AN65921 CY7C634XX And CY7C635XX USB Keyboard Families: Power-On Reset Considerations Author: Nelson Zhang Associated Project: No Associated Part Family: NA Software Version: NA Related Application Notes: None To get the latest version of this application note, or the associated project file, please visit http://www.cypress.com/go/AN65921. This application note provides Power-On Reset considerations for CY7C634XX and CY7C635XX family of devices when used for USB and non-USB applications. Introduction There are two important cases of initial VCC voltage considerations that the system designer using our USB Keyboard Microcontroller must be aware of and plan for. These are: (1) the slow voltage rise during a cold powerup of the host PC and (2) the fast voltage rise during hotplugging into an already stable USB bus. The USB Keyboard families have built-in circuitry that resets the Microcontroller during an initial VCC voltage ramp. This internal circuitry can accommodate VCC voltage ramp rates between 10 µs and 200 ms, but is not able to differentiate between these four decades of variation. Our other USB Microcontroller families go immediately into suspend after the Power-On-Reset event (POR), however, the 7C634XX/5XX families have an extra mode that allows them to operate in non-USB applications. To support those applications, one of the LED ports pins (Port 3 bit 7) is used only during the POR event to distinguish between USB and non-USB applications. This is documented in the datasheet in section 8.1, and is illustrated in the schematic diagram Figure 1, and is reviewed as follows: 1. Cable and connector resistance can be as small as 0.1 Ohms and as large as 0.6 Ohms in typical applications. 2. The correct sequence for a USB hot-plug event is as follows: The RC time constant to the VCC pin must be greater than 10 µs. 3. 1. Microcontroller experiences VCC ramp (which must be constrained to be greater than 10 µs). The LEDs and R1 must be tied to the lowest resistance path to VCC. 4. 2. Port 3 bit 7 is at logic high with respect to VCC during the voltage ramp (see next section). The VCC pin must be bypassed for high-frequency noise (0.01 µF). 5. 3. No USB traffic occurs after the ramp so the device goes into suspend. The USB VCC entry point should be bypassed for high-frequency noise (C2). 6. 4. A Bus Reset takes the device out of suspend and the Microcontroller begins at the reset vector. Port 3 pin 7 must be pulled to VCC during initial VCC voltage ramp, for USB operation. 7. VPP must be tied to VSS. 5. Device awaits for enumeration or loss of keepalive. 8. LEDs are driven by sinking current, and require current limiting resistors (R5, R6, R7). www.cypress.com Document No. 001-65921 Rev. *A 1 CY7C634XX and CY7C635XX USB Keyboard Families: Power-On Reset Considerations To use the 7C634XX/5XX Microcontrollers in applications which require that the unit not go into suspend upon POR, the Port 3 bit 7 pin must be pulled to VSS during POR (this mode is not shown in Figure 1). This precludes this pin from functioning as a LED driver, but does not destroy the utility of this pin. The value of the pull-down resistor from Port 3 bit 7 to VSS can be any convenient value from 470K to 0 Ohms. Figure 1. 7C634XX and 7C635XX Families USB Keyboard Components Summary This application note provides Power On Reset considerations for CY7C634XX and CY7C635XX family of devices when used for USB and non-USB applications. Also it documents the RC time constant values during cold boot or hot plug event. www.cypress.com About the Author Name: Nelson Zhang Title: Principal Systems Engineer Document No. 001-65921 Rev. *A 2 CY7C634XX and CY7C635XX USB Keyboard Families: Power-On Reset Considerations Document History Document Title: CY7C634XX and CY7C635XX USB Keyboard Families: Power-On Reset Considerations - AN65921 Document Number: 001-65921 Revision ** ECN 3108802 Orig. of Change NXZ Submission Date 12/13/2010 Description of Change Obtain spec# for note to be added to spec system. This note had no technical updates. Kindly replace existing .pdf file on cypress.com *A 4228601 PRVE 12/21/2013 Added Summary. Updated in new template. Completing Sunset Review. www.cypress.com Document No. 001-65921 Rev. *A 3 CY7C634XX and CY7C635XX USB Keyboard Families: Power-On Reset Considerations Worldwide Sales and Design Support Cypress maintains a worldwide network of offices, solution centers, manufacturer’s representatives, and distributors. To find the office closest to you, visit us at Cypress Locations. PSoC® Solutions Products Automotive cypress.com/go/automotive psoc.cypress.com/solutions Clocks & Buffers cypress.com/go/clocks PSoC 1 | PSoC 3 | PSoC 4 | PSoC 5LP Interface cypress.com/go/interface Lighting & Power Control cypress.com/go/powerpsoc cypress.com/go/plc Memory cypress.com/go/memory PSoC cypress.com/go/psoc Touch Sensing cypress.com/go/touch USB Controllers cypress.com/go/usb Wireless/RF cypress.com/go/wireless Cypress Developer Community Community | Forums | Blogs | Video | Training Technical Support cypress.com/go/support PSoC is a registered trademark of Cypress Semiconductor Corp. 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The inclusion of Cypress’ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. Use may be limited by and subject to the applicable Cypress software license agreement. www.cypress.com Document No. 001-65921 Rev. *A 4