AS8228 datasheet(14/123 Pages) AMSCO

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Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 14 of 123

8. Detailed Functional Description

The AS8218 / AS8228 integrated circuits have a dedicated measurement front end, which is capable of

measuring active and reactive energy, RMS mains voltage, RMS mains current as well as power factor. There

are two completely separate differential current channel inputs, for measurement of both the Live and Neutral

currents. The two current inputs may be connected to a shunt resistor (I1) and a current transformer (I2); of

which the secondary winding is terminated with a burden resistor. Both current channels have programmable

gains; thus it is possible to connect the shunt resistor to any of the two differential current inputs. The option to

use two current transformers is also available. The AS8218 / AS8228 ICs may be programmed to accept either

of the two measured currents for the energy calculation, or may be programmed to accept the larger of the two

currents for the energy calculation.

The AS8218 / AS8228 ICs may also be used for conventional 1-phase single current measurement

applications, where only the Live current is measured. In this case, the I2P and I2N pins are left unconnected

and the second current channel modulator can be powered down.

The voltage channel input for measurement of the line voltage is also differential and is connected to a tap of a

resistive divider of the line voltage. The resistive divider can be set to accommodate any line voltage standard

(Vmains) including 100V, 110V, 220V, 230V and 240V.

A 3.0 to 4.0MHz low power oscillator generates the system clock for the AS8218 / AS8228 ICs. The absolute

clock frequency may be calibrated on-chip. A low power divider is used to generate a 1Hz clock for the on-chip

real time clock/calendar (RTC). The supply voltage to the low power oscillator, the low power divider and the

RTC may be buffered with an external battery in case of mains power dips or failures, which results in the

AS8218 / AS8228 ICs power supply being interrupted.

The LCD driver (LCDD) signals LSD0 … LSD23 and LBP0 … LBP3 can be directly connected to a liquid crystal

display (LCD), which is used to display the various measured parameters. A total of 80 LCD segments may be

driven by the AS8218 IC and 96 LCD segments may be driven by the AS8228 IC. The measurement data and

display annunciators are fully programmable. The meter system designer should define the annunciators so

that the end customer’s specific meter system requirements are met.

A maximum of twelve programmable multi-purpose input/output (MPIO) pins are available for various meter

functions, for example light-emitting diodes (LED) to signal energy consumption, energy direction, fault

condition, etc. These I/O pins may also be programmed for use as bi-directional communication channels such

as an optical interface or an additional Universal Asynchronous Receiver/Transmitter (UART2) Interface,

should it so be required. The AS8218 has 9 x MPIO pins, while the AS8228 has 12 x MPIO pins.

A dedicated Serial Peripheral Interface (SPI) is also provided for the direct connection to an external EEPROM

memory with a compatible serial peripheral interface. Depending on the meter system requirements, the

external EEPROM memory capacity may be selected from 1kB up to 32kB, in binary steps.

The on-chip 8051 compatible microcontroller performs all the required calculations and enables the user to

customize the input and output configuration of the meter. The microcontroller has 24kB of program memory,

1kB data memory, a square root calculation facility and a second UART (UART2) for debugging purposes.

A programmable watchdog timer is provided to automatically initiate a system reset when a regular hold-off

signal is not detected by the watchdog timer. The watchdog timer is an optional function which is software

enabled.

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