FRQCNT2 Frequency Counter

Transcription

1 Summary This core reference describes how to place and use the Frequency Counter instrument in an FPGA design. Core Reference CR0101 (v1.0) January 08, 2004 The FRQCNT2 device provides a two-channel, three-mode frequency counter instrument for use in an FPGA design. For each input signal, the device counts the number of edges rising or falling - detected within a specified gating period. Depending on the mode of operation selected, each channel can display the frequency of the signal, its period, or the total number of edges counted. Features 2-Channel, 3-Mode counter Frequency Mode (with a range of 0 to (TIMEBASE / 2)) Period Mode (with a resolution of 1 / (TIMEBASE / 2) Seconds) Event (edge) Counter Mode (able to count up to 9,999,999,999 edges) Selectable gating period Ability to count rising or falling edges of incoming signals Available devices The FRQCNT2 device can be found in the FPGA Instruments integrated library (\Program Files\Altium2004\Library\Fpga\FPGA Instruments.IntLib). CR0101 (v1.0) January 08,

2 Functional description Symbol Figure 1. FRQCNT2 Symbol Pin description Table 1. FRQCNT2 Pin description Name Type Polarity/ Bus size Description FREQA I Rise/Fall Channel A input signal FREQB I Rise/Fall Channel B input signal TIMEBASE I Rise External reference clock signal use for synchronization and as the base from which to derive the actual gating periods. 2 CR0101 (v1.0) January 08, 2004

3 Operation Placing the FRQCNT2 device in a design The FRQCNT2 device enables the accurate frequency measurement of periodic signals in a design. Figure 2 shows an example of how the FRQCNT2 device is wired into a design. Figure 2. Using the FRQCNT2 Frequency Counter in a design. In the example circuit above, the Frequency Counter takes the NanoBoard s system clock (CLK_BRD) as its time-base and is used to measure the frequency of the signal output from a Frequency Generator and the output of an embedded counter in a TSK165B_D microcontroller. Note: A JTAG Connector and corresponding Nexus Processor JTAG Chain Connector must also been placed in the schematic for the design, as shown in Figure 3. Figure 3. Required JTAG devices. These two devices must be placed on the top schematic sheet of the design project where the instrument symbol(s) themselves are placed in order for correct communication to the instrument(s) via the JTAG chain. These devices can be found in the FPGA NanoBoard Port-Plugin integrated library (\Program Files\Altium2004\Library\Fpga\FPGA NanoBoard Port-Plugin.IntLib). CR0101 (v1.0) January 08,

4 Accessing the Frequency Counter Controls The host computer is connected to the target Frequency Counter instrument using the IEEE (JTAG) standard interface. This is the physical interface, providing connection to physical pins of the FPGA device in which the instrument has been embedded. The Nexus 5001 standard is used as the protocol for communications between the host and all devices that are debug-enabled with respect to this protocol. This includes the Frequency Counter, as well as other Nexus-compliant devices such as OCD-version microcontrollers, digital I/O modules, logic analyzers and frequency generators. All such devices are connected in a chain the Soft Devices chain which is determined when the design has been implemented within the target FPGA device and presents in the Devices view. It is not a physical chain, in the sense that you can see no external wiring the connections required between the Nexus-enabled devices are made internal to the FPGA itself. For instruments such as FRQCNT2, the Nexus protocol enables you to access the registers used for controlling the device. These register are not exposed as such, rather input to them is provided through an instrument panel, which allows you to: Set the time-base frequency Control operation of the Counter, including changing the Counter's mode of operation (Frequency, Period, Event Counter) Request a desired gating period either from a predefined selection, or by entering a specific value View the actual gating period that is generated based on the requested gating period and the timebase. The controls for a FRQCNT2 device used in a design can be accessed from the Devices view. Simply double click on the icon representing the FRQCNT2 device whose controls you wish to access, in the Soft Devices region of the view. The Soft Devices Instrument Rack will appear. Each FRQCNT2 device that you have included in the design will appear, when double-clicked, as an Instrument in the rack (along with any other Nexus-enabled devices that have been added to the rack). 4 CR0101 (v1.0) January 08, 2004

5 Figure 4. Accessing the Frequency Counter Instrument Panel. Operating the Frequency Counter Controls The instrument panel for the FRQCNT2 device (Figure 5) contains various controls and options that allow you to effectively use the instrument in your design. The following sections discuss the available controls in greater detail. Figure 5. The Frequency Counter Instrument Panel. CR0101 (v1.0) January 08,

