DS2490 查看數據表（PDF） - Maxim Integrated

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DS2490

USB to 1-Wire Bridge Chip

Maxim Integrated 

ACTIVE PULLUP Figure 3

5V

VIAPTO

tAPUOT

DS2490

VIAPO

0V

1-Wire bus

is discharged t 1

t2 t3

The circuit operates as follows: At t1, the pulldown (induced by the DS2490 or a device on the bus) ends.

From this point on the 1-Wire bus is pulled high by the weak pullup current IWEAKPU provided by the

DS2490. The slope is determined by the load on the bus and the value of the pullup current. At t2, the

voltage crosses the threshold voltage VIAPO. Now, the DS2490 switches over from the weak pullup

current IWEAKPU to the higher current IACTPU. As a consequence, the voltage on the bus now rises faster.

As the voltage on the bus crosses the threshold VIAPTO at t3, a timer is started. As long as this timer is on

(tAPUOT), the IACTPU current will continue to flow. After the timer is expired, the DS2490 will switch back

to the weak pullup current.

Falling Edges (DS2490-initiated)

Whenever the DS2490 begins pulling the 1-Wire bus low to initiate a time slot, for example, it first turns

off the weak pullup current IWEAKPU. Then, at regular and overdrive speed it will generate a falling edge at

a slew rate of typically 15 V/µs. This value is acceptable for short 1-Wire busses and adequate for

communication at overdrive speed. For 1-Wire networks of more than roughly 30m length, flexible speed

should always be used. One of the parameters that is adjustable at flexible speed is the slew rate of

DS2490-initiated falling edges. The effect of the slew rate control is shown in Figure 4.

SLEW RATE CONTROL Figure 4

tF

Target for

long lines:

5V

4 ± 0.5 µs

low slew rate

high slew rate

0.8 V

0V

1-Wire bus

is pulled up

weak pull-up ends,

t 1 pull-down begins

Extensive tests have shown that 1-Wire networks with lengths of up to 300m will perform best if the fall

time tF is in the range of 4 ± 0.5µs. This translates into a slew rate of approximately 1V/µs. This slew rate

is typically achieved by selecting a PULLDOWN SLEW RATE parameter code of 0 x 4 (see MODE

COMMANDS). If the actual measured fall time is longer than the target value, a parameter code of 0 x 3

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