71M6533 Datasheet, PDF (38/132 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6533/G/H and 71M6534/H Data Sheet
FDS_6533_6534_004
The PLL has a 2x emulator clock which is controlled by the ECK_DIS bit. Since clock noise from this feature
may disturb the ADC, it is recommended that this option be avoided when possible.
The MPU clock frequency CKMPU is determined by another divider controlled by the I/O RAM field
MPU_DIV[2:0] and can be set to MCK/2^(MPU_DIV+2) Hz where MPU_DIV[2:0] varies from 0 to 6. The circuit
also generates the 2 x CKMPU clock for use by the emulator. The emulator clock is not generated when
ECK_DIS is asserted.
During a power-on reset, [M40MHZ, M26MHZ] defaults to [0,0], and the MCK divider is set to divide by 4.
When [M40MHZ, M26MHZ] = [1,0], the CE clock frequency may be set to ~5 MHz (4.9152 MHz) or ~10
MHz (9.8304 MHz), using the I/O RAM register CE10MHZ. In this mode, the ADC and FIR clock frequencies
remain at ~5 MHz. When [M40MHZ, M26MHZ] = [0,1], the CE, ADC, FIR, and MPU clock frequencies are
shifted to ~6.6 MHz (6.5536 MHz). This increases the ADC sample rate by 33%.
CE codes are tailored to particular clock frequencies. Changing the clock frequency for a
particular CE code may render it unusable.
In SLEEP mode, the M40MHZ and M26MHZ inputs to the clock generator are forced low. In BROWNOUT
mode, the clocks are derived from the crystal oscillator, and the clock frequencies are scaled by 7/8.
1.5.3 Real-Time Clock (RTC)
The RTC is driven directly by the crystal oscillator. It is powered by the net RTC_NV (battery-backed up
supply). The RTC consists of a counter chain and output registers. The counter chain consists of registers
for seconds, minutes, hours, day of week, day of month, month, and year. The RTC is capable of
processing leap years. Each counter has its own output register. The RTC registers will not be affected
by the reset pin, watchdog timer resets, or by transitions between the battery modes and mission mode.
Whenever the MPU reads the seconds register, all other output registers are automatically updated.
Since the RTC clock is not coherent to the MPU clock, the MPU must read the seconds register until two
consecutive reads are the same (this requires either 2 or 3 reads). At this point, all RTC output registers
will have the correct time. Regardless of the MPU clock speed, RTC reads require one wait state.
RTC time is set by writing to the RTC_SEC[5:0] through RTC_YR registers. Each write operation must be
preceded by a write operation to the WE register in I/O RAM. The value written to the WE register is
unimportant.
Time adjustments are written to the RTCA_ADJ[6:0], PREG[16:0] and QREG[1:0] registers. Updates to
PREG[16:0] and QREG[1:0] must occur after the one second interrupt, and must be finished before reaching
the next one-second boundary. The new values are loaded into the counters at the next one-second
boundary.
PREG[16:0] and QREG[1:0] are separate registers in the device hardware, but the bits are 16-bit contiguous
so the MPU firmware can treat them as a single register. A single binary number can be calculated and
then loaded into them at the same time.
The 71M6533 and 71M6534 have two rate adjustment mechanisms. The first is an analog rate adjustment,
using RTCA_ADJ[6:0], which trims the crystal load capacitance. Setting RTCA_ADJ[6:0] to 00 minimizes
the load capacitance, maximizing the oscillator frequency. Setting RTCA_ADJ[6:0] to 0x7F maximizes the
load capacitance, minimizing the oscillator frequency. The adjustable capacitance is approximately:
C _ADJ
=
RTCA _ ADJ
128
⋅16.5 pF
The typical adjustment range is approximately -15 ppm. The precise amount of adjustment will depend
on the crystal and board properties. The adjustment may occur at any time, and the resulting clock frequency
can be measured over a one-second interval.
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