CPC5902

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3.2 Calculating Minimum Pull-Up Resistor Values

supply voltage range and the weakest load current

sinking device on the bus. Note: Systems that do not

need maximum bandwidth and busses with lower

capacitive loading can use a higher value for the

pull-up resistor to reduce power consumption.

inputs below 0.4V for outputs rated at 3mA or 0.6V for

For example, if the weakest device is only guaranteed

to sink 3mA then the maximum allowed logic low

output voltage will be 0.4V. For designs with

pull-up resistor is:

the minimum value of the pull-up resistor is easily

calculated as:

Chose a standard value resistor that will not violate

this minimum value over tolerance and temperature,

such as a 1.1k

, 1% tolerance, 100ppm/C

temperature coefficient resistor.

If all the non-CPC5902 devices on the Side A bus are

Fast-mode compliant (400pF capacitive loading

capable) with the required 6mA current sink capability,

then the bus can be configured for Fast-mode.

Resistor selection for Fast-mode is similar to the

example given above but because the logic low output

level is greater (0.6V) then the voltage across the

pull-up resistor will be less. Calculation of the

compliant Fast-mode bus minimum pull-up resistor

value is given by:

The minimum E96 standard value 1% tolerance,

100ppm/

C temperature coefficient resistor is 511.

Calculating the pull-up resistor for Side B is similar to

the process used for Side A but with some additional

considerations.

Before proceeding, it must be pointed out that Side B

of the CPC5902 is Fast-mode compliant with

outputs are 6mA capable allowing bus operation of

400kb/s with up to 400pF of capacitive loading. For

are only rated for 3mA but can be operated at

Fast-mode speeds of 400kb/s whenever the bus

loading of the Side B bus limits the CPC5902 to data

rates of 100kb/s.

First, it must be determined if the Side B bus will be

configured for 3mA or 6mA operation. This is done by

evaluating the external (non-CPC5602) devices on the

Side B bus and the operational capabilities of the

CPC5902. There are three possibilities:

1) One or more of the external devices is limited to

3mA of output current sink.

2) All of the external devices are rated at 6mA of

output current sink and the Side B minimum supply

3) All of the external devices are rated at 6mA of

output current sink and the Side B minimum supply

For conditions 1 and 2 above the bus must be

configured for 3mA. Condition 3 is the only situation

where the bus can be configured for 6mA, a

Fast-mode requirement when capacitive bus loading is

an issue.

Second, it is necessary to configure the Side B bus to

be compatible with the CPC5902’s lower logic low

input threshold:

As discussed earlier, this lower input threshold

requirement is to ensure the CPC5902 can drive a

devices on the bus, but will not accept it’s own logic

low output. This prevents latching of the CPC5902.

Additionally, this implies there can be no more than

one limited drive (Side B) CPC5902 interface on the

Side B bus, and that all other devices on the Side B

thresholds. Because the CPC5902 Side A inputs are

compatible with this requirement, any number of