Datasheet

71M6521DE/DH/FE Data Sheet
Page: 10 of 107 Rev 3
HARDWARE DESCRIPTION
Hardware Overview
The Teridian 71M6521DE/DH/FE single-chip energy meter integrates all primary functional blocks required to
implement a solid-state electricity meter. Included on chip are an analog front end (AFE), an independent digital
computation engine (CE), an 8051-compatible microprocessor (MPU) which executes one instruction per clock cycle
(80515), a voltage reference, a temperature sensor, LCD drivers, RAM, flash memory, a real time clock (RTC), and a
variety of I/O pins. Various current sensor technologies are supported including Current Transformers (CT), and
Resistive Shunts.
In a typical application, the 32-bit compute engine (CE) of the 71M6521DE/DH/FE sequentially processes the
samples from the voltage inputs on pins IA, VA, IB, VB and performs calculations to measure active energy (Wh),
reactive energy (VARh), A
2
h, and V
2
h for four-quadrant metering. These measurements are then accessed by the
MPU, processed further and output using the peripheral devices available to the MPU.
In addition to advanced measurement functions, the real time clock function allows the 71M6521DE/DH/FE to record
time of use (TOU) metering information for multi-rate applications and to time-stamp tamper events. Measurements
can be displayed on 3.3V LCD commonly used in low temperature environments. Flexible mapping of LCD display
segments will facilitate integration of existing custom LCD. Design trade-off between number of LCD segments vs.
DIO pins can be implemented in software to accommodate various requirements.
In addition to the temperature-trimmed ultra-precision voltage reference, the on-chip digital temperature com-
pensation mechanism includes a temperature sensor and associated controls for correction of unwanted temperature
effects on measurement and RTC accuracy, e.g. to meet the requirements of ANSI and IEC standards. Temperature
dependent external components such as crystal oscillator, current transformers (CTs), and their corresponding signal
conditioning circuits can be characterized and their correction factors can be programmed to produce electricity
meters with exceptional accuracy over the industrial temperature range.
The 71M6521DH is trimmed at +85°C in addition to the trim at room temperature, which provides a set of
individualized trim fuse values that enable temperature compensation with accuracy better than ±20 PPM/°C.
One of the two internal UARTs is adapted to support an Infrared LED with internal drive and sense configuration, and
can also function as a standard UART. The optical output can be modulated at 38 kHz. This flexibility makes it
possible to implement AMR meters with an IR interface. A block diagram of the IC is shown in Figure 1. A detailed
description of various functional blocks follows.
Analog Front End (AFE)
The AFE of the 71M6521DE/DH/FE is composed of an input multiplexer, a delta-sigma A/D converter and a voltage
reference.
Input Multiplexer
The input multiplexer supports up to four input signals that are applied to pins IA, VA, IB and VB of the device.
Additionally, using the alternate mux selection, it has the ability to select temperature and the battery voltage. The
multiplexer can be operated in two modes:
• During a normal multiplexer cycle, the signals from the IA, IB, VA, and VB pins are selected.
• During the alternate multiplexer cycle, the temperature signal (TEMP) and the battery monitor are selected,
along with the signal sources shown in Table 1. To prevent unnecessary drainage on the battery, the battery
monitor is enabled only with the BME bit (0x2020[6]) in the I/O RAM.
The alternate mux cycles are usually performed infrequently (e.g. every second) by the MPU. In order to prevent
disruption of the voltage tracking PLL and voltage allpass networks, VA is not replaced in the ALT mux selections.
Table 1 details the regular and alternative MUX sequences. Missing samples due to an ALT multiplexer sequence are
filled in by the CE.