RV5C387A_03 Datasheet, PDF (23/49 Pages) RICOH electronics devices division – I2C bus SERIAL INTERFACE REAL-TIME CLOCK IC WITH VOLTAGE MONITORING FUNCTION

RV5C387A
Cautions on Determining RP Resistance
(1) Voltage drop at RP due to sum of input current or output current at off conditions on each IC pin
connected to the I²C bus shall be adequately small.
(2) Rising time of each signal shall be kept short even when all capacity of the bus is driven.
(3) Current consumed in I²C bus is small compared to the consumption current permitted for the entire
system.
When all ICs connected to I²C bus are CMOS type, condition (1) may usually be ignored since input current
and off state output current is extremely small for the many CMOS type ICs.
Thus the maximum resistance of RP may be determined based on (2) while the minimum on (3) in most
cases.
In actual cases a resistor may be place between the bus and input/output pins of each IC to improve noise
margins in which case the RP minimum value may be determined by the resistance.
Consumption current in the bus to review (3) above may be expressed by the formula below:
Bus consumption current ^.= ^{(Sum of input current and off state output current of all devices in stand-by mode) × Bus stand-by duration}
Bus stand-by duration + bus operation duration
Supply voltage × bus operation duration × 2
+ _{RP resistance × 2 × (bus stand-by duration + bus operation duration)}
+ supply voltage × bus capacity × charging/discharging times per unit time
Operation of “× 2” in the second member denominator in the above formula is derived from assumption that
“L” duration of SDA and SCL pins are the half of bus operation duration. “× 2” in the numerator of the same
member is because there are two pins of SDA and SCL. The third member, (charging/discharging times per
unit time) means number of transition from “H” to “L” of the signal line.
Calculation example is shown below:
Pull-up resistor (RP)=10kΩ, Bus capacity=50pF (both for SCL and SDA), VDD=3V
In as system with sum of input current and off state output current of each pin=0.1µA, I²C bus is used for
10ms every second while the rest of 990ms is in the stand-by mode. In this mode number of transitions of the
SCL pin from “H” to “L” state is 100 while SDA 50, every second.
Bus consumption current ^.= ^{0.1µA × 990ms}
990ms + 10ms
3V × 10ms × 2
+ _{10kΩ × 2 × (990ms + 10ms)}
+ 3V × 50pF × (100 + 50)
= 0.099µA + 3.0µA + 0.0225µA = 3.12µA
Generally, the second member of the above formula is larger enough than the first and the third members,
bus consumption current may be determined by the second member in many cases.
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