P16F877 datasheet(96/218 Pages) MICROCHIP

96 / 218 page

PIC16F87X

DS30292C-page 94

 2001 Microchip Technology Inc.

9.3

Connection Considerations for

I2C Bus

For standard-mode I2C bus devices, the values of

resistors Rp and Rs in Figure 9-27 depend on the fol-

lowing parameters:

• Supply voltage

• Bus capacitance

• Number of connected devices

(input current + leakage current)

The supply voltage limits the minimum value of resistor

Rp, due to the specified minimum sink current of 3 mA at

VOL max = 0.4V, for the specified output stages. For

example, with a supply voltage of VDD = 5V±10% and

VOL max = 0.4V at 3 mA, Rp min = (5.5-0.4)/0.003 = 1.7 kΩ.

VDD as a function of Rp is shown in Figure 9-27. The

desired noise margin of 0.1VDD for the low level limits

the maximum value of Rs. Series resistors are optional

and used to improve ESD susceptibility.

The bus capacitance is the total capacitance of wire,

connections, and pins. This capacitance limits the max-

imum value of Rp due to the specified rise time

(Figure 9-27).

The SMP bit is the slew rate control enabled bit. This bit

is in the SSPSTAT register, and controls the slew rate

of the I/O pins when in I2C mode (master or slave).

FIGURE 9-27:

SAMPLE DEVICE CONFIGURATION FOR I2C BUS

VDD + 10%

SDA

SCL

DEVICE

Cb=10 - 400 pF

Note: I2C devices with input levels related to VDD must have one common supply line to which the pull-up resistor is also

connected.

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