AD7993 datasheet(18/27 Pages) AD

18 / 27 page

AD7994/AD7993

–18–

REV. PrF

PRELIMINARYTECHNICAL DATA

LIMIT REGISTERS

The AD7994/AD7993 has four pairs of limit registers,

each to store high and low conversion limits for each ana-

log input channel. Each pair of limit registers has one

associated hysteresis register. All twelve registers are 16-

bits wide, only the 12 LSBs of the Registers are used for

the AD7994/AD7993, However on the AD7993 the 2

LSBs, D1 and D0, should contain 0s. On power-up, the

contents of the DATAHIGH Register for each channel will

be fullscale, while the contents of the DATALOW registers

will be zeroscale by default. The Limit Registers can be

used to monitor the conversion results on each on the

Analog input channels. The AD7994/AD7993 will signal

an Alert ( in either hardware or software or both depend-

ing on configuration) if the result moves outside the upper

or lower limit set by the limit registers.

DATAHIGH REGISTER CH1/CH2/CH3/CH4

The DATAHIGH Register for each channel is a 16-bit read/

write register, only the 12 LSBs of the Register are used.

The Registers store the upper limit that will activate the

ALERT output and/or the Alert_Flag bit in the Conver-

sion Result Register. Therefore, if the value in the Con-

version Result Register is greater than the value in the

DATAHIGH Register, then the Alert_Flag bit is set to 1

and the ALERT pin is activated (the latter is true if

ALERT is enabled in the Configuration Register). When

the conversion result returns to a value at least N LSBs

below the DATAHIGH Register value the ALERT output

pin and Alert_Flag bit will be reset. The value of N is

taken from the 12-bit Hysteresis register associated with

that channel.

The ALERT pin can also be reset by writ-

ing to bits D2, D1 in the Configuration Register. For the

AD7993, D1 and D0 of the DATAHIGH register should

contain 0’s.

Table VIIIa.

DATAHIGH Register (First Read/Write)

D15

D14

D13 D12

D11

D10

Alert_Flag 0

B11

B10

B 9

B 8

Table VIIIb.

DATAHIGH Register (Second Read/Write)

DATALOW REGISTER CH1/CH2/CH3/CH4

The DATALOW Register for each channel is a 16-bit read/

write register, of which only the 12 LSBs are used. The

ALERT output and/or the Alert_Flag bit in the conver-

sion result register. Therefore, if the value in the Conver-

sion Result Register is less than the value in the

DATALOW Register, then the Alert_Flag bit is set to 1 and

the ALERT pin is activated (the latter is true if ALERT is

enabled in the Configuration Register). When the Conver-

sion result returns to a value at least N LSBs above

the

DATALOW Register value the ALERT ouput pin and

Alert_Flag bit will be reset. The value of N is taken from

the 12-bit Hysteresis register associated with that channel.

The ALERT pin can also be reset by writing to bit D2,D1

in the Configuration Register. For the AD7993 D1 and

D0 of the DATALOW register should contain 0’s.

Table IXa.

DATALOW Register (First Read/Write)

D15

D14

D13 D12

D11

D10

Alert_Flag 0

B11

B10

B 9

B 8

Table IXb.

DATALOW Register (Second Read/Write)

HYSTERESIS REGISTER (CH1/CH2/CH3/CH4)

Each Hysteresis Register is a 16-bit read/write register,

only the 12 LSBs of the register are used. The Registers

store the hysteresis value, N when using the limit registers.

Each pair of Limit registers has a dedicated hysteresis

for the ALERT pin/Alert_Flag if a violation of the limits

has occurred. If a hysteresis value of say 8 LSBs is re-

quired on the upper and lower limits of channel 1 then the

12 bit word, 0000 0000 0000 1000, should be written to

the Hysteresis Register CH1, the address of which is

shown in Table III. On power up, the Hysteresis Registers

will contain a value of 8 LSBs for the AD7994 and 2

LSBs for the AD7993. If a different hysteresis value is

required then that value must be written to the Hysteresis

and D0 of the Hysteresis Register should contain 0’s.

Table Xa.

Hysteresis Register (First Read/Write)

D15

D14

D13 D12

D11

D10

Alert_Flag 0

B11

B10

B 9

B 8

Table Xb.

Hysteresis Register (Second Read/Write)

Using the Limit Registers to Store Min/Max Conversion

Results

If fullscale, i.e. all 1s, is written to the Hysteresis register

for a particular channel then the DATAHIGH and

DATALOW Registers for that channel will no longer act as

Limit registers as previously described, but instead they

will act as storage registers for the maximum and mini-

mum conversion results returned from conversions on a

channel over any given period of time. This function is

useful in applications where the widest span of actual con-

version results is required rather than using the ALERT to

signal an intervention is necessary, e.g. monitoring tem-

perature extremes during refrigerated goods transporta-

tion. When using the limit registers to store the min and

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