Cypress AN49947 User Manual

Undershoot Effect in RTC Circuit  
of 0.25μ nvSRAM  
AN49947  
Associated Part Family: CY14BXXXK/STK17TX8  
Application Note Abstract  
This application note describes the undershoot effect in the RTC circuit of the 256K and 1M nvSRAM in 0.25μ technology. The  
part numbers affected are CY14B256K/STK17T88 and CY14B101K/STK17TA8.  
Introduction  
Effect on RTC Oscillator  
Under certain noise conditions, the Real Time Clock (RTC)  
circuit of CY14BXXXK/STK17TX8 can be disturbed to the  
point that the oscillator circuit stops.  
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.  
Undershoot in Application  
The RTC devices in 0.25μ specify 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.  
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 A0-A3 and data pin DQ0 are located in the area  
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.  
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.  
Figure 1. Signal Undershoot  
Recommendation  
Undershoot can be reduced by adding a Schottky diode (VF <  
0.4V with IF at 100 mA) connected with the anode at ground  
3.3V  
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 A0, A1, A2, A3, and DQ0. A scope  
0V  
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.  
ESD Diode  
Conduction  
-2V  
November 06, 2008  
Document # 001-49947 Revision **  
1

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