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AD7997/AD7998
Hysteresis Register (CH1/CH2/CH3/CH4)
Each hysteresis register is a 16-bit read/write register, of which
only the 12 LSBs are used. The hysteresis register stores the
hysteresis value,
N,
when using the limit registers. Each pair of
limit registers has a dedicated hysteresis register. The hysteresis
value determines the reset point for the ALERT pin/Alert_Flag
if a violation of the limits has occurred. For example, if a
hysteresis value of 8 LSBs is required on the upper and lower
limits of Channel 1, the 12-bit word, 0000 0000 0000 1000,
should be written to the hysteresis register of CH1, the address
of which is shown in Table 8. On power-up, the hysteresis
registers contain a value of 2 for the AD7997 and a value of 8
for the AD7998. If a different hysteresis value is required, that
value must be written to the hysteresis register for the channel
in question. For the AD7997, D1 and D0 of the hysteresis
register should contain 0s.
Table 19. Hysteresis Register (First Read/Write)
D15
D14
D13
D12
0
0
0
0
Table 20. Hysteresis Register (Second Read/Write)
D7
D6
D5
D4
B7
B6
B5
B4
Using the Limit Registers to Store Min/Max Conversion
Results for CH1 to CH4
If full scale, that is, all 1s, is written to the hysteresis register for
a particular channel, the DATA
HIGH
and DATA
LOW
registers for
that channel no longer act as limit registers as previously
described, but instead act as storage registers for the maximum
and minimum 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 conversion
results is required rather than using the ALERT to signal that an
intervention is necessary. This function could be useful for
monitoring temperature extremes during refrigerated goods
transportation. It must be noted that on power-up, the contents
of the DATA
HIGH
register for each channel are full scale, while the
contents of the DATA
LOW
registers are zero scale by default.
Therefore, minimum and maximum conversion values being
stored in this way are lost if power is removed or cycled.
Rev. 0 | Page 21 of 32
D11
B11
D10
B10
D9
B9
D8
B8
D3
B3
D2
B2
D1
B1
D0
B0
ALERT STATUS REGISTER (CH1 TO CH4)
The alert status register is an 8-bit, read/write register that
provides information on an alert event. If a conversion result
activates the ALERT pin or the Alert_Flag bit in the conversion
result register, as described in the Limit Registers section, the
alert status register may be read to gain further information.
The Alert Status Register contains two status bits per channel,
one corresponding to the DATA
HIGH
limit and the other to the
DATA
LOW
limit. The bit with a status of 1 shows where the
violation occurred—that is, on which channel—and whether
the violation occurred on the upper or lower limit. If a second
alert event occurs on the other channel between receiving the
first alert and interrogating the alert status register, the
corresponding bit for that alert event is also set.
The alert status register only contains information for CH1 to
CH4 because these are the only channels with associated limit
registers.
The entire contents of the alert status register can be cleared by
writing 1,1, to Bits D2 and D1 in the configuration register, as
shown in Table 12. This may also be done by writing all 1s to
the alert status register itself. Thus, if the alert status register is
addressed for a write operation, which is all 1s, the contents of
the alert status register are cleared or reset to all 0s.
Table 21. Alert Status Register
D7
D6
D5
D4
CH4
HI
CH4
LO
CH3
HI
CH3
LO
CH2
HI
CH2
LO
Table 22. Alert Status Register Bit Function Description
Bit
Mnemonic
If bit is set to 1, violation of…
D0
CH1
LO
DATA
LOW
limit on Channel 1.
No violation if bit is set to 0.
D1
CH1
HI
DATA
HIGH
limit on Channel 1.
No violation if bit is set to 0.
D2
CH2
LO
DATA
LOW
limit on Channel 2.
No violation if bit is set to 0.
D3
CH2
HI
DATA
HIGH
limit on Channel 2.
No violation if bit is set to 0.
D4
CH3
LO
DATA
LOW
limit on Channel 3.
No violation if bit is set to 0.
D5
CH3
HI
DATA
HIGH
limit on Channel 3.
No violation if bit is set to 0.
D6
CH4
LO
DATA
LOW
limit on Channel 4.
No violation if bit is set to 0.
D7
CH4
HI
DATA
HIGH
limit on Channel 4.
No violation if bit is set to 0.
D3
D2
D1
CH1
HI
D0
CH1
LO