Using the LS7166 Quadrature Counter - Revision history

Thomas Peterson: /* Overview of the LS7166 */

2010-04-30T04:51:31Z

Overview of the LS7166

Thomas Peterson: /* Introduction */

2010-04-30T00:45:32Z

Introduction

Thomas Peterson at 00:40, 30 April 2010

2010-04-30T00:40:03Z

Thomas Peterson: /* Code */

2010-04-30T00:33:20Z

Code

Thomas Peterson: /* 7166_lib.c */

2010-04-22T06:16:46Z

7166_lib.c

Thomas Peterson: /* 7166_test.c */

2010-04-22T06:16:38Z

7166_test.c

Thomas Peterson: /* Code */

2010-04-22T06:07:50Z

Code

Thomas Peterson: /* 7166_test.c */

2010-04-22T06:06:29Z

7166_test.c

Thomas Peterson: /* Code */

2010-04-22T06:06:06Z

Code

Thomas Peterson: /* Code */

2010-04-22T06:05:12Z

Code

Show changes

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 ==Overview of the LS7166==
-The LS7166 is a 24 bit quadrature counter. The A and B inputs normally serve as quadrature inputs, like those which come from a quadrature encoder like those on most motors. The A and B inputs can also server as up / down counter inputs, so that the LS7166 can be used for other applications as well. The internal counter can count in binary, BCD, or in time units (hours/minutes/seconds). In any motor-related application, binary counting is necessary. The chip can also operate in divide-by-n mode, which loads the count register with a fixed value on rollover. This is useful for motor applications, since the fixed value can be set to the number of pulses in a full rotation, and then whenever the count value is read, the exact position of the motor can be known no matter the amount of time that has passed since the last read. The chip has several other features that can be read about in the datasheet. The code in this document only covers divide-by-n binary quadrature mode, and for more advanced features the support code will need to be modified.
+The LS7166 is a 24 bit quadrature counter. The A and B inputs normally serve as quadrature inputs, like those which come from a quadrature encoder like those on most motors. The A and B inputs can also serve as up / down counter inputs, so that the LS7166 can be used for other applications as well. The internal counter can count in binary, BCD, or in time units (hours/minutes/seconds). In any motor-related application, binary counting is necessary. The chip can also operate in divide-by-n mode, which loads the count register with a fixed value on rollover. This is useful for motor applications, since the fixed value can be set to the number of pulses in a full rotation, and then whenever the count value is read, the exact position of the motor can be known no matter the amount of time that has passed since the last read. The chip has several other features that can be read about in the datasheet. The code in this document only covers divide-by-n binary quadrature mode, and for more advanced features the support code will need to be modified.
 The LS7166 communicates with a microcontroller via a bi-directional 8-bit data bus and several control bits. Four of the contorl bits, /RD (read), /WR (write), C/D (control or data), and /CS (chip select), are required for communication. Two other control lines, LCTR/LLTC and ABGT/RCTR are for more complex features and can usually be tied to Vdd or GND as needed. There are also two pins which output flags when programmed events occur, such as an overflow. For more info on these features, see the datasheet. Note that most pins are active low, and thus must be kept at '''high''' at all times unless in use. If multiple chips are used (for interfacing to multiple motors), all the communication lines can be reused except /CS (chip select).

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 In this document, the LS7166 is interfaced with a PIC32, but the code could be ported to work with a PIC18.
+A chip with similar capability is the [[Using the LS7366R SPI Quadrature Counter| LS7366R]], which uses the SPI bus to communicate instead of in parallel, is discussed on separate wiki page.
 ==Overview of the LS7166==

