Understanding USB frequency involves looking at the different speeds and how data is transmitted. There isn't one single "USB frequency" that applies to all versions, as USB standards have evolved to offer much higher data rates over time.
Based on the provided reference, we can look specifically at the frequencies associated with the USB Full-Speed standard.
For USB Full-Speed:
- The data rate is 12 MHz.
- The interface itself runs at a higher clock speed. As stated in the reference, "The interface runs at four times this frequency and must be clocked from an accurate 48 MHz clock source."
This means that while the data is transmitted at a rate equivalent to 12 million bits per second (often simplified as 12 Mbps for data rate, though MHz here refers to signaling rate), the underlying clock controlling the data transfer mechanism for USB Full-Speed operates at 48 MHz.
USB Full-Speed Frequencies Explained
Let's break down the frequencies mentioned for USB Full-Speed:
- Data Rate (12 MHz): This refers to the rate at which data is effectively transmitted. For USB Full-Speed, this rate is 12 million transfers per second, allowing for a maximum theoretical throughput of 12 Mbps (Megabits per second).
- Interface Clock (48 MHz): This is the fundamental clock signal that times the operations of the USB interface circuit itself. The reference states this clock runs at four times the data rate frequency, sourced from a precise 48 MHz oscillator. This higher frequency clock is necessary for the detailed timing and control signals required by the USB protocol, enabling efficient and reliable data transfer at the specified 12 MHz data rate.
It's important to note that newer USB standards like High-Speed (USB 2.0), SuperSpeed (USB 3.x), and SuperSpeed+ offer significantly higher data rates, each operating with different signaling methods and underlying clock frequencies to achieve speeds ranging from 480 Mbps up to 40 Gbps or more. However, the reference specifically details the frequencies for the USB Full-Speed mode.
