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DS32B35-33IND Datasheet(PDF) 10 Page - Maxim Integrated Products |
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DS32B35-33IND Datasheet(HTML) 10 Page - Maxim Integrated Products |
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10 / 21 page  Accurate I2C RTC with Integrated TCXO/Crystal/FRAM 10 ____________________________________________________________________ Operation The Block Diagram shows the main elements of the DS32x35. The nine blocks can be grouped into six functional groups: TCXO, power control, pushbutton function, RTC, I2C interface, and FRAM. Their opera- tions are described separately in the following sections. 32kHz TCXO The temperature sensor, oscillator, and control logic form the TCXO. The controller reads the output of the on-chip temperature sensor and uses a lookup table to determine the capacitance required, adds the aging correction in the AGE register, and then sets the capacitance selection registers. New values, including changes to the AGE register, are loaded only when a change in the temperature value occurs, or when a user-initiated temperature conversion is completed. The temperature is read on initial application of VCC and once every 64 seconds afterwards while the device is powered by either VCC or VBAT. Power Control This function is provided by a temperature-compensat- ed voltage reference and a comparator circuit that monitors the VCC level. When VCC is greater than VPF, the part is powered by VCC. When VCC is less than VPF but greater than VBAT, the RTC is powered by VCC. If VCC is less than VPF and is less than VBAT, the device is powered by VBAT. See Table 1. The RTC can be accessed when the device is powered by either VCC or VBAT. The FRAM is only accessible when the device is powered by VCC. The FRAM must not be accessed when VCC < VCC(MIN). To preserve the battery, the first time VBAT is applied to the device, the oscillator will not start up until VCC exceeds VPF, or until a valid I2C address is written to the part. Typical oscillator startup time is less than one sec- ond. Approximately 2 seconds after VCC is applied, or a valid I2C address is written, the device makes a temper- ature measurement and applies the calculated correc- tion to the oscillator. Once the oscillator is running, it continues to run as long as a valid power source is avail- able (VCC or VBAT), and the device continues to mea- sure the temperature and correct the oscillator frequency every 64 seconds. Pushbutton Reset Function The DS32x35 provides for a pushbutton switch to be con- nected to the RST output pin. When the device is not in a reset cycle, it continuously monitors the RST signal for a low going edge. If an edge transition is detected, the device debounces the switch by pulling RST low. After the internal timer has expired (PBDB), the device contin- ues to monitor the RST line. If the line is still low, the device continuously monitors the line looking for a rising edge. Upon detecting release, the device forces the RST pin low and holds it low for tRST. RST is also used to indicate a power-fail condition. When VCC is lower than VPF, an internal power-fail sig- nal is generated, which forces the RST pin low. When VCC returns to a level above VPF, the RST pin is held low for tREC to allow the power supply to stabilize. If the oscillator is not running (see the Power Control section) when VCC is applied, tREC is bypassed and RST imme- diately goes high. The state of RST does not affect the operation of the TCXO, I2C interface, FRAM, or RTC functions. Real-Time Clock With the clock source from the TCXO, the RTC provides seconds, minutes, hours, day, date, month, and year information. The date at the end of the month is automati- cally adjusted for months with fewer than 31 days, includ- ing corrections for leap year. The clock operates in either the 24-hour or 12-hour format with an AM/PM indicator. The clock provides two programmable time-of-day alarms and a programmable square-wave output. The INT/SQW pin either generates an interrupt due to alarm condition or outputs a square-wave signal and the selection is controlled by the bit INTCN. I2C Interface The FRAM I2C interface is accessible whenever VCC is at a valid level. The RTC I2C interface is accessible whenever either VCC or VBAT is at a valid level. If a microcontroller connected to the device resets because of a loss of VCC or other event, it is possible that the microcontroller and the RTC I2C communications could become unsynchronized, e.g., the microcontroller resets while reading data from the RTC. When the microcon- troller resets, the RTC I2C interface may be placed into a known state by toggling SCL until SDA is observed to be at a high level. At that point the microcontroller should pull SDA low while SCL is high, generating a START condition. SUPPLY CONDITION ACTIVE SUPPLY FRAM ACCESS* RTC ACCESS VCC < VPF, VCC < VBAT VBAT No Yes VCC < VPF, VCC > VBAT VCC No Yes VCC > VPF, VCC < VBAT VCC Yes Yes VCC > VPF, VCC > VBAT VCC Yes Yes Table 1. Device Operation * Read/write access is not inhibited by the device, but must not be done to avoid FRAM data errors. |
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