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 The LS7166, as mentioned above, connects to the PIC via an 8-bit data bus, 4 control bits, 2 optional control bits, and 2 optional flags. In the code below and example schematic, pins B0-B7 are used for the data bus. These pins can be changed, but the data bus read and write functions will need to be changed to interface with bit lines instead of bytes. Since this was put in the separate function, other code won't have to be changed, other than TRIS setups.
-Other than PIC connections, the LS7166 also needs power (3.3v or 5v), ground, and connections to the A and B inputs of the encoder. Also note that in the case of this example, the optional control bit /ABGT is tied to ground, and the optional control bit /LCTR is tied to the flag /CY.
+Other than PIC connections, the LS7166 also needs power (3.3v or 5v), ground, and connections to the A and B inputs of the encoder. Also note that in the case of this example the optional control bit /ABGT is tied to ground..
 As mentioned above, multiple LS7166 can be used which are connected to the same data and control lines, except for chip select, which needs to be separate for each chip. The code will also have to be modified to toggle specific /CS lines during reads.
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  	}
  }
+==Shortfalls / Next Steps==
+The biggest problem with the system above is that in divide-by-n mode the chip does not reset the counter to zero when CNTR = PR. Because of this, if running the motor backwards continuously (counting down), the value of CNTR will always be between 0 and PR at the current position. However, when running forward (counting up) the value of CNTR goes past PR. Because of this, the pic software must be written so that the read value of CNTR is modded by the maximum count number to find the position. For instance, if the maximum count is 4000, and CNTR is 4200, then the actual position is 200 (out of 4000). Eventually the CNTR will overflow if not reduced. In this case, the /CY flag can be set to interrupt the PIC so that the correct value of CNTR can be reloaded. Alternatively, at each read the PIC could send back the actual count value. Note that this would require writing to PR and then doing a PR->CNTR transfer using the control register.
+If possible, the 7133R may be a better choice since it does bounded counting correctly.

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 ==Code==
-'''NOTE: this code still has a slight bug: at overflow the CNTR doesn't roll to 0. This will be fixed soon'''
 The code consists of three files:
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 All three can be download from [[Media:7166.zip | this zip file]].
 ===7166_lib.c===
-This c file contains various functions used to interface with the LS7166. The init_7166() function should be called before using any other functions. The init function can be modified to utilize other special features of the chip not used here. Note that at the end of the init function, the counter limit value is loaded into the PR register. This number should be changed based on the full rotation count of the encoder in use.
+This c file contains various functions used to interface with the LS7166. The init_7166() function should be called before using any other functions. The init function can be modified to utilize other special features of the chip not used here. Note that at the end of the init function, the counter limit value is loaded into the PR register. This number should be changed based on the full rotation count of the encoder in use. The value of the PR register is loaded whenever the counter borrows (downticks from 0). Note that the code does '''not''' reset to 0 when it reaches PR. To find the actual position, the software will have to take count MOD PR.
 The other primary function that will be used is read_reg(OL), which returns the value from CNTR after transferring CNTR to OL. Most other functions will usually not be needed unless special features are needed.
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 ===7166_test.c===
-This c file simply loops continously and reads values from the LS7166. Note that to use the 7166, the only steps are to setup the pins, call the init function, then call the read function to get a value.
+This c file simply loops continuously and reads values from the LS7166. Note that to use the 7166, the only steps are to setup the pins, call the init function, then call the read function to get a value.
  /***********************************************************

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   *
   * Creation date: 20-APR-2010
-  * Last modified: 22-APR-2010
+  * Last modified: 21-APR-2010
   *
   * Thomas Peterson
@@ Line 193: / Line 193: @@
     asm("nop");
  }
 ===7166_test.c===

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 *7166_lib.c - This c file contains functions that interface with the 7166
 *7166_test.c - This c file contains the main function and is a simple example of how to use the library functions
-All three can be download from [[7166.zip|this zip file]].
+All three can be download from [[Media:7166.zip | this zip file]].
 ===7166_lib.c===
 This c file contains various functions used to interface with the LS7166. The init_7166() function should be called before using any other functions. The init function can be modified to utilize other special features of the chip not used here. Note that at the end of the init function, the counter limit value is loaded into the PR register. This number should be changed based on the full rotation count of the encoder in use.