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CY14B108K-ZS45XI Datasheet(PDF) 10 Page - Cypress Semiconductor |
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CY14B108K-ZS45XI Datasheet(HTML) 10 Page - Cypress Semiconductor |
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10 / 34 page  CY14B108K, CY14B108M Document Number: 001-47378 Rev. *I Page 10 of 34 Setting the Clock A write access to the RTC device stops updates to the time keeping registers and enables the time to be set when the write bit ‘W’ (in the Flags register at 0xFFFF0) is set to ‘1’. The correct day, date, and time is then written into the registers and must be in 24 hour BCD format. The time written is referred to as the “Base Time”. This value is stored in nonvolatile registers and used in the calculation of the current time. When the write bit ‘W’ is cleared by writing ‘0’ to it, the values of timekeeping registers are transferred to the actual clock counters after which the clock resumes normal operation. If the time written to the timekeeping registers is not in the correct BCD format, each invalid nibble of the RTC registers continue counting to 0xF before rolling over to 0x0 after which RTC resumes normal operation. Note After ‘W’ bit is set to ‘0’, values written into the timekeeping, alarm, calibration, and interrupt registers are transferred to the RTC time keeping counters in tRTCp time. These counter values must be saved to nonvolatile memory either by initiating a Software/Hardware STORE or AutoStore operation. While working in AutoStore disabled mode, perform a STORE operation after tRTCp time while writing into the RTC registers for the modifications to be correctly recorded. Backup Power The RTC in the CY14B108K is intended for permanently powered operation. The VRTCcap or VRTCbat pin is connected depending on whether a capacitor or battery is chosen for the application. When the primary power, VCC, fails and drops below VSWITCH the device switches to the backup power supply. The clock oscillator uses very little current, which maximizes the backup time available from the backup source. Regardless of the clock operation with the primary source removed, the data stored in the nvSRAM is secure, having been stored in the nonvolatile elements when power was lost. During backup operation, the CY14B108K consumes a 0.35 µA (Typ) at room temperature. The user must choose capacitor or battery values according to the application. Backup time values based on maximum current specifications are shown in the following Table 2. Nominal backup times are approximately two times longer. Using a capacitor has the obvious advantage of recharging the backup source each time the system is powered up. If a battery is used, a 3 V lithium is recommended and the CY14B108K sources current only from the battery when the primary power is removed. However, the battery is not recharged at any time by the CY14B108K. The battery capacity must be chosen for total anticipated cumulative down time required over the life of the system. Stopping and Starting the Oscillator The OSCEN bit in the calibration register at 0xFFFF8 controls the enable and disable of the oscillator. This bit is nonvolatile and is shipped to customers in the “enabled” (set to ‘0’) state. To preserve the battery life when the system is in storage, OSCEN must be set to ‘1’. This turns off the oscillator circuit, extending the battery life. If the OSCEN bit goes from disabled to enabled, it takes approximately one second (two seconds maximum) for the oscillator to start. While system power is off, if the voltage on the backup supply (VRTCcap or VRTCbat) falls below their respective minimum level, the oscillator may fail.The CY14B108K has the ability to detect oscillator failure when system power is restored. This is recorded in the Oscillator Fail Flag (OSCF) of the Flags register at the address 0xFFFF0. When the device is powered on (VCC goes above VSWITCH) the OSCEN bit is checked for the ‘enabled’ status. If the OSCEN bit is enabled and the oscillator is not active within the first 5 ms, the OSCF bit is set to ‘1’. The system must check for this condition and then write ‘0’ to clear the flag. Note that in addition to setting the OSCF flag bit, the time registers are reset to the ‘Base Time’, which is the value last written to the timekeeping registers. The control or calibration registers and the OSCEN bit are not affected by the ‘oscillator failed’ condition. The value of OSCF must be reset to ‘0’ when the time registers are written for the first time. This initializes the state of this bit which may have become set when the system was first powered on. To reset OSCF, set the write bit ‘W’ (in the Flags register at 0xFFFF0) to a ‘1’ to enable writes to the Flags register. Write a ‘0’ to the OSCF bit and then reset the write bit to ‘0’ to disable writes. Calibrating the Clock The RTC is driven by a quartz controlled crystal with a nominal frequency of 32.768 kHz. Clock accuracy depends on the quality of the crystal and calibration. The crystals available in market typically have an error of +20 ppm to +35 ppm. However, CY14B108K employs a calibration circuit that improves the accuracy to +1/–2 ppm at 25 °C. This implies an error of +2.5 seconds to –5 seconds per month. The calibration circuit adds or subtracts counts from the oscillator divider circuit to achieve this accuracy. The number of pulses that are suppressed (subtracted, negative calibration) or split (added, positive calibration) depends upon the value loaded into the five calibration bits found in Calibration register at 0xFFFF8. The calibration bits occupy the five lower order bits in the Calibration register. These bits are set to represent any value between ‘0’ and 31 in binary form. Bit D5 is a sign bit, where a ‘1’ indicates positive calibration and a ‘0’ indicates negative calibration. Adding counts speeds the clock up and subtracting counts slows the clock down. If a binary ‘1’ is loaded into the register, it corre- sponds to an adjustment of 4.068 or –2.034 ppm offset in oscil- lator error, depending on the sign. Calibration occurs within a 64-minute cycle. The first 62 minutes in the cycle may, once every minute, have one second shortened by 128 or lengthened by 256 oscillator cycles. If a binary ‘1’ is loaded into the register, only the first two minutes of the 64-minute cycle are modified. If a binary 6 is loaded, the first 12 are affected, and so on. Therefore, each calibration step has the Table 2. RTC Backup Time Capacitor Value Backup Time 0.1 F 72 hours 0.47 F 14 days 1.0 F 30 days |
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