R2025S/D
Interfacing with the CPU
The R2025S/D employs the I2C-Bus system to be connected to the CPU via 2-wires. Connection and system of
I2C-Bus are described in the following sections.
Connection of I2C-Bus
2-wires, SCL and SDA pins that 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 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 resistor to each
signal line as shown in the figure below. Each IC is designed not to affect SCL and SDA signal lines when power
to each of these is turned off separately.
VDD1
*
For data interface, the following
conditions must be met:
VCC4≥VCC1
VDD2
VDD3
VDD4
VCC4≥VCC2
VCC4≥VCC3
Rp
Rp
*
When the master is one, the
micro-controller is ready for driving
SCL to “H” and Rp of SCL may not be
required.
SCL
SDA
Other
Peripheral
Device
Micro-
Controller
R2025S/D
Cautions on determining Rp resistance,
(1) Dropping voltage 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 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 standby mode ) × Bus standby duration
Bus stand-by duration + the 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”
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RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
