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LM12454 Datasheet(PDF) 35 Page - Texas Instruments |
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LM12454 Datasheet(HTML) 35 Page - Texas Instruments |
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35 / 45 page  LM12454, LM12458, LM12H458 www.ti.com SNAS079A – MAY 2004 – REVISED FEBRUARY 2006 3.4 DMA The DMA works in tandem with Interrupt 2. An active DMA Request on pin 32 (DMARQ) requires that the FIFO interrupt be enabled. The voltage on the DMARQ pin goes high when the number of conversions in the FIFO equals the 5-bit value stored in the Interrupt Enable register (bits 11–15). The voltage on the INT pin goes low at the same time as the voltage on the DMARQ pin goes high. The voltage on the DMARQ pin goes low when the FIFO is emptied. The Interrupt Status register must be read to clear the FIFO interrupt flag in order to enable the next DMA request. DMA operation is optimized through the use of the 16-bit data bus connection (a logic “0” applied to the BW pin). Using this bus width allows DMA controllers that have single address Read/Write capability to easily unload the FIFO. Using DMA on an 8-bit data bus is more difficult. Two read operations (low byte, high byte) are needed to retrieve each conversion result from the FIFO. Therefore, the DMA controller must be able to repeatedly access two constant addresses when transferring data from the LM12(H)454/8 to the host system. 4.0 FIFO The result of each conversion stored in an internal read-only FIFO (First-In, First-Out) register. It is located at 1100 (A4–A1, BW = 0) or 1100x (A4–A0, BW = 1). This register has 32 16-bit wide locations. Each location holds 13-bit data. Bits 0–3 hold the four LSB's in the 12 bits + sign mode or “1110” in the 8 bits + sign mode. Bits 4–11 hold the eight MSB's and Bit 12 holds the sign bit. Bits 13–15 can hold either the sign bit, extending the register's two's complement data format to a full sixteen bits or the instruction address that generated the conversion and the resulting data. These modes are selected according to the logic state of the Configuration register's Bit 5. The FIFO status should be read in the Interrupt Status register (Bits 11–15) to determine the number of conversion results that are held in the FIFO before retrieving them. This will help prevent conversion data corruption that may take place if the number of reads are greater than the number of conversion results contained in the FIFO. Trying to read the FIFO when it is empty may corrupt new data being written into the FIFO. Writing more than 32 conversion data into the FIFO by the ADC results in loss of the first conversion data. Therefore, to prevent data loss, it is recommended that the LM12(H)454/8's interrupt capability be used to inform the system controller that the FIFO is full. The lower portion (A0 = 0) of the data word (Bits 0–7) should be read first followed by a read of the upper portion (A0 = 1) when using the 8-bit bus width (BW = 1). Reading the upper portion first causes the data to shift down, which results in loss of the lower byte. Bits 0–12 hold 12-bit + sign conversion data. Bits 0–3 will be 1110 (LSB) when using 8-bit plus sign resolution. Bits 13–15 hold either the instruction responsible for the associated conversion data or the sign bit. Either mode is selected with Bit 5 in the Configuration register. Using the FIFO's full depth is achieved as follows. Set the value of the Interrupt Enable register's Bits 11–15 to 11111 and the Interrupt Enable register's Bit 2 to a “1”. This generates an external interrupt when the 31st conversion is stored in the FIFO. This gives the host processor a chance to send a “0” to the LM12(H)454/8's Start bit (Configuration register) and halt the ADC before it completes the 32nd conversion. The Sequencer halts after the current (32) conversion is completed. The conversion data is then transferred to the FIFO and occupies the 32nd location. FIFO overflow is avoided if the Sequencer is halted before the start of the 32nd conversion by placing a “0” in the Start bit (Configuration register). It is important to remember that the Sequencer continues to operate even if a FIFO interrupt (INT 2) is internally or externally generated. The only mechanisms that stop the Sequencer are an instruction with the PAUSE bit set to “1” (halts before instruction execution), placing a “0” in the Configuration register's START bit, or placing a “1” in the Configuration register's RESET bit. 5.0 Sequencer The Sequencer uses a 3-bit counter (Instruction Pointer, or IP, in Figure 9) to retrieve the programmable conversion instructions stored in the Instruction RAM. The 3-bit counter is reset to 000 during chip reset or if the current executed instruction has its Loop bit (Bit 1 in any Instruction RAM “00”) set high (“1”). It increments at the end of the currently executed instruction and points to the next instruction. It will continue to increment up to 111 unless an instruction's Loop bit is set. If this bit is set, the counter resets to “000” and execution begins again with the first instruction. If all instructions have their Loop bit reset to “0”, the Sequencer will execute all eight instructions continuously. Therefore, it is important to realize that if less than eight instructions are programmed, the Loop bit on the last instruction must be set. Leaving this bit reset to “0” allows the Sequencer to execute “unprogrammed” instructions, the results of which may be unpredictable. Copyright © 2004–2006, Texas Instruments Incorporated Submit Documentation Feedback 35 Product Folder Links: LM12454 LM12458 LM12H458 |
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