Specifications

ManualsBrandsCypress Semiconductor ManualsComputer equipmentCY7C63413C

Designing a Dual-Interface Keyboard with the Cypress

Semiconductor CY7C63413 USB Microcontroller

Revision 1.3 02/13/99

Added 3D mouse documentation and endpoint 2 boot protocol description

Revision 1.2 11/09/98

Added power management documentation

Revision 1.1 10/29/98

Added slave mouse documentation

Introduction

The PS/2 auxiliary device interface is an established standard in the personal computer industry. It is used for the

attachment of peripherals to the computer such as a keyboard, mouse, joystick, etc. In recent years, the Universal Serial

Bus (USB) standard has been developed in an attempt to address the shortcomings of PS/2 and other common desktop

computer electrical interfaces, including limited bandwidth, data integrity and the limitation of a single slave device per port.

USB addresses bandwidth limitations by offering 1.5Mbit (low speed) and 12Mbit (full speed) serial data transfer rates. USB

provides several levels of data integrity validation, including cyclic redundancy code (CRC) checking, and defines procedures

for failed-transfer recovery. In addition, the USB architecture provides for the connection of up to 127 devices on a single

root hub. While the USB standard has been embraced by the computer industry, and is now included by default on most

desktop motherboards produced for sale, there remains a large installed base of computers and operating system software

which do not yet support USB. This application note address the design of a keyboard that supports both the USB and PS/2

interfaces using the Cypress Semiconductor single-chip CY7C63413 USB microcontroller.

This application note starts by addressing the basic requirements for key-scan and other interface-independent keyboard

functionality, followed by an introduction to the CY7C63413 USB controller. A schematic of the Dual-Interface keyboard and

its connection details can be found in the Hardware Implementation Section.

The software section describes the architecture of the firmware required to implement the keyboard function. The source

code of the demonstration keyboard firmware is available with the Cypress CY3651 Development Kit. Please contact your

local Cypress Sales Office for details.

This application note assumes that the reader is reasonably familiar with the CY7C63413 USB controller, the Universal

Serial Bus and the PS/2 device interface. The CY7C63413 data sheet is available from the Cypress web site at

http://www.cypress.com. USB documentation can be found at the USB Implementers Forum web site at http://www.usb.org/.

Information regarding the PS/2 keyboard interface can be found in IBM publication 84F9735, IBM Personal System/2

Hardware Technical Reference – Common Interfaces, which may be purchased directly from IBM.

Keyboard Basics

Key Switches and Scan Matrix

A computer keyboard typically has between 101 and 104 keys that are uniquely positioned in a scan matrix. The scan matrix

consists of M rows and N columns, all of which are electrically isolated from each other. The number of rows (M) is typically

no greater than 8, and the number of columns (N) is no greater than 20. Each key sits over two isolated contacts of its

corresponding row and column in the scan matrix. When a key is pressed, the two contacts are shorted together, and the

row and column of the key are electrically connected (Figure 1).

1 Key Switch

Summary of content (25 pages)

