Data Sheet

User manual Rev. 6 — 4 April 2014 58 of 64

NXP Semiconductors

UM10204

C-bus specification and user manual

The switched pull-up circuit in Figure 44 is for a supply voltage of V

=5V± 10 % and a

maximum capacitive load of 400 pF. Since it is controlled by the bus levels, it needs no

additional switching control signals. During the rising/falling edges, the bilateral switch in

the HCT4066 switches pull-up resistor R

on/off at bus levels between 0.8 V and 2.0 V.

Combined resistors R

and R

can pull up the bus line within the maximum specified

rise time (t

) of 300 ns.

Series resistors R

are optional. They protect the I/O stages of the I

C-bus devices from

high-voltage spikes on the bus lines, and minimize crosstalk and undershoot of the bus

line signals. The maximum value of R

is determined by the maximum permitted voltage

drop across this resistor when the bus line is switched to the LOW level in order to switch

off R

Additionally, some bus buffers contain integral rise time accelerators. Stand-alone rise

time accelerators are also available.

7.3 Series protection resistors

As shown in Figure 45, series resistors (R

) of, for example, 300 Ω can be used for

protection against high-voltage spikes on the SDA and SCL lines (resulting from the

flash-over of a TV picture tube, for example). If series resistors are used, designers must

add the additional resistance into their calculations for R

and allowable bus capacitance.

The required noise margin of 0.1V

for the LOW level, limits the maximum value of R

s(max)

as a function of R

is shown in Figure 46. Note that series resistors affect the

output fall time.

Fig 45. Series resistors (R

) for protection against high-voltage spikes

mbc627

SDA

SCL

DEVICE