71M6531D_10 Datasheet, PDF (36/120 Pages) Teridian Semiconductor Corporation – Energy Meter IC

Data Sheet 71M6531D/F-71M6532D/F
FDS 6531/6532 005
1.5 On-Chip Resources
1.5.1 Oscillator
The oscillator of the 71M6531D/F and 71M6532D/F drives a standard 32.768 kHz watch crystal. These
crystals are accurate and do not require a high-current oscillator circuit. The oscillator of the 71M6531D/F
and 71M6532D/F has been designed specifically to handle these crystals and is compatible with their
high impedance and limited power handling capability.
Oscillator calibration can improve the accuracy of both the RTC and metering. Refer to Section 1.5.3
Real-Time Clock (RTC) for more information.
The oscillator is powered directly and only from VBAT, which therefore must be connected to a DC voltage
source. The oscillator requires approximately 100 nA, which is negligible compared to the internal leakage
of a battery.
The oscillator may appear to work when VBAT is not connected, but this mode of operation is not
recommended.
If VBAT is connected to a drained battery or disconnected, a battery test that sets BME may drain
VBAT’s supply and cause the oscillator to stop. A stopped oscillator may force the device to reset.
Therefore, an unexpected reset during a battery test should be interpreted as a battery failure.
1.5.2 Internal Clocks
Timing for the device is derived from the 32.768 kHz crystal oscillator output. On-chip timing functions
include:
• The MPU clock (CKMPU)
• The emulator clock (2 x CKMPU)
• The clock for the CE (CKCE)
• The clock driving the delta-sigma ADC along with the FIR (CKADC, CKFIR)
• A real time clock (RTC).
The two general-purpose counter/timers contained in the MPU are controlled by CKMPU (see Section
1.4.7 Timers and Counters). Table 37 provides a summary of the available clock functions.
Clock
CKPLL
MCK
CKCE
CKADC / CKFIR
CKMPU maximum
Table 37: Clock System Summary
Derived
From
MCK Divider / [M40MHZ, M26MHZ]
÷2 / [1,0]
÷3 / [0,1] ÷4^** / [0,0]
Crystal
78.6432 MHz
78.6432
MHz
78.6432
MHz
CKPLL
39.3216 MHz
26.2144
MHz
19.6608
MHz
MCK
4.9152
MHz ^†
9.8304
MHz ^†
6.5536MHz 4.9152 MHz
MCK
4.9152 MHz
6.5536 MHz 4.9152 MHz
MCK
9.8304 MHz^***
6.5536 MHz 4.9152 MHz
***
***
Brownout
Mode
off
112 kHz
off
28 kHz
28 kHz
CK32
MCK
32.768 kHz
32.768 kHz 32.768 kHz
** _{Default state at power-up}
*** _{The maximum CKMPU frequency. CKMPU can be reduced from this rate using MPU_DIV[2:0].}
† _{CKCE = 9.8304 MHz when CE10MHZ is set, 4.9152 MHz otherwise.}
The master clock, MCK, is generated by an on-chip PLL that multiplies the oscillator output frequency
(CK32) by 2400 to provide approximately 80 MHz (78.6432 MHz). A divider controlled by the I/O RAM
bits M40MHZ and M26MHZ permits scaling of MCK by ½, ⅓ and ¼. All other clocks are derived from this
scaled MCK output (making them multiples of 32768 Hz), and the clock skew is matched so that the rising
edges of CKADC, CKCE, CK32 and CKMPU are aligned.
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