Datasheet

Data Sheet 71M6531D/F-71M6532D/F FDS 6531/6532 005
88 Rev 2
4.3 CE Interface Description
4.3.1 CE Program
The CE performs the precision computations necessary to accurately measure energy. Different code
variations are used for EQU[2:0] = 0 and EQU[2:0] = 1 or 2. The computations include offset cancellation,
products, product smoothing, product summation, frequency detection, VAR calculation, sag detection,
peak detection and voltage phase measurement. All data computed by the CE is dependent on the
selected meter equation as given by EQU[2:0]. Although EQU[2:0] = 0 and EQU[2:0] = 2 have the same
element mapping, the MPU code can use the value of EQU[2:0] to decide if element 2 is used for tamper
detection (typically done by connecting VB to VA) or as a second independent element.
The CE program is supplied by Maxim as a data image that can be merged with the MPU operational
code for meter applications. Typically, the CE program covers most applications and does not need to be
modified. Other variations of CE code may be available from Maxim. The description in this section ap-
plies to CE code revision CE31A04 (for EQU[2:0] = 0). Deviations for code revision CE31A03 (for
EQU[2:0] = 1 or 2) are noted where applicable.
4.3.2 CE Data Format
All CE words are 4 bytes. Unless specified otherwise, they are in 32-bit two’s complement format
(-1 = 0xFFFFFFFF). Calibration parameters are defined in flash memory (or external EEPROM) and
must be copied to CE data memory by the MPU before enabling the CE. Internal variables are used in
internal CE calculations. Input variables allow the MPU to control the behavior of the CE code. Output
variables are outputs of the CE calculations. The corresponding MPU address for the most significant
byte is given by 0x0000 + 4 x CE_address and by 0x0003 + 4 x CE_address for the least significant byte.
4.3.3 Constants
Constants used in the CE Data Memory tables are:
• F
S
= 32768 Hz/13 = 2520.62 Hz.
• F
0
is the fundamental frequency.
• IMAX is the external rms current corresponding to 250 mV pk at the inputs IA and IB.
• VMAX is the external rms voltage corresponding to 250 mV pk at the VA and VB inputs.
• N
ACC, the accumulation count for energy measurements is PRE_SAMPS[1:0]*SUM_CYCLES[5:0].
• The duration of the accumulation interval for energy measurements is
PRE_SAMPS[1:0]*SUM_CYCLES[5:0]/F
S
• ln_8 is a gain constant of the current channel, n. Its value is 8 or 1 and is controlled by In_SHUNT.
• X is a gain constant of the pulse generators. Its value is determined by PULSE_FAST and PULSE_SLOW.
• Voltage LSB for sag detection = VMAX * 7.8798*10
-6
V.
The system constants IMAX and VMAX are used by the MPU to convert internal quantities (as used by
the CE) to external, i.e. metering quantities. Their values are determined by the off-chip scaling of the
voltage and current sensors used in an actual meter. The LSB values used in this document relate digital
quantities at the CE or MPU interface to external meter input quantities. For example, if a SAG threshold
of 80 V peak is desired at the meter input, the digital value that should be programmed into SAG_THR
would be 80/SAG_THR
LSB, where SAG_THRLSB is the LSB value in the description of SAG_THR.
The parameters EQU[2:0], CE_E, PRE_SAMPS[1:0] and SUM_CYCLES[5:0] essential to the function of the
CE are stored in I/O RAM (see Section 4.2 I/O RAM Description – Alphabetical Order).
4.3.4 Environment
Before starting the CE using the CE_E bit, the MPU has to establish the proper environment for the CE by
implementing the following steps (for CE31A04 code):
• Load the CE data into RAM.
• Establish the equation to be applied in EQU[2:0]. The CE code has to match the selected equation.
• Establish the accumulation period and number of samples in PRE_SAMPS[1:0] = 0 (multiplier = 42) and
SUM_CYCLES[5:0] = 0x3C (60).
• Set PLS_INTERVAL[7:0] to 81.
• Select the values for FIR_LEN[1:0] = 2 and MUX_DIV[3:0] = 4.