PAGE 1
Designing a Dual-Interface Keyboard with the Cypress Semiconductor CY7C63413 USB Microcontroller Revision 1.3 02/13/99 Added 3D mouse documentation and endpoint 2 boot protocol description Revision 1.2 11/09/98 Added power management documentation Revision 1.1 10/29/98 Added slave mouse documentation Introduction The PS/2 auxiliary device interface is an established standard in the personal computer industry.
PAGE 2
Intelligent Controller The typical computer keyboard keyboard contains an embedded controller that performs the necessary functions to scan the key matrix and implement the communication interface to the host computer. The essential task of the controller is to monitor the keys and report key presses and/or releases to the host computer. The basic key-matrix scanning approach is, for the most part, independent of the host communications interface.
PAGE 3
Configuration Bits 11 10 Interrupt Bit 10 01 00 1 X X X 0 Polarity Resistive CMOS Output CMOS Input Open Drain Open Drain disabled disabled + Ports 0 to 2 offer low current drive with a typical current sink capability of 7 mA. Port 3 offers higher current drive, with a typical current sink of 12 mA which can be used to drive LEDs. Each General Purpose I/O (GPIO) is capable of generating an interrupt to the RISC core.
PAGE 4
Hardware Implementation Figure 8 is the schematic for a dual-interface keyboard application. This design may be used to implement a USB-only keyboard, a dual-interface (USB and PS/2) keyboard, or a USB keyboard with the capability to support a slave PS/2 mouse. The latter design is the subject of Cypress Application Note XXXXX.
PAGE 5
the nominal 5VDC rail. Cypress recommends this termination method as an alternative to the method described in the USB specification, as it yields compatible signal levels and does not require the use of a separate 3V regulator. In the event that a dual-interface design is being implemented, JP1 is left open, and transistor Q1 is installed to allow firmware control over the USB pullup resistor via Bit 3 of Port 3. On power-up, port pin 3.
PAGE 6
PAGE 7
Firmware Implementation The reference firmware supports both the USB and PS/2 interfaces. For each interface, the firmware can be conceptually divided into the following sections: host command processing, scan code conversion, and keyboard matrix scanning. Of these tasks, the keyboard matrix scanning is common to both interfaces and, as such, is organized as a set of callable functions which scan the keyboard and return interface-independent information about the state of the keys.
PAGE 8
high. A host computer will not typically attempt to transmit data to the keyboard until it has received a power up status indication from the keyboard. The D+/D- lines are then polled approximately every 2uS for a duration of 1mS. If, on any four consecutive samples, either or both of the D+ or D- lines are sampled as non-zero, the interface is determined to be PS/2. Otherwise (both D+ and Dremain low), the D- pullup is engaged by enabling Q1, causing a USB connect condition.
PAGE 9
• Responds to SETUP packet according to the parsing structure Figure 2. USB Standard Request Parsing Structure Figure 2. USB HID Class Request Parsing Structure Data Acquisition/Transfer The firmware periodically writes scan patterns to the scan matrix columns, and reads the row result to determine which keys are pressed. The scan codes of the keys pressed are sent to the host using endpoint 1 (see Figure 15).
PAGE 10
Bit 7 Right Right Right Right Left GUI Alt Shift Ctrl GUI Bit 0 Left AltLeft Left Byte 0Shift Ctrl Modifier Figure 2. IN Data Organization for USB Keyboard Bit 7 Bit 0 Con Con Con Kana Com Scroll Caps Num Byte 0stant stant stant pose Lock Lock Lock LED report Figure 2. LED Report for USB Keyboard The byte order and bit field positions are defined by the HID report descriptor (discussed below), and are also consistent with the Boot Protocol requirements for legacy systems.
PAGE 11
Configuration Descriptor The configuration descriptor is 9 bytes in length and gives the configuration information for the device. It is possible to have more than one configuration for each device. When the host requests a configuration descriptor, it will continue to read these descriptors until all configurations have been received.
PAGE 12
Endpoint Descriptor The endpoint descriptor describes each endpoint, including the attributes and the address of each endpoint. It is possible to have more than one endpoint for each interface. This de scriptor is 7 bytes long and is set up as follows: • • • • Endpoint address (1 byte) Endpoint attributes.
PAGE 13
found in the example called a Usage Page. The reason for the Usage Page is that it is necessary to allow for more than 256 possible Usage tags. Usage Page tags are used as a second byte which allows for up to 65536 Usages. Logical Minimum and Logical Maximum are used to bound the values that a device will return. For example, a keyboard that will return the values 0 to 101 for the scan code of each key press will have a Logical Minimum (0) and Logical Maxi mum (101).
PAGE 14
N-Key Rollover In the case where multiple keys are pressed and held down, only the last key pressed should be repeated. To accomplish this, the last key press is stored in theLastTx buffer and is sent again in a separate report to the host whenever a multi ple key report is sent. Once the same key is released, a report notifying it of the release is sent to the host (all 6 key array positions are filled with 0’s).
PAGE 15
usage page (generic desktop) usage (mouse) collection (application) report id (1) usage (pointer) collection (linked) usage page (buttons) usage minimum (1) usage maximum (3) logical minimum (0) logical maximum (1) report count (3 bytes) report size (1) input (3 button bits) report count (1) report size (5) input (constant 5 bit padding) usage page (generic desktop) usage (X) usage (Y) usage (wheel) logical minimum (-127) logical maximum (127) report size (8) report count (3) input (3 position bytes X & Y &
PAGE 16
Finally, to support boot protocol, the firmware will not transmit power key status on endpoint 2 when the boot protocol is selected via the host command; instead, it reverts to transmitting a standard 2-axis mouse packet without the packet id header: i.e., a three-byte packet consisting of button data, X, and Y coordinates. When report protocol is re-selected, the packet structure is as described above.
PAGE 17
the clock line (corresponding to D+) low while floating the data line (corresponding to D-), for instance, would be accomplished with a setting of ‘110’in Control Bits 2~0. The PS/2 firmware can be be viewed as two layers: an interface layer and a command layer. The interface layer processes the bit-level transaction management between the host and the keyboard, and the command layer deals with the higher-level implementation of keyboard commands and responses.
PAGE 18
Functionality Main Loop After power-up housekeeping is performed, the main loop of the PS/2 firmware is entered. This loop invokes the keyboard scanning task at periodic (4msec) intervals and sends to the host any key presses that were detected, and also checks for the presence of a host command request. Command requests are honored immediately: the firmware receives and parses the command and invokes the proper subroutine to perform the command and return any required data to the host.
PAGE 19
released, is considered to be caused by switch bounces, and is disregarded. Different keyboards will have varying key switch sensitivity, and so these debounce times are declared as constants in the firmware to facilitate modification. Phantom Keys A phantom fourth key press (D) will be detected in a scan matrix if two keys (A and B) in the same column are pressed, along with a third key (C) pressed in the same row as A or B (see Figure 19).
PAGE 20
Figure 20. USB Main Loop Figure 21.
PAGE 21
Figure 22. PS/2 Main Loop Main Loop Figure 23.
PAGE 22
Figure 24.
PAGE 23
Figure 25.
PAGE 24
Figure 26.
PAGE 25
Conclusion This Application Note, together with the hardware reference design and reference firmware (available from Cypress) provides a superset of the functionality necessary for the implementation of a USB, PS/2 or dual-interface keyboard using the Cypress CY7C63413 processor. The built-in USB functionality of the CY7C63413, combined with its fast instruction execution time and flexible I/O configuration make it an unbeatable choice for low-speed USB and dual-interface applications.