Reverse Compatibility of the D

文章摘要：POL serves the same function in both the DS1621 and the DS1721; that is, it defines the active polarity of the TOUT thermostat output. However, there is a possible compatibility issue with this bit. In the DS1621 the POL bit is stored in EEPROM and will thus power-up containing the last data written

Reverse Compatibility of the D,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

POL serves the same function in both the DS1621 and the DS1721; that is, it defines the active polarity of the TOUT thermostat output. However, there is a possible compatibility issue with this bit. In the DS1621 the POL bit is stored in EEPROM and will thus power-up containing the last data written to it. On the other hand, in the DS1721 this bit will always power-up as a 1 (TOUT = active high). Thus, if a DS1621 application presets the POL bit to 0 (TOUT = active low), and does not write a 0 to the POL bit as part of every power-up sequence, software (or hardware) changes will be required when transitioning to the DS1721 in order for TOUT to have the correct polarity.

1SHOT also serves the same function on both the DS1721 and DS1621. It defines whether temperature conversions occur continuously or only once (i.e., one-shot mode) per Start Convert T command. However, again there is a possible compatibility issue with this bit. The 1SHOT bit in the DS1621 is stored in EEPROM and will power-up containing the last data written to it. However, in the DS1721, the 1SHOT bit will always power-up as a 0 (continuous conversion mode). Thus, if a DS1621 application presets the 1SHOT bit to a 1 (one-shot mode) and does not rewrite a 1 to that that bit as part of every power-up sequence, there will be an incompatibility that requires software modifications when transitioning to the DS1721—namely, the DS1721 will perform continuous temperature conversions instead of just one conversion when the Start Convert T command (EEh) is issued.

Although the DS1721 will function properly in continuous conversion mode, this could present a problem in applications where one-shot mode is desired for maximum power savings. The DS1721 uses up to 1mA (max) operating current when it is actively converting, whereas the standby current when conversions are not taking place is 1µA. This 1000X increase in power supply current is probably unacceptable for power-sensitive applications. Another issue that can arise if the DS1721 is operating in continuous mode instead of one-shot mode involves applications that use data from the slope accumulator and counter registers for high-resolution temperature calculations as per AN105. In order for these registers to function properly, the device must be in one-shot mode (same as in the DS1621).

Thermostat Trip Point Registers (TH and TL)

Both the DS1621 and DS1721 utilize the same protocols to access the TH (Access TH=A1h) and TL (Access TL=A2h) registers, so writing to and reading from these registers is identical for both devices. However, there is a possible compatibility issue with these registers. On the DS1621 the TH and TL registers are stored in EEPROM and will always power-up containing the most recent values written to them by the application. On the other hand, in the DS1721 these registers will always power up with TH=80°C and TL=75°C. Thus, if a DS1621 application presets the thermostat trip points but does not rewrite these registers as part of every power-up sequence, software changes will be required when transitioning to the DS1721 to ensure correct thermostat operation.

Temperature-to-Digital Conversion Time

The maximum conversion time specification for the DS1621 is 1 second. The maximum conversion time specification for the DS1721 is 1.2 seconds for 12-bit resolution. When the DS1721 is used in 1621 mode, the conversion time will be 1.2 seconds because 12-bit conversions are performed in this mode.

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