High-performance data acquisition system-on-chip LM12H458 and application

LM12H458 is a highly integrated data acquisition system DAS chip, which will sample and hold, A/D conversion is integrated on a chip, which greatly reduces the external circuit design.

The 8-channel analog signal input can be used as single-ended input and two two composition differential input. Device internal provided a 2.5V reference voltage, 8 × 48bit instruction RAM and 32 × 16bit of FIFO significantly reduces the burden of the microprocessor. LM12H458 working voltage of 3 ~ 5.5V, power consumption is less than 34mW, standby mode power consumption of only 50 μ W. In addition, there are the following main performance LM12H458:

There are three operating modes: respectively, signed 13-bit mode, signed 9-bit mode and watchdog mode;

8 analog input channels, the analog signal can be single-ended input, or differential input;

Built-in sample-and-hold and 2.5V reference voltage;

Contains 32 × 16bit of FIFO;

Sampling time and conversion rate programmable;

With the calibration and Diagnostics;

With 8-bit or 16-bit data bus.

2 pin feature and function description

LM12H458 pin functions as listed in table 1.

Figure 1 for its internal function block diagrams. LM12H458 is a multifunction data acquisition system, its internal charge redistribution ADC uses capacitance ladder network instead of the normal resistance ladder network and sent using the gradual approximation of

Save the DAC to VREF-and VREF + between an intermediate voltage, the voltage and input voltage phase comparison of sampling produces digital outputs each, the number of intermediate voltage and comparison of number corresponds to the ADC resolution ADC through the calibration of capacitance in the network can be calibrated digital output of each and every precision.

LM12H458 has two different calibration modes: one is the compensation offset voltage or zero error, in which only the measuring time offset error, and the establishment of correction factor; the other is a fixed offset errors and ADC linear calibration error, as the whole. The mode of offset error measuring eight times and averaged to establish a correction factor. The above two patterns of correction factor is stored in the internal offset correction registers. LM12H458 linear correction through modified internal DAC's mismatching capacitor, in LM12H458 internal ROM in calibration algorithm, each capacitance calibration 8 times and averaged to produce linear correction factor. Once calibrated, the internal arithmetic logic unit (ALU) you can use the offset error correction coefficient and linearity correction factor to amend each of the conversion result. Watchdog mode is used to monitor single-ended input or differential input signal amplitude. Each sample signals have upper and lower threshold, the input signal exceeds or drops below a certain threshold value generates interrupts.

　　

　　

Table 1 LM12H458 feet symbols and functions

　　

　　

LM12H458 is a multifunction data acquisition system, internal 28 16bit registers, various registers function as follows:

Configuration register is das control center, can be used to control the sequence of start and stop, reset RAM pointer and flags, set the standby state, the alignment offset and linearity error, select RAM area, etc.

Instruction RAM is divided into three areas: extents, threshold zone 1 and zone 2 threshold.

Each one instruction (48bit = 3X16bit) scattered in three 16-bit width of RAM area, the three areas selected by the configuration registers 2bit RAM pointer to control. Extents to set channel options, work pattern, sampling time and cycle. Other two areas used to set the upper and lower threshold values. DAS can execute from command 0 continuous all setup instructions executed last one instruction cycle bit is 1, then return to the instruction of 0. Instruction execution period, the microprocessor does not have access to the instruction RAM, processor terminate after instruction cycle.

FIFO is read-only registers that can be used to store the result.

Interrupt enable register will enable the user to activate 8 interrupt sources, the register of high byte and interrupt 1, 2.

　　

　　

Figure 2 LM12H458 and 80C51 interface circuit

Interrupt status register and threshold status register is used to indicate the DAS interrupt sources, and whether the input signal exceeds the threshold limit or next door.

Timer register is used to set the wait time before.

Whereas Directive registers the bit9 can enable or disable insert wait time.

LM12H458 eight interrupt sources, all interrupts have the same priority level, the interrupt enable register can enable or disable the corresponding interrupt, when the interrupt occurs, the interrupt status register corresponding position 1.

Each interrupt the corresponding function is as follows:

INT0: analog input signals within the defined threshold values generate interrupts.

INT1: sequence generator implementation to a directive, the directive addresses equal to the interrupt enable register bit8 ~ bit10 set values, generate interrupts.

INT2: A/D conversion results are saved in the FIFO, FIFO conversion results in a number equal to the interrupt enable register bit 11 ~ 15 set values, generate interrupts.

INT3: automatic calibration complete single shot after the break.

INT4: completed

Complete calibration after break.

INT5: when executed, the command to pause when the bit is 1.

INT6: power indicates that interrupt the power supply voltage when the chip is smaller than 4V, generate interrupts.

INT7: 10ms from waiting mode returns after interruption.

　　

3 application

3.1 hardware circuit design

LM12H458 flexible bus interface simplifies the interface with a variety of microprocessors, it can be connected with 8-bit processor, and can be easily connected with the 16-bit microprocessor.

Figure 2-AT89C51 and DAS interface circuit diagram. The circuit uses the entire address decoding mode to produce the films selected signal DAS CS, its DAS mapped address space is 0000-001F, data bus width is 8bit. Diagram of 74HC373 for latch low 8-bit address range comparators 8bit is available to address the high 8 bits for decoding, by comparing the addresses and address ranges selected input logic can produce U5 (74HC138) selected communication number, Y0 as 74HC138 of DAS slice selected signal. DAS INT port used to drive the INT0 interrupt AT80C51, while at the same time it also allows DAS request service interruption.

3.2 software programming

LM12H458 although flexible, widely, but the basic workflow, Figure 3 is its typical programming processes.

After initialization, the processor should write a suitable instruction DAS to initialize the DAS, in order to set the sampling time, operating mode, channel selection, etc. Complete a full calibration need 4944 clocks, to set the interrupt-enabled after the calibration should be generated interrupt to tell the microprocessor. All calibration affects internal flag and DAS, thus affecting the implementation of the directive. Therefore, you must reset after calibration. The configuration registers bit0 is set to 1 to start the sequence. Flowchart of p represents a user-defined different work modes. Figure 3 (a) to the initialization and DAS sequence of start procedure, Figure 3 (b) to the interrupt service routine. When entering the interrupt service configuration registers should be cleared of bit0 to stop A/D conversion and then handle the interruption of service.

4 conclusions

LM12H458 data acquisition system-on-chip application with powerful, flexible, and can be widely used for data logging, measurement instruments, inertial guidance, process control, energy management, etc.

Because of space constraints, this article only briefly describes the detailed technical information LM12H458 in National Semiconductor's Web site. The author has used PI network LM12H458 quartz crystal units of measurement system. Use proof: LM12H458 conversion accuracy are high, and stable and reliable.