USB remains the dominant peripheral interface, but its low-speed and full-speed modes are often overlooked in favor of high-speed variants. This deep-dive series starts by examining the electrical characteristics—such as voltage levels, signaling rates, and cable requirements—that define these foundational modes. It then walks through the communication protocol, including packet structures, handshaking, and bus states, which are critical for anyone implementing USB in resource-constrained embedded systems. Understanding these basics is not just academic; many devices like keyboards, mice, and simple sensors still rely on low-speed USB for cost and power efficiency. The article also clarifies common misconceptions about termination resistors and pull-up/pull-down configurations, which are frequent sources of hardware bugs. For firmware developers, the protocol section provides a clear mental model of how transactions are initiated and completed, aiding in driver development and debugging. While the series promises more advanced topics later, this first installment is a self-contained primer that will benefit both newcomers and experienced engineers refreshing their knowledge.
A technical series begins with the electrical and protocol basics of USB low-speed and full-speed modes, essential for embedded engineers.