figure 3.0 below.
USB type-A connector
GVND+D-
CD
C
Figure 3.0 USB and PS/2 peripheral connectors
Overall circuit
A schematic of the overall circuit is shown inAppendix A of this document. Appendix B liststhe bill of materials.
Firmware Implementation
The firmware for this reference design is writtenin the C language and requires the ByteCraft M8C-compiler. The following files are required tocompile the mouse firmware
Chip.h – interrupt table, register and portconstants for the CY7C63723 microcontrollerCombi.h – main mouse firmware
Macros.h – general macros used with this designPs2defs.h – PS/2 interface constantsUsb_desc.h – USB descriptor tablesUsbdefs.h – USB interface constants
At power up, the firmware tests the host interfaceand automatically determines if the mouse ifplugged into a USB or a PS/2 host connection.After interface detection, the host firmwareconfigures itself to operate on the detectedinterface.
All USB Human Interface Device (HID) classapplications follow the same USB start-upprocedure. The procedure is as follows
1. Device Plug-in
When a USB device is first connected to the bus,it is powered and is running firmware, butcommunications on the USB remain non-functional until a USB bus reset by the host.2.Bus Reset
The pull-up resistor on D– notifies the hub that alow-speed (1.5 Mbps) device has just been
connected. The host recognizes the presence ofa new USB device and initiates a bus reset tothat device.
3. Enumeration
The host initiates SETUP transactions that revealgeneral and device specific information about themouse. When the description is received, thehost assigns a new and unique USB address tothe mouse. The mouse begins responding tocommunication with the newly assigned address,while the host continues to ask for informationabout the device description, configuration
description and HID report description. Using theinformation returned from the mouse, the hostnow knows the number of data endpointssupported by the mouse (2). At this point, theprocess of enumeration is completed. The USBdescriptors used during enumeration are listed inAppendix C.
4. Post Enumeration Operation
Once the communications connection betweenthe host and mouse is established, the peripheralnow has the task of sending and receiving dataon the control AND data endpoints. In this case,when the host configures endpoint 1, the mousestarts to transmit button and motion data back tothe host when there is data to send. At any timethe peripheral may be reset or reconfigured bythe host.
USB Requests – Endpoint 0
Endpoint 0 acts as the control endpoint for thehost. At power up endpoint 0 is the default
communication channel for all USB devices. Thehost initiates Control-Read and Control-Write(see Chapter 8 of the USB specification) to
determine the device type and how to configurecommunications with this device. In this
particular design, only Control-Read transactions