DS2482-800: Eight-Channel 1-Wire Master

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Presence Pulse Masking (PPM)

The PPM bit controls whether the DS2482 will mask the leading edge (falling) of presence pulses. When PPM = 0,

masking is disabled. Presence pulse masking applies only to standard 1-Wire speed (1WS = 0); this bit has no

function if 1WS = 1 (Overdrive speed). Presence pulse masking can improve the performance of large 1-Wire

networks since it prevents the fast falling edge of a presence pulse generated by a 1-Wire slave device from

propagating through the network and getting reflected. Reflections can cause glitches in the network that in turn

may cause slave devices to lose synchronization with the 1-Wire master.

Figure 3 shows the timing references for the PPM. If enabled (PPM = 1), the DS2482 begins pulling the 1-Wire line

will be pulling the 1-Wire line low. The falling edge of the presence pulse mask is slew rate controlled.

Figure 3. Presence Pulse Masking

Pull-up

DS2482 Pull-down

1-W Slave Pull-down

DS2482 pull-down with PPM = 1; Standard speed only

RESET PULSE

PRESENCE PULSE

0V

APU con-

trolled edge

Resistive

pull-up

Strong Pullup (SPU)

bit of either a 1-Wire Write Byte command or after a 1-Wire Single Bit command has completed. The strong

pullup feature is commonly used with 1-Wire EEPROM devices when copying scratchpad data to the main memory

or when performing a SHA-1 computation, and with parasitically powered temperature sensors or A-to-D

converters. The respective device data sheets specify the location in the communications protocol after which the

strong pullup should be applied. The SPU bit in the configuration register of the DS2482 must be set immediately

prior to issuing the command that puts the 1-Wire device into the state where it needs the extra power.

If SPU is 1, the DS2482 applies active pullup to the rising edge of the time slot in which the strong pullup starts,

regardless of the APU bit setting. However, in contrast to setting APU = 1 for active pullup, the low-impedance

until: a) the next 1-Wire communication command (the typical case), b) by writing to the Configuration Register with

the SPU bit being 0 (alternative), or c) by issuing the Device Reset command. Additionally, when the pullup ends,

the SPU bit is automatically reset to 0. Using the strong pullup does not change the state of the APU bit in the

Configuration Register.