having different data rates. It also results in reduced buffer resource (memory) requirements, and reduced system latency. In redundant systems it also simplifies the direct vs. spare selection mechanism.
1. Apparatus comprising:
a transmitter; and
a rate limiter coupled to the transmitter for limiting an actual transmit data rate at which data are transmitted by the transmitter, wherein the rate limiter constrains transmission of the data in accordance with credits accumulated in proportion to a reference clock rate and expended upon transmission of the data, wherein the rate limiter comprises:
a rate register for storing at least one value corresponding to an accumulation rate at which the credits are accumulated; and
a credit counter for maintaining the credits, wherein the at least one value comprises:
an upper value corresponding to an upper accumulation rate; and
a lower value corresponding to a lower accumulation rate, wherein the rate limiter operates at the upper accumulation rate for a first period of time and at the lower accumulation rate for a second period time such that, over the first and second periods of time, the rate limiter operates at an average rate adapted to conform to a maximum average receive rate at which the data may be received.
2. The apparatus of claim 1 wherein the rate limiter is adapted to be responsive to a backpressure flow control signal.
3. The apparatus of claim 1 wherein, during a first transmit cycle, the transmitter is adapted to provide a first bit of the data along a first link and a second bit of the data along a second link, wherein the rate limiter is adapted to be responsive to at least one backpressure flow control signal pertaining to the first link and the second link.
4. The apparatus of claim 1 wherein, during a first transmit cycle, the transmitter is adapted to provide a first bit of the data along a first link to a first receiver and along a second link to a second receiver, wherein the rate limiter is adapted to be responsive to at least one backpressure flow control signal pertaining to the first link and the second link.
5. The apparatus of claim 1 wherein the transmitter is adapted to provide a first bit of the data along a first link to a first receiver, wherein the credits are accumulated at a credit accumulation rate that is frequency locked to a receiver clock of the receiver.