Is there a way to manually send messages to a USB keyboard, under Windows?

Question

I'm working on a personal project that involves MCU - Windows communications. The MCU is configured to act as a HID keyboard (no serial coms).

After reading OS Dev's HID specifications, as well as a couple of questions here on Stack, I noticed that my MCU can detect, with no sister piece of software running on my machine, when caps lock is pressed (presumably works for all the other “led keys”). So when I press Caps Lock, a led turns on my MCU.

To set the LED lamps, the driver sends a SetReport request to the device using a standard USB Setup Transaction, with a one-byte data stage. The setup packet's request type should contain 0x21, the request code for SetReport is 0x09. The value field of the setup packet contains the report ID in the low byte, which should be zero. The high byte contains the report type, which should be 0x02 to indicate an output report, or a report that is being sent from the software to the hardware. The index field should contain the interface number of the USB keyboard, which is the number present in the interface descriptor which indicated this device was a USB keyboard at all. The data stage should be 1 byte, which is a bitfield. This Setup Transaction should be transferred to control endpoint zero, which would work on all hardware. Other hardware may or may not support the optional interrupt OUT endpoint. If the hardware supports the interrupt OUT endpoint, you can just transfer the 1 byte data stage to the interrupt OUT endpoint, without the extra overhead of the SETUP stage and STATUS stage. If the hardware support the interrupt OUT endpoint, you should avoid the control endpoint when possible, as the interrupt OUT endpoint is faster and can be programmed with interrupt transfers instead of setup transfers. The format of the 1-byte data stage (for SETUP transaction) or 1-byte interrupt OUT transfer is shown below. When a bit is set to 1, the corresponding LED is turned on.

I'm wondering if it would be possible to manually send a message to the MCU, just like blinking a led works, but with my own contents. Any help would be greatly appreciated.

selbie selbie · Accepted Answer · 2019-11-10T13:16:57

You can use SendInput to toggle the caps lock (or num lock) state as follows:

INPUT inputs[2] = {};

const WORD wScanCodeCapsLock = 0x3a;
const WORD wScanCodeNumLock = 0x45;

inputs[0].type = INPUT_KEYBOARD;
inputs[0].ki.wScan = wScanCodeCapsLock;
inputs[0].ki.dwFlags = KEYEVENTF_SCANCODE;
inputs[0].ki.time = 1;

inputs[1].type = INPUT_KEYBOARD;
inputs[1].ki.wScan = wScanCodeCapsLock;
inputs[1].ki.dwFlags = KEYEVENTF_SCANCODE | KEYEVENTF_KEYUP;
inputs[1].ki.time = 100;
SendInput(2, inputs, sizeof(INPUT));

Check this answer out here for how to detect the current state of these keys.

Is there a way to manually send messages to a USB keyboard, under Windows?

1 Answers