BU99901GUZ-W_10 Datasheet, PDF (13/17 Pages) Rohm – WL-CSP EEPROM family I2C BUS

BU99901GUZ-W
Technical Note
●Cautions on microcontroller connection
○Rs
In I²C BUS, it is recommended that SDA port is of open drain input/output. However, when to use CMOS input / output of
tri state to SDA port, insert a series resistance Rs between the pull up resistance Rpu and the SDA terminal of EEPROM.
This is controls over current that occurs when PMOS of the microcontroller and NMOS of EEPROM are turned ON
simultaneously. Rs also plays the role of protection of SDA terminal against surge. Therefore, even when SDA port is
open drain input/output, Rs can be used.
ACK
SCL
RS
SDA
'H' output of microcontroller
'L' output of EEPROM
Microcontroller
EEPROM
Over current flows to SDA line by 'H' output of microcontroller
and 'L' output of EEPROM.
Fig.45 I/O circuit diagram
Fig.46 Input/output collision timing
○Maximum value of Rs
The maximum value of Rs is determined by following relations.
(1) SDA rise time to be determined by the capacity (CBUS) of bus line of Rpu and SDA shoulder be tR or below.
And AC timing should be satisfied even when SDA rise time is late.
(2) The bus electric potential ○^A to be determined by Rpu and Rs the moment when EEPROM outputs 'L' to SDA bus
should sufficiently secure the input 'L' level (V_IL) of microcontroller including recommended noise margin 0.1Vcc.
A
RS
IOL
VCC
RPU=10ｋΩ
VOL
(VCC－V OL)× RS
RPU+RS
+ VOL+0.1VCC≦VIL
∴
RS ≦
VIL－VOL－0.1VCC
1.1VCC－V IL
×
RPU
Bus line
VIL
capacity CBUS
Microcontroller
EEPROM
Fig.47 I/O circuit diagram
Example）When VCC=3V,　VIL=0.3VCC,　VOL=0.4V,　RPU=10kΩ,
from(2),
RS
≦
0.3×3－0.4－0.1×3
1.1×3－0.3×3
×
10×10³
≦ 0.835［kΩ］
○Maximum value of Rs
The minimum value of Rs is determined by over current at bus collision. When over current flows, noises in power source
line, and instantaneous power failure of power source may occur. When allowable over current is defined as I, the
following relation must be satisfied. Determine the allowable current in consideration of impedance of power source line
in set and so forth. Set the over current to EEPROM 10mA or below.
R_PU=10Ω
'L' output
RS
'H' output
Over current I
Microcontroller
EEPROM
Fig.48 I/O circuit diagram
VCC
RS
≦
I
∴
RS
≧
VCC
I
Example）When VCC=3V, I=10mA
RS
≧
3
10×10^-3
≧ 300［Ω］
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13/16
2010.09 - Rev.A