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DS2480S Datasheet(PDF) 7 Page - Dallas Semiconductor |
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DS2480S Datasheet(HTML) 7 Page - Dallas Semiconductor |
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7 / 26 page  DS2480 042498 7/26 ator control flag is to be set. If the flag is set to a 1 (on) the device translates every byte received in data mode into a 12–bit sequence on the 1–Wire bus. For details on how the Search Accelerator works please refer to the section Search Accelerator Operation. Before activat- ing the Search Accelerator, one must make sure that the strong pull–up after every byte is disarmed (see Pulse Command). The Search Accelerator command does not generate a command response byte. Although the Search Accelerator Control command itself does not generate any 1–Wire activity, it can be used to select the communication speed on the 1–Wire bus. The speed selection (if different from the previous setting, e.g., from a Reset command) will take effect immediately. Pulse The Pulse command serves several functions that are selected by the contents of bit 1 and bit 4 of the com- mand code. The main functions are generating a strong pull–up to 5V and generating 12V programming pulses for EPROM devices (if the 12V are available at the VPP pin). The secondary function of the pulse command is arming and disarming a strong pull–up after every sub- sequent byte in data mode. The arm/disarm function is controlled by bit 1 of the command code. Bit 4 deter- mines whether the device will generate a strong pull–up to 5V or a 12V programming pulse. The table below summarizes these options. BIT 4 BIT 1 FUNCTION 0 0 strong pull–up to 5V and disarm 1 0 12V Programming Pulse and disarm 0 1 strong pull–up to 5V and arm 1 1 12V Programming Pulse and arm The strong pull–up to 5V is required to program EEPROM devices or to operate special function devices that require a higher current for a limited time after having received a “go and convert” command. Therefore and because it significantly reduces the effective data throughput on the 1–Wire bus, the strong pull–up is disarmed most of the time. Although arming or disarming is simultaneously possible while generating a programming pulse, this is not recommended since it is likely to destroy the DS2480 if non–EPROM devices are connected to the 1–Wire bus. The duration of the strong pull–up or programming pulse is determined by configuration parameters and ranges from a few microseconds up to unlimited (see section Configuration Commands). However, unlimited duration is not allowed in conjunction with arming the strong–pull–up after every byte. As long as the DS2480 is in Command Mode the host may terminate a strong pull–up or programming pulse prematurely at any time by sending the command code F1h. The response byte is generated as soon as the strong pull–up or programming pulse is over (either because the predefined time has elapsed or due to termination). The response byte mainly returns the command code as sent by the host, but the two least significant bits are undefined. If the strong pull–up is armed and the device is in Data Mode, the end of the strong pull–up will be signaled as code F6h if the most significant bit of the preceding data byte on the 1–Wire bus was a 1 and 76h otherwise. The host will see this response byte in addition to the response on the data byte sent (see also section Wave Forms later in this document). SEARCH ACCELERATOR INTRODUCTION The Search Accelerator is a logic block inside the DS2480 that allows using the Search ROM function very efficiently under modern operating systems such as Windows and Windows 95/NT. Without the DS2480 all 1–Wire port adapters have to involve the computer’s CPU for every single time slot or pulse to be generated on the 1–Wire bus. Under DOS, accessing peripherals such as the UART or parallel port is very straight forward and therefore fast. Under Windows the situation is different and it may take several milliseconds or more to get the first time slot generated on the 1–Wire bus. Every subsequent time slot will be generated in much less time, since the computer simply sends out (“streams”) a long chain of bytes. This works reasonably well when reading or writ- ing large blocks of data. |
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