USB Behind the Scenes: Hands-on HID Firmware Development

Identify USB 2.0 speed by noting host pull-downs on D+ and D− data lines, and device pull-ups on D− for low speed or D+ for full/high speed.

Explain how frames, marked by the start of frame signal every one millisecond in full-speed USB, allocate minimum portions to each device, balancing camera, keyboard, and mouse bandwidth.

Understand how USB transmits data in packets, with packet structure, endpoint-based size decisions, and signaling from sync to end, including bit stuffing and least significant bit first.

Understand USB packet identifiers (PIDs) and the PID field, outlining token, data, and handshake packets, their in and out directions, and how a PID value indicates packet type.

Explore how USB is host driven, where the host triggers all transactions with out and setup tokens, and the host requests device data using in tokens.

Understand the four USB transfer types—interrupt, bulk, isochronous, and control—and how endpoints, including the control endpoint zero, perform transfers. The host learns endpoint configuration and descriptors during device enumeration.

Explore how USB control transfer enables device enumeration and configuration through endpoint zero, detailing setup, data, and status stages and their handshaking for error-free transmission.

Organize a logging helper for hid firmware by adding include and source helpers and implementing logger c and h with error, information, and debug levels, and set system_log_level in system_init.

Configure the microcontroller clock to feed a fixed 48 MHz USB clock, using an 8 MHz external oscillator, HSE, PLL multipliers/dividers, and prescalers, with CubeMX guiding the calculations.

Explain how to configure MCO one source mux to output the PLL clock on PA8, routing USB and system clocks, and verify a 36 MHz signal on the MCO1 pin.

Configure flash memory read latency as wait states before raising the CPU clock to 72 MHz, by modifying the flash ACR latency field using standard macros and datasheet guidance.

Use the fld2val macro to convert a value into a field with proper bit shifting and masking, then examine the first register modification and USB-related code glimpses.

Configure mco1 to output clock on pin a8 by selecting the clock source, setting prescaler to two, enabling gpio a, and setting pin a8 to alternate function with medium speed.

Discover a three-layer USB firmware architecture—driver layer, framework (protocol-specific) layer, and application layer—balancing hardware access, host interactions, endpoints, and FIFO buffer management.

Use two firmware functions to connect or disconnect the USB device by setting the software disconnect bit in the device control register and toggling the power down bit for transceivers.

Explore argument validation within the USB HID driver abstraction and learn to document code with doxygen, balancing hardware specific checks with forward progress in the driver layer.

Implement the rx fifo not-empty interrupt handler by popping the status word, decoding endpoint and byte count, and routing setup versus out data packets.

Understand how setup and out transfer statuses from the fifo may appear without data, and how to re-enable endpoints after transfers in USB HID firmware.

Resolve compilation errors by including the USB standards header, then reorder functions so definitions appear before use, enabling the project to compile successfully.

Define a usb_device structure in header to store device state, default address, endpoint zero control transfer stage, current configuration, and data buffers; declare a usb_events variable in the driver header.

Initialize the USB device by passing and storing a USB device pointer in a static global variable, then set an out buffer in main.c and compile.

investigate the host's first usb request by examining the get device descriptor transaction, showing an eight-byte setup with bm request 0x80, bRequest 6, descriptor type 1, length 64.

Define the USB standard device descriptor and other descriptor types, explain get descriptor requests, and show how host requests specify descriptor types using the value byte.

Define standard USB request macros in the USB standards header and handle get descriptor requests by extracting the descriptor type from the w value field.

Implement the function to process the control transfer stage and handle the USB pulled event to manage setup and data in stages, including get device descriptor, in a bus-polling loop.

Define and attach the in transfer completed handler to USB events, check the in data buffer, and resume the in data stage to send the next packet to endpoint zero.

Set address requests are handled in the outer switch with no data stage; read W value, update device address via the driver, then switch to in status stage.

Demonstrate viewing a USB set address request and its immediate response in Wireshark, alongside capturing device descriptor traffic and debugging USB timeouts during firmware development.

Explore usb device classes and how peripherals like mice interact with hosts, and implement a simple hid mouse firmware that moves the cursor periodically, with buttons and position sensor enhancements.

Write the USB configuration descriptor for a mouse HID device, defining the configuration, interfaces, and endpoints, calculating the total length, and setting attributes, max power, and wake capability.

Add the USB ID descriptor for HID, create a dedicated ID standards header, link to an ID report descriptor, and clarify descriptor versus report descriptor to prepare testing.

Describe HID buttons by setting the usage page to button, defining min/max and logical ranges, and specifying report size, count, input as absolute, with padding for compatibility.

Implement handling of standard USB interface requests, including get report descriptor, to deliver HID report data via a control transfer and endpoint three.

Instantiate a mouse report, fill the x and y values, and write it to endpoint three after a successful transfer using the in transfer completed handler.