6 Obtaining accurate results time-base definition The TIMEBASE input to the device is essentially a reference clock, either the external system clock (on the NanoBoard) or some other clock source in the design. The TIMEBASE signal is used as the base for frequency and period calculations and is divided down to produce the range of gating periods that the Counter can use. In order to achieve accurate measurements, the time base frequency for the device must be set to the frequency of the clock signal wired to the TIMEBASE input. For example, if the external system clock is used as the TIMEBASE input and the frequency of this clock has been set to 50 MHz, then the time base frequency for the device should, for accurate results, be set to 50 MHz as well. The time base frequency for the device is set in the Counter Module - Options dialog, accessed by pressing the Counter Options button on the Counter s Instrument Panel, as shown in Figure 6. Figure 6. Setting the time base frequency for the device. Selecting a gating period Access is provided on the Instrument Panel to specify the desired gating period for each channel of the Counter. This is the period of time in which cycles of an input signal are captured, in order to calculate the frequency or period of that signal. Bare in mind though that this is a requested gating period and may not necessarily be the actual gating period that can be used. The range of actual gating periods that can be used is based on the value of the chosen time base frequency (which in turn is set to equal the frequency of the incoming TIMEBASE signal). The Counter can only use gating periods that are the reciprocals of even integer divisors of this time base frequency. The gating period can therefore be expressed as follows: Gating Period = 1 / (Time Base Frequency / n) where n is an even integer in the range 0 to (2 32-1). 6 CR0101 (v1.0) January 08, 2004

7 Choosing a gating period for Channel A or Channel B of the Counter can be carried out in the following ways: Selecting a predefined gating period Use the available buttons on the Panel to choose from the following selection of predefined and commonly used gating periods : 0.1s 0.5s 1s 5s 10s As you click on a button, the actual gating period that is used will be displayed in the small display window to the right of the gating-related buttons. Requesting your own gating period If the gating period you require is not available in the predefined selections, you can enter a specific gating period of your own. Simply click on the Gating button and enter a value for the period (in seconds) in the Choose Gating Period dialog that appears. Once again, the actual gating period used will be displayed on the Panel and may differ from that requested, depending on whether or not the period you enter is the reciprocal of an even integer divisor of the time base frequency. Selecting the mode of operation The FRQCNT2 device supports three operating modes Frequency, Period and Count. The current mode is toggled, in that order, using the Mode button. As the mode is changed, the display for the channel will change accordingly and textual confirmation of the current mode will appear in the top-left corner of the display. Note: As the two channels of the device are mutually exclusive, Channel A can be configured to operate in the same or different mode to Channel B. Frequency Mode In this mode, the number of edges (rising or falling) of the input signal counted during the specified gating period, is used to calculate the actual frequency of the signal. Frequency = Number of edges / Gating Period The frequency range supported by the device is: 0 to TIMEBASE / 2 Hz CR0101 (v1.0) January 08,

8 Figure 7. Channel B of FRQCNT2 in Frequency Mode. Period Mode In this mode, the number of edges (rising or falling) of the input signal counted during the specified gating period, is used to calculate the actual period of the signal. Period = Gating Period / Number of Edges The resolution supported by the device is: 1 / (TIMEBASE / 2) Seconds Figure 8. Channel B of FRQCNT2 in Period Mode Count Mode This mode is used to display a running total for the number of edges (rising or falling) of the signal that have been captured. This is the only mode that does not depend on the gating period it is purely just an event (edge) counter. In this mode, the Counter is able to display the number of edges in the range 0 to 9,999,999,999. Upon reaching the maximum count limit, the Counter will rollover to 0 and continue. Figure 9. Channel B of FRQCNT2 in Count Mode 8 CR0101 (v1.0) January 08, 2004

9 In each mode, either the rising or falling edges of the input signal can be detected. The edge detection mode is toggled by clicking on the edge detection button: - Rising edge detection currently being used. - Falling edge detection currently being used. Running the Frequency Counter When a design that incorporates one or more FRQCNT2 instruments is programmed into an FPGA device, each channel of each instrument is, by default, in the Run mode. To toggle between normal Run and Paused modes, simply click on the Run button for the respective channel on the Instrument Panel: - Frequency Counter channel in normal operating (Run) mode - Frequency Counter channel in Paused (non-running) mode. Confirmation text will appear in the main display window of the Instrument Panel for the channel and measurement will cease. Using the Reset button for a channel will reset the gating period (when the operational mode is either Frequency or Period) and reset the edge count to zero (when the operational mode is Count). In each case, the affected channel of the Frequency Counter will enter Paused mode. Revision History Date Version No. Revision 08-Jan New product release Software, hardware, documentation and related materials: Copyright 2004 Altium Limited. All rights reserved. You are permitted to print this manual provided that (1) the use of such is for personal use only and will not be copied or posted on any network computer or broadcast in any media, and (2) no modifications of the manual is made. Unauthorized duplication, in whole or part, of this document by any means, mechanical or electronic, including translation into another language, except for brief excerpts in published reviews, is prohibited without the express written permission of Altium Limited. Unauthorized duplication of this work may also be prohibited by local statute. Violators may be subject to both criminal and civil penalties, including fines and/or imprisonment. Altium, CAMtastic, Design Explorer, DXP, LiveDesign, NanoBoard, Nexar, nvisage, P-CAD, Protel, Situs, TASKING, and Topological Autorouting and their respective logos are trademarks or registered trademarks of Altium Limited or its subsidiaries. All other registered or unregistered trademarks referenced herein are the property of their respective owners and no trademark rights to the same are claimed. CR0101 (v1.0) January 08,