Designing a USB Keyboard with the Cypress Semiconductor ...

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Designing a USB Keyboardbmrequest typehost to devdev recip0x00host to devinter recip0x01host to devendp recip0x02dev to hostdev recip0x80dev to hostinter recip0x81dev to hostendp recip0x82get status0x00clr feature0x01set feature0x3set addr0x05get desc0x06set desc0x07get config0x08set config0x09get inter0x0Aset inter0x0Bsynch0x0CbrequestFigure 13. USB Standard Request Parsing Structurebmrequest typehost to devinter recip0x21dev to hostinter recip0xA1get_report0x01get_idle0x02get_protocol0x03set_report0x09set_idle0x0Aset_protocol0x0BbrequestFigure 14. USB HID Class Request Parsing StructureData Acquisition/TransferThe firmware periodically writes scan patterns to the scanmatrix columns, and reads the row result to determine whichkeys are pressed. The scan codes of the keys pressed aresent to the host using endpoint 1 (see Figure 15). When thehost issues IN packets to retrieve data from the device, thedevice returns eight bytes of data. These eight bytes hold thekeyboard control data (see Figure 16). Usage codes for eachkey can be found in Appendix A.3 of the Device Class Definitionfor Human Interface Devices (HID). When one of the LEDbuttons (i.e. Num Lock, Caps Lock, Scroll Lock) are pressedor released, the host issues a SETUP packet with aSet_Report request through the control pipe to End Point 0,followed by an OUT packet with 1 Data byte indicating whichLED should be on or off (see Figure 17.)6
Designing a USB KeyboardFigure 15. Endpoint 1 Interrupt Service RoutineByte 7 - Key 6Byte 6 - Key 5Byte 5 - Key 4Byte 4 - Key 3Byte 3 - Key 2Byte 2 - Key 1Byte 1 - ReservedBit 7 Bit 0RightGUIEnd Point 1RightAltReceivedACK fromlast TxYesRightShiftRightCtrlNoClear End Point 1 FIFOToggle Data PIDLeftGUILeftAltLeftShiftReturnLeftCtrlByte 0-ModifierFigure 16. IN Data Organization for USB KeyboardBit 7 Bit 0ConstantConstantConstantKana ComposeScrollLockCapsLockNumLockByte 0-LED reportFigure 17. LED Report for USB KeyboardThe byte order and bit field positions are defined by the HIDreport descriptor (discussed below), and are also consistentwith the Boot Protocol requirements for legacy systems.USB DescriptorsAs stated earlier, the USB descriptors hold information aboutthe device. There are several types of descriptors, which willbe discussed in detail below. All descriptors have certaincharacteristics in common. Byte 0 is always the descriptorlength in bytes and Byte 1 is always the descriptor type. Discussionof these two bytes will be omitted from the followingdescriptions. The rest of the descriptor structure is dependenton the descriptor type. An example of each descriptor will begiven. Descriptor types are device, configuration, interface,endpoint, string, report, and several different class descriptors.Device DescriptorThis is the first descriptor the host requests from the device.It contains important information about the device. The sizeof this descriptor is 18 bytes. A list follows:• USB Specification release number in binary-coded decimal(BCD) (2 bytes)• Device class (1 byte)• Device subclass (1 byte)• Device protocol (1 byte)• Max packet size for Endpoint 0 (1 byte)• Vendor ID (2 bytes)• Product ID (2 bytes)• Device release number in BCD (2 bytes)• Index of string describing Manufacturer (Optional) (1 byte)• Index of string describing Product (Optional) (1 byte)• Index of string containing serial number (Optional) (1 byte)• Number of configurations for the device (1 byte)Example of a device descriptorDescriptor Length (18 bytes)Descriptor Type (Device)Complies to USB Spec Release (1.00)Class Code (insert code)Subclass Code (0)Protocol (No specific protocol)Max Packet Size for endpt 0 (8 bytes)Vendor ID (Cypress)Product ID (USB Keyboard)Device Release Number (1.03)String Describing Vendor (None)String Describing Product (None)String for Serial Number (None)Possible Configurations (1)7
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