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FM31T376 Folha de dados(PDF) 7 Page - Ramtron International Corporation |
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FM31T376 Folha de dados(HTML) 7 Page - Ramtron International Corporation |
7 / 26 page FM31T372/374/376/378-G Rev. 1.1 Apr. 2011 Page 7 of 26 The serial number is located in registers 11h to 18h. The lock bit is SNL, register 0Bh bit 7. Setting the SNL bit to „1‟ disables writes to the serial number registers, and the SNL bit cannot be cleared. Real-Time Clock (TCXO) Operation The real-time clock is a timekeeping function that can be battery or capacitor backed for continuous operation. The RTC is operated by a temperature compensated crystal oscillator (TCXO) based on an embedded 32.768 kHz crystal. The RTC consists of an oscillator, clock divider, and a register system for user access. It divides down the 32.768 kHz time-base and provides a minimum resolution of seconds (1Hz). Static registers provide the user with read/write access to the time values. It includes registers for seconds, minutes, hours, day-of-the-week, date, months, and years. A block diagram (Figure 8) illustrates the RTC function. The user registers are synchronized with the timekeeper core using R and W bits in register 00h described below. Changing the R bit from „0‟ to „1‟ transfers timekeeping information from the core into holding registers that can be read by the user. If a timekeeper update is pending when R is set, then the core will be updated prior to loading the user registers. The registers are frozen and will not be updated again until the R bit is cleared to „0‟. R is used for reading the time. Setting the W bit to „1‟ locks the user registers. Clearing it to „0‟ causes the values in the user registers to be loaded into the timekeeper core. W is used for writing new time values. Users should be certain not to load invalid values, such as FFh, to the timekeeping registers. Updates to the timekeeping core occur continuously except when locked. Backup Power The real-time clock/calendar is intended to be permanently powered. When the primary system power fails, the voltage on the VDD pin will drop. When VDD is less 2.4V the RTC (and event counters) will switch to the backup power supply on VBAK. The clock operates at extremely low current in order to maximize battery or capacitor life. However, an advantage of combining a clock function with F-RAM memory is that data is not lost regardless of the backup power source. The IBAK current varies with temperature and voltage (see DC parametric table). The following graph shows IBAK as a function of VBAK. These curves are useful for calculating backup time when a capacitor is used as the VBAK source. Figure 8. IBAK vs. VBAK Voltage The minimum VBAK voltage varies linearly with temperature. The user can expect the minimum VBAK voltage to be 1.23V at +85°C and 1.90V at -40°C. The tested limit is 1.55V at +25°C. The minimum VBAK voltage has been characterized at -40°C and +85°C but is not 100% tested. Figure 9. VBAK (min.) vs. Temperature Trickle Charger To facilitate capacitor backup, the VBAK pin can optionally provide a trickle charge current. When the VBC bit, register 0Bh bit 2, is set to „1‟ the VBAK pin will source approximately 80µA until VBAK reaches VDD or 3.75V whichever is less. In 3V systems, this charges the capacitor to VDD without an external diode and resistor charger. There is a Fast Charge mode which is enabled by the FC bit (register 0Bh, bit 5). In this mode the trickle charger current is set to approximately 1 mA, allowing a large backup capacitor to charge more quickly. In the case where no battery is used, the VBAK pin should be tied to VDD. Although VBAK may be connected to VSS, this is not recommended if the companion is used. None of the companion functions will operate below approximately 2.4V. |
Nº de peça semelhante - FM31T376 |
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Descrição semelhante - FM31T376 |
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