ECE 381
Lab 4 – Square Wave Generator
Square Wave Generator
Objective
Learn about PSoC User Modules, clock sources, and system configuration registers.
Description
Configure the PSoC to produce a square wave whose frequency is adjusted using an optical encoder.
Two momentary push-button switches are used. One, when pressed, halts the output of the square
wave by disabling the counter module that produces the output. The other, when pressed, toggles
between two frequency ranges.
Configuration
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Use the same optical encoder configuration as in the previous lab. An easy
way to do this is to create a new PSoC project and use Clone Project at the
top of the Select Project Type –Chip level window.
Place the LCD module, which will be used to display information to the user.
Connect momentary pushbutton switches to Port1[4] and Port1[5] as shown
in the figure at the right.
In the Global Resources window, set VC3 Source to SysClk/1 and VC3 Divider
to 4.
Place a 16-bit Counter module and configure the following Parameters
o Name: Counter16
o Clock: VC3
o Enable: Route to Port1[5]
o CompareOut: Route to Port1[0].
o TerminalCountOut: None
o Period: To be determined by you
o CompareValue: To be determined by you
o CompareType: Less Than
o InterruptType: Terminal Count (Note: Parameters must not be left blank, even if unused)
o ClockSync: Sync to SysClock
o InvertEnable: Invert
Calculate and initialize the Period and CompareValue Parameters of the Counter16 so that it
produces an output of 1kHz square wave at 50% duty cycle on startup or after reset.
Part I Software
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SW2 is not used in the part.
SW1 controls the Enable input of the Counter16 User Module. When the switch is pressed, the
user module halts operation directly in hardware and requires no software intervention.
ECE 381
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Lab 4 – Square Wave Generator
The optical encoder ISR should adjust both the Period and CompareValue of Counter16 so that
the output frequency ranges from 100Hz to 10kHz, always with a 50% duty cycle.
For this application to be practical, one “tick” of the optical encoder should change the Period
and CompareValue by more than 1 integer value. Consider a logarithmic scale. You do not need
to hit every frequency in the range and the range should be reachable in a reasonable number of
turns. Reasonable will be determined by the professor and/or the lab TA.
The LCD should display the frequency of the signal sent to Port1[0] in units of Hz. You will need
to do floating point calculations for this to work. Be sure to ask the instructor about string
manipulation in class.
Floating point math routines are available. See the C Language Compiler User Guide
Remember: Do not update the LCD inside of an interrupt service routine! Keep the optical
encoder ISR small. It is bad programming practice for an ISR to do more than is necessary to
service the interrupt (e.g. updating the value on the LCD). Instead consider using a variable that
will let the main loop know that something has changed. For instance, if the main loop sees that
the Period value has changed, it can update the information on the LCD, however, it should not
update the LCD unless it has actually changed.
Test your design to make sure it meets all of the above requirements.
Part II
In this part, SW2 will be used to toggle the output between two different ranges: 100Hz to 10kHz
and 10Hz to 1kHz.
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Since there is only one GPIO interrupt that is shared by all of the GPIO pins, we will use a
DigBuf User Module to provide a unique interrupt for SW2.
Place a DigBuf User Module and configure it with the following Parameters:
o Name: SW2Buf
o Default Load Status: Enable
o Input1: Route SW2 (Port1[5]) to this input
o Input2: Disable
o Input2 ClockSync: Sync to SysClock
o Output1: Disable
o Output2: Disable
o InvertInput1: Normal
Implement an ISR for the DigBuf User Module that, when called, toggles the VC3 Divider
between two different Divider values so that the output frequency ranges from either 10Hz
to 1kHz or 100Hz to 10kHz. To do this, you will need to modify the register named OSC_CR3
which is described in the Technical Reference Manual on page 287. You will also need to
modify the LCD display routine to reflect the frequency change and the current mode of
operation.
Test your design to make sure it meets all of these additional requirements.
ECE 381
Lab 4 – Square Wave Generator
Part III
The system clock (SySClk) is generated internally by the PSoC. It is not a precise oscillator and has a
tolerance around its nominal frequency of 24MHz. This means that the actual output frequency of your
square wave also shares this tolerance.
You will measure the actual frequency of your specific PSoC’s system clock. To do this, you can use
register OSC_GO_EN which is described in the Technical Reference Manual on page 285. Modifying this
register will allow you to place the SysClk signal on GOE[2]. From there you can connect either Port0[2]
or Port2[2]. However, since the LCD is connected to Port2[2] on the evaluation board, it is unavailable
for this purpose.
Measure your PSoC’s actual SysClk frequency as accurately as you can and calculate the percentage it
deviates from the 24MHz nominal frequency. Look in the datasheet for your PSoC part number:
DataSheet_CY8C29x66_I.pdf and find the minimum and maximum values of the System Clock; it is listed
as Internal main oscillator (IMO). Compare your measured frequency with this range.
This model of PSoC is capable of operating at voltages from 3.00V to 5.25V. Suppose you had a design
that required the PSoC to operate at 3.3V. Look at the datasheet and determine the maximum CPU
Frequency (CPU_Clock) that can be used at this voltage.