Datasheet

LM12L458
www.ti.com SNAS085B –JULY 1999–REVISED MARCH 2013
Writing a “1” to Bit 3 initiates a complete calibration process that includes a “long” auto-zero offset voltage
correction (this calibration averages eight samples of the comparator offset voltage when creating a correction
coefficient) followed by an ADC linearity calibration. This complete calibration is started after the currently
running instruction is completed if the Sequencer is running when Bit 3 is set to “1”. Bit 3 is reset automatically to
a “0” and an interrupt flag (Bit 4, in the Interrupt Status register) will be generated at the end of the calibration
procedure (4944 clock cycles). After completion of a full auto-zero and linearity calibration, the Sequencer
fetches the next instruction as pointed to by the Instruction RAM's pointer and resumes execution. If the
Sequencer is stopped, a full calibration is performed immediately at the time requested.
Bit 4 is the Standby bit. Writing a “1” to Bit 4 immediately places the LM12L458 in Standby mode. Normal
operation returns when Bit 4 is reset to a “0”. The Standby command (“1”) disconnects the external clock from
the internal circuitry, decreases the LM12L458's internal analog circuitry power supply current, and preserves all
internal RAM contents. After writing a “0” to the Standby bit, the LM12L458 returns to an operating state identical
to that caused by exercising the RESET bit. A Standby completion interrupt is issued after a power-up
completion delay that allows the analog circuitry to settle. The Sequencer should be restarted only after the
Standby completion is issued. The Instruction RAM can still be accessed through read and write operations while
the LM12L458 are in Standby Mode.
Bit 5 is the Channel Address Mask. If Bit 5 is set to a “1”, Bits 13–15 in the conversion FIFO will be equal to the
sign bit (Bit 12) of the conversion data. Resetting Bit 5 to a “0” causes conversion data Bits 13 through 15 to hold
the instruction pointer value of the instruction to which the conversion data belongs.
Bit 6 is used to select a “short” auto-zero correction for every conversion. The Sequencer automatically inserts
an auto-zero before every conversion or “watchdog” comparison if Bit 6 is set to “1”. No automatic correction will
be performed if Bit 6 is reset to “0”.
The LM12L458's offset voltage, after calibration, has a typical drift of 0.1 LSB over a temperature range of −40°C
to +85°C. This small drift is less than the variability of the change in offset that can occur when using the auto-
zero correction with each conversion. This variability is the result of using only one sample of the offset voltage to
create a correction value. This variability decreases when using the full calibration mode because eight samples
of the offset voltage are taken, averaged, and used to create a correction value.
Bit 7 is used to program the SYNC pin (29) to operate as either an input or an output. The SYNC pin becomes
an output when Bit 7 is a “1” and an input when Bit 7 is a “0”. With SYNC programmed as an input, the rising
edge of any logic signal applied to pin 29 will start a conversion or “watchdog” comparison. Programmed as an
output, the logic level at pin 29 will go high at the start of a conversion or “watchdog” comparison and remain
high until either have finished. See Instruction RAM “00”, Bit 8.
Bits 8 and 9 form the RAM Pointer that is used to select each of a 48-bit instruction's three 16-bit sections during
read or write actions. A “00” selects Instruction RAM section one, “01” selects section two, and “10” selects
section three.
Bit 10 activates the Test mode that is used only during production testing. Leave this bit reset to “0”.
Bit 11 is the Diagnostic bit and is available only in the LM12L458. It can be activated by setting it to a “1” (the
Test bit must be reset to a “0”). The Diagnostic mode, along with a correctly chosen instruction, allows
verification that the LM12L458's ADC is performing correctly. When activated, the inverting and non-inverting
inputs are connected as shown in Table 4. As an example, an instruction with “001” for both V
IN+
and V
IN−
while
using the Diagnostic mode typically results in a full-scale output.
INTERRUPTS
The LM12L458 has eight possible interrupts, all with the same priority. Any of these interrupts will cause a
hardware interrupt to appear on the INT pin (31) if they are not masked (by the Interrupt Enable register). The
Interrupt Status register is then read to determine which of the eight interrupts has been issued.
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