RS5C372B-E2(1997) View Datasheet(PDF) - RICOH Co.,Ltd.

Part Name

Description

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RS5C372B-E2
(Rev.:1997)

I2C-bus Serial Interface Real Time Clock (8pin SSOP)

RICOH Co.,Ltd. 

RS5C372B

s Functional Description

1. Interfacing with the CPU

The RS5C372B employs the I2C-bus system to be connected to the CPU via two signal lines. Connection

and transfer system of I2C-bus are described in the following section

1.1. Connection of I2C-bus

Two signal lines, SCL and SDA which are connected to I2C-bus are used for transmit clock pulses and data

respectively. All ICs that are connected to these lines are designed that will be not be clamped when a voltage

beyond supply voltage is applied to input or output pins. Open drain pins are used for output. This construction

allows communication of signals between ICs with different supply voltages by adding a pull-up resistance to each

signal line as shown in the figure below. Each IC is de-signed not to affect SCL and SDA signal lines when power

to each of these is turned off separately.

VDD1

VDD2

VDD3

VDD4

Rp

SCL

SDA

Micro-

Controller

Rp

RS5C372B

Other

Peripheral

Device

*) For data interface, the

following conditions must

be met:

VDD4≥VDD1

VDD4≥VDD2

VDD4≥VDD3

*) When the master is one, the

micro controller is ready for

driving SCL to “H” and Rp of

SCL may not be required.

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 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 “x 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. “x 2” in the numerator of the

same member is because there are two pins of SDA and SCL. The third member, (charging/discharging

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