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LM628 Datasheet(PDF) 18 Page - National Semiconductor (TI) |
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LM628 Datasheet(HTML) 18 Page - National Semiconductor (TI) |
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18 / 24 page  Data Reporting Commands (Continued) This command reads the instantaneous desired (current temporal ) position output of the profile generator. This is the “setpoint” input to the position-loop summing junction. The bytes are read in most-to-least significant order. RDRP COMMAND: ReaD Real Position Command Code: 0A Hex Bytes Read: Four Data Range: C0000000 to 3FFFFFFF Hex Executable During Motion: Yes This command reads the current actual position of the motor. This is the feedback input to the loop summing junction. The bytes are read in most-to-least significant order. RDDV COMMAND: ReaD Desired Velocity Command Code: 07 Hex Bytes Read: Four Data Range: C0000001 to 3FFFFFFF Executable During Motion: Yes This command reads the integer and fractional portions of the instantaneous desired (current temporal ) velocity, as used to generate the desired position profile. The bytes are read in most-to-least significant order. The value read is properly scaled for numerical comparison with the user-supplied (commanded) velocity; however, because the two least-significant bytes represent fractional velocity, only the two most-significant bytes are appropriate for compari- son with the data obtained via command RDRV (see below). Also note that, although the velocity input data is constrained to positive numbers (see command LTRJ), the data returned by command RDDV represents a signed quantity where negative numbers represent operation in the reverse direc- tion. RDRV COMMAND: ReaD Real Velocity Command Code: 0B Hex Bytes Read: Two Data Range: C000 to 3FFF Hex, See Text Executable During Motion: Yes This command reads the integer portion of the instanta- neous actual velocity of the motor. The internally maintained fractional portion of velocity is not reported because the re- ported data is derived by reading the incremental encoder, which produces only integer data. For comparison with the result obtained by executing command RDDV (or the user-supplied input value), the value returned by command RDRV must be multiplied by 2 16 (shifted left 16 bit positions). Also, as with command RDDV above, data returned by com- mand RDRV is a signed quantity, with negative values repre- senting reverse-direction motion. RDSUM COMMAND: ReaD Integration-Term SUMmation Value Command Code: 0D Hex Bytes Read: Two Data Range: 00000 Hex to ± the Current Value of the Integration Limit Executable During Motion: Yes This command reads the value to which the integration term has accumulated. The ability to read this value may be help- ful in initially or adaptively tuning the system. Typical Applications Programming LM628 Host Handshaking (Interrupts) A few words regarding the LM628 host handshaking will be helpful to the system programmer. As indicated in various portions of the above text, the LM628 handshakes with the host computer in two ways: via the host interrupt output (Pin 17), or via polling the status byte for “interrupt” conditions. When the hardwired interrupt is used, the status byte is also read and parsed to determine which of six possible condi- tions caused the interrupt. When using the hardwired interrupt it is very important that the host interrupt service routine does not interfere with a command sequence which might have been in progress when the interrupt occurred. If the host interrupt service rou- tine were to issue a command to the LM628 while it is in the middle of an ongoing command sequence, the ongoing com- mand will be aborted (which could be detrimental to the ap- plication). Two approaches exist for avoiding this problem. If one is us- ing hardwired interrupts, they should be disabled at the host prior to issuing any LM628 command sequence, and re-enabled after each command sequence. The second ap- proach is to avoid hardwired interrupts and poll the LM628 status byte for “interrupt” status. The status byte always re- flects the interrupt-condition status, independent of whether or not the interrupts have been masked. Typical Host Computer/Processor Interface The LM628 is interfaced with the host computer/processor via an 8-bit parallel bus. Figure 12 shows such an interface and a minimum system configuration. As shown in Figure 12, the LM628 interfaces with the host data, address and control lines. The address lines are de- coded to generate the LM628 CS input; the host address LSB directly drives the LM628 PS input. Figure 12 also shows an 8-bit DAC and an LM12 Power Op Amp interfaced to the LM628. LM628 and High Performance Controller (HPC) Interface Figure 13 shows the LM628 interfaced to a National HPC High Performance Controller. The delay and logic associated with the WR line is used to effectively increase the write-data hold time of the HPC (as seen at the LM628) by causing the WR pulse to rise early. Note that the HPC CK2 output pro- vides the clock for the LM628. The 74LS245 is used to de- crease the read-data hold time, which is necessary when in- terfacing to fast host busses. Interfacing a 12-Bit DAC Figure 14 illustrates use of a 12-bit DAC with the LM628. The 74LS378 hex gated-D flip-flop and an inverter demultiplex the 12-bit output. DAC offset must be adjusted to minimize DAC linearity and monotonicity errors. Two methods exist for making this adjustment. If the DAC1210 has been socketed, remove it and temporarily connect a 15 k Ω resistor between Pins 11 and 13 of the DAC socket (Pins 2 and 6 of the LF356) and adjust the 25 k Ω potentiometer for 0V at Pin 6 of the LF356. If the DAC is not removable, the second method of adjust- ment requires that the DAC1210 inputs be presented an all-zeros code. This can be arranged by commanding the ap- www.national.com 18 |
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