7220 Datasheet, PDF (231/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
6.4 Countermeasures against noise
(3) Wiring to CNVss pin
Connect the CNVSS pin to the VSS pin with the
shortest possible wiring.
Reason
The processor mode of a microcomputer is
influenced by a potential at the CNVSS pin. If
a potential difference is caused by the noise
between pins CNVSS and VSS, the processor
mode may become unstable. This may cause
a microcomputer malfunction or a program
runaway.
Noise
CNVSS
VSS
CNVSS
VSS
N.G.
O.K.
Fig.6.4.3 Wiring for CNVSS pin
(4) Wiring to VPP pin of One Time PROM version
and EPROM version
When the VPP pin is also used as the CNVSS
pin^V1
Connect an approximately 5 kW resistor to
the VPP pin the shortest possible in series and
also to the VSS pin. When not connecting the
resistor, make the length of wiring between
the VPP pin and the VSS pin the shortest possible
(refer to “countermeasure example 1 of
Figure 6.4.4”)
V1 When a microcomputer has the CNVSS pin,
the VPP pin is also used as the CNVSS pin.
Note: Even when a circuit which included an
approximately 5 kW resistor is used in
the Mask ROM version, the micro-
computer operates correctly.
CNVSS/VPP
Approximately
5kΩ
VSS
In the shortest
distance
Fig.6.4.4 Wiring for VPP pin of One Time
PROM and EPROM version
Reason
The VPP pin of the One Time PROM and the
EPROM version is the power source input pin
for the built-in PROM. When programming in
the built-in PROM, the impedance of the VPP
pin is low to allow the electric current for writing
flow into the PROM. Because of this, noise
can enter easily. If noise enters the VPP pin,
abnormal instruction codes or data are read
from the built-in PROM, which may cause a
program runaway.
6-12
7220 Group User’s Manual