lm12458mel-883 National Semiconductor Corporation, lm12458mel-883 Datasheet - Page 24

• Manufacturer Part Number: lm12458mel-883
• Description: 12-bit Sign Data Acquisition System With Self-calibration
• Manufacturer: National Semiconductor Corporation

www.national.com
2.0 Internal User-Programmable
Registers
2.1 INSTRUCTION RAM
The instruction RAM holds up to eight sequentially execut-
able instructions. Each 48-bit long instruction is divided into
three 16-bit sections. READ and WRITE operations can be
issued to each 16-bit section using the instruction’s address
and the 2-bit “RAM pointer” in the Configuration register. The
eight instructions are located at addresses 0000 through
0111 (A4–A1, BW = 0) when using a 16-bit wide data bus or
at addresses 00000 through 01111 (A4–A0, BW = 1) when
using an 8-bit wide data bus. They can be accessed and
programmed in random order.
Any Instruction RAM READ or WRITE can affect the se-
quencer’s operation:
The three sections in the Instruction RAM are selected by
the Configuration Register’s 2-bit “RAM Pointer”, bits D8 and
D9. The first 16-bit Instruction RAM section is selected with
the RAM Pointer equal to “00”. This section provides multi-
plexer channel selection, as well as resolution, acquisition
time, etc. The second 16-bit section holds “watchdog” limit
#1, its sign, and an indicator that shows that an interrupt can
be generated if the input signal is greater or less than the
programmed limit. The third 16-bit section holds “watchdog”
limit #2, its sign, and an indicator that shows that an interrupt
can be generated if the input signal is greater or less than the
programmed limit.
Instruction RAM “00”
Bit 0 is the LOOP bit. It indicates the last instruction to be
executed in any instruction sequence when it is set to a “1”.
The next instruction to be executed will be instruction 0.
Bit 1 is the PAUSE bit. This controls the Sequencer’s opera-
tion. When the PAUSE bit is set (“1”), the Sequencer will stop
after reading the current instruction and before executing it,
and the start bit in the Configuration register is automatically
reset to a “0”. Setting the PAUSE also causes an interrupt to
be issued. The Sequencer is restarted by placing a “1” in the
Configuration register’s Bit 0 (Start bit).
After the Instruction RAM has been programmed and the
RESET bit is set to “1”, the Sequencer retrieves Instruction
000, decodes it, and waits for a “1” to be placed in the
Configuration’s START bit. The START bit value of “0” “over-
rides” the action of Instruction 000’s PAUSE bit when the
Sequencer is started. Once started, the Sequencer executes
Instruction 000 and retrieves, decodes, and executes each
of the remaining instructions. No PAUSE Interrupt (INT 5) is
The Sequencer should be stopped by setting the RESET
bit to a “1” or by resetting the START bit in the Configura-
tion Register and waiting for the current instruction to
finish execution before any Instruction RAM READ or
WRITE is initiated. Bit 0 of the Configuration Register
indicates the Sequencer Status. See paragraph 2.2 for
information on the Configuration Register.
A soft RESET should be issued by writing a “1” to the
Configuration Register’s RESET bit after any READ or
WRITE to the Instruction RAM.
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generated the first time the Sequencer executes Instruction
000 having a PAUSE bit set to “1”. When the Sequencer
encounters a LOOP bit or completes all eight instructions,
Instruction 000 is retrieved and decoded. A set PAUSE bit in
Instruction 000 now halts the Sequencer before the instruc-
tion is executed.
Bits 2–4 select which of the eight input channels (“000” to
“111” for IN0–IN7) will be configured as non-inverting inputs
to the LM12(H)458’s ADC. (See Table 1.) They select which
of the four input channels (“000” to “011” for IN0–IN4) will be
configured as non-inverting inputs to the LM12454’s ADC.
(See Table 2.)
Bits 5–7 select which of the seven input channels (“001” to
“111” for IN1 to IN7) will be configured as inverting inputs to
the LM12(H)458’s ADC. (See Table 1.) They select which of
the three input channels (“001” to “011” for IN1–IN4) will be
configured as inverting inputs to the LM12454’s ADC. (See
Table 2.) Fully differential operation is created by selecting
two multiplexer channels, one operating in the non-inverting
mode and the other operating in the inverting mode. A code
of “000” selects ground as the inverting input for single
ended operation.
Bit 8 is the SYNC bit. Setting Bit 8 to “1” causes the Se-
quencer to suspend operation at the end of the internal S/H’s
acquisition cycle and to wait until a rising edge appears at
the SYNC pin. When a rising edge appears, the S/H ac-
quires the input signal magnitude and the ADC performs a
conversion on the clock’s next rising edge. When the SYNC
pin is used as an input, the Configuration register’s “I/O
Select” bit (Bit 7) must be set to a “0”. With SYNC configured
as an input, it is possible to synchronize the start of a
conversion to an external event. This is useful in applications
such as digital signal processing (DSP) where the exact
timing of conversions is important.
When the LM12(H)454/8 are used in the “watchdog” mode
with external synchronization, two rising edges on the SYNC
input are required to initiate two comparisons. The first rising
edge initiates the comparison of the selected analog input
signal with Limit #1 (found in Instruction RAM “01”) and the
second rising edge initiates the comparison of the same
analog input signal with Limit #2 (found in Instruction RAM
“10”).
Bit 9 is the TIMER bit. When Bit 9 is set to “1”, the Se-
quencer will halt until the internal 16-bit Timer counts down
to zero. During this time interval, no “watchdog” comparisons
or analog-to-digital conversions will be performed.
Bit 10 selects the ADC conversion resolution. Setting Bit 10
to “1” selects 8-bit + sign and when reset to “0” selects 12-bit
+ sign.
Bit 11 is the “watchdog” comparison mode enable bit. When
operating in the “watchdog” comparison mode, the selected
analog input signal is compared with the programmable
values stored in Limit #1 and Limit #2 (see Instruction RAM
“01” and Instruction RAM “10”). Setting Bit 11 to “1” causes
two comparisons of the selected analog input signal with the
two stored limits. When Bit 11 is reset to “0”, an 8-bit + sign
or 12-bit + sign (depending on the state of Bit 10 of Instruc-
tion RAM “00”) conversion of the input signal can take place.