90E21 datasheet(18/57 Pages) IDT

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90E21/22/23/24

SINGLE-PHASE HIGH-PERFORMANCE WIDE-SPAN ENERGY METERING IC

Interface

January 10, 2012

4.1.3

TIMEOUT AND PROTECTION

Timeout occurs if SCLK does not toggle for 6ms in both four-wire and

three-wire modes. When timeout, the read or write operation is aborted.

If there are more than 24 SCLK cycles when CS is driven low in four-

wire mode or between two starts in three-wire mode, writing operation is

prohibited while normal reading operation can be completed by taking

the first 24 SCLK cycles as the valid ones. However, the reading result

might not be the intended one.

A read access to an invalid address returns all zero. A write access

to an invalid address is discarded.

Table-9 and Table-10 list the read or write result in different condi-

tions.

4.2

WARNOUT PIN FOR FATAL ERROR WARNING

Fatal error warning is raised through the WarnOut pin in two cases:

checksum calibration error and voltage sag.

Calibration Error

The 90E21/22/23/24 performs diagnosis on a regular basis for impor-

tant parameters such as calibration parameters and metering configura-

tion. When checksum is not correct, the CalErr[1:0] bits (SysStatus,

01H) are set, and both the WarnOut pin and the IRQ pin are asserted.

When checksum is not correct, the metering part does not work to pre-

vent a large number of pulses during power-on or any abnormal situa-

tion upon incorrect parameters.

Voltage Sag

Voltage sag is detected when voltage is continuously below the volt-

age sag threshold for one cycle which starts from any zero-crossing

point. Voltage threshold is configured by the SagTh register (03H). Refer

to section 6.5.

When voltage sag occurs, the SagWarn bit (SysStatus, 01H) is set

and the WarnOut pin is asserted if the FuncEn register (02H) enables

voltage sag warning through the WarnOut pin. This function helps

reduce power-down detection circuit in system design. In addition, the

method of judging voltage sag by detecting AC side voltage eliminates

the influence of large capacitor in traditional rectifier circuit, and can

detect voltage sag earlier.

4.3

LOW COST IMPLEMENTATION IN ISOLATION

WITH MCU

The following functions can be achieved at low cost when the 90E21/

22/23/24 is isolated from the MCU:

SPI: MCU can perform read and write operations through low speed

optocoupler (e.g. NEC2501) when the 90E21/22/23/24 is isolated from

the MCU. The SPI interface can be of 3-wire or 4-wire.

Energy Pulses CFx: Energy can be accumulated by reading values

in corresponding energy registers. CFx can also connect to the optocou-

pler and the energy pulse light can be turned on by CFx.

Fatal Error WarnOut: Fatal error can be acquired by reading the CalE

rr[1:0] bits (SysStatus, 01H).

IRQ: IRQ interrupt can be acquired by reading the SysStatus register

(01H).

Reset: The 90E21/22/23/24 is reset when ‘789AH’ is written to the

software reset register (SoftReset, 00H).

Table-9 Read / Write Result in Four-Wire Mode

Condition

Result

Operation

Timeout

SCLK Cycles

note 1

Read/Write

Status

LastSPIData

Read

note 2

>=24

Normal Read

Yes

note 2

<24

Partial Read

Write

=24

Normal Write

Yes

!=24

No Write

Yes

No Write

Note 1:

The number of SCLK cycles when CS is driven low or the number of

SCLK cycles before timeout if any.

Note 2:

'-' stands for Don't Care.

Table-10 Read / Write Result in Three-Wire Mode

Condition

Result

Operation

Timeout

SCLK Cycles

note 1

Read/Write

Status

LastSPIData

Read

>=24

note 2

Normal Read

Yes

Timeout after

24 cycles

>24

Normal Read

Yes

Timeout

before 24

cycles

note 3

Partial Read

Timeout at 24

cycles

=24

Normal Read

Yes

Write

=24

Normal Write

Yes

!=24

No Write

Yes

No Write

Note 1:

The number of SCLK cycles between 2 starts or the number of SCLK

cycles before timeout if any.

Note 2:

There is no such case of less than 24 SCLK cycles when there is no

timeout in three-wire mode, because the first few SCLK cycles in the next oper-

ation is counted into this operation. In this case, data is corrupted.

Note 3:

'-' stands for Don't Care.

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