Cypress AutoStore STK17T88 Montage- und Bedienungsanleitung

November 06, 2008 Document # 001-49947 Revision ** 1

Undershoot Effect in RTC Circuit

of 0.25μnvSRAM

Application Note Abstract

This application note describes the undershoot effect in the RTC circuit of the 256K and 1M nvSRAM in 0.25μtechnology. The

part numbers affected are CY14B256K/STK17T88 and CY14B101K/STK17TA8.

Introduction

Under certain noise conditions, the Real Time Clock (RTC)

circuit of CY14BXXXK/STK17TX8 can be disturbed to the

point that the oscillator circuit stops.

Undershoot in Application

The RTC devices in 0.25μspecify in the data sheet that

inputs may not undershoot by more than –0.5V. This is diffi-

cult to achieve in systems where signal fall times are fast (1

to 3 ns). As a result of these fast fall times, signal undershoot

greater than –0.5V is quite common.

Signal undershoot greater than –0.5V causes the ESD

diodes on the device inputs to conduct current. This current

travels through the substrate until it reaches collection points

(guard bands with substrate contacts). However, as this cur-

rent flows within the substrate, it adds noise to the overall

noise floor on the device.

Figure 1. Signal Undershoot

Effect on RTC Oscillator

The RTC oscillator circuit, which is embedded on the 256K

and 1M nvSRAM family is designed to be a very low power

circuit. To achieve this low power operation, the automatic

gain circuit of the oscillator is designed to operate on very

small currents (nano-Amperes). This makes the gain circuit

sensitive to on-chip noise, which in turn makes the RTC oscil-

lator circuit very sensitive to substrate currents caused by sig-

nal undershoot.

There have been reports of the oscillator stopping in some

customer systems. Investigation showed that these systems

all had a significant amount of undershoot. Correcting the

undershoot on the input pins corrected the problem. The

address pins A

-A

and data pin DQ

are located in the area

of the RTC oscillator circuit. Undershoot on these address

pins has the greatest effect on the RTC circuit operation. It is

recommended that customers experiencing similar problems

investigate and correct undershoot issues.

Recommendation

Undershoot can be reduced by adding a Schottky diode (V

0.4V with I

at 100 mA) connected with the anode at ground

and cathode on the signal line as close to the device pin as

possible. Layout must route the signals to connect to the

diode first then the pin if possible. The anode to ground

should go directly to the ground plane. The only signals that

require this treatment are A

, A

, and DQ

. A scope

must be used to confirm controlled undershoot as some fast

edge rates may need more effective termination. It is critical

that the voltage does not go below 0.6V as substrate currents

begin to flow.

AN49947

Associated Part Family: CY14BXXXK/STK17TX8

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AN49947

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Phone: 408-943-2600

Fax: 408-943-4730

http://www.cypress.com

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unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in

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November 06, 2008 Document # 001-49947 Revision ** 2

Document History

All trademarks or registered trademarks referenced herein are the property of their respective owners.

Document Title: Undershoot Effect in RTC Circuit of 0.25μ nvSRAM

Document Number: 001-49947

Revision ECN No. Orig. of

Change Submission

Date Description of Change

** 2606848 NXR 11/13/08 New application note

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