Contents
Objective
In this tutorial we are going to discuss the PWM module of LPC1768.
First we will see how to configure the PWM registers to generate signals of required PWM, At the end we will see how to use the ExploreEmdedded PWM library.
LPC1768 PWM Module
LPC1768 has 6 PWM output pins which can be used as 6-Single edged or 3-Double edged. There as seven match registers to support these 6 PWM output signals. Below block diagram shows the PWM pins and the associated Match(Duty Cycle) registers.
| Adc Channel | Port Pin | Pin Functions | Associated PINSEL Register | Corresponding Match Register |
|---|---|---|---|---|
| PWM_1 | P2.0 | 0-GPIO, 1-PWM1[1], 2-TXD1, 3- | 0,1 bits of PINSEL4 | MR1 |
| PWM_2 | P2.1 | 0-GPIO, 1-PWM1[2], 2-RXD1, 3- | 2,3 bits of PINSEL4 | MR2 |
| PWM_3 | P2.2 | 0-GPIO, 1-PWM1[3], 2-CTS1, 3-TRACEDATA[3] | 4,5 bits of PINSEL4 | MR3 |
| PWM_4 | P2.3 | 0-GPIO, 1-PWM1[4], 2-DCD1, 3-TRACEDATA[2] | 6,7 bits of PINSEL4 | MR4 |
| PWM_5 | P2.4 | 0-GPIO, 1-PWM1[5], 2-DSR1, 3-TRACEDATA[1] | 8,9 bits of PINSEL4 | MR5 |
| PWM_6 | P2.5 | 0-GPIO, 1-PWM1[6], 2-DTR1, 3-TRACEDATA[0] | 10,11 bits of PINSEL4 | MR6 |
LPC7168 PWM Registers
The below table shows the registers associated with LPC1768 PWM.
| Register | Description |
|---|---|
| IR | Interrupt Register: The IR can be read to identify which of eight possible interrupt sources are pending. Writing Logic-1 will clear the corresponding interrupt. |
| TCR | Timer Control Register: The TCR is used to control the Timer Counter functions(enable/disable/reset). |
| TC | Timer Counter: The 32-bit TC is incremented every PR+1 cycles of PCLK. The TC is controlled through the TCR. |
| PR | Prescalar Register: This is used to specify the Prescalar value for incrementing the TC. |
| PC | Prescale Counter: The 32-bit PC is a counter which is incremented to the value stored in PR. When the value in PR is reached, the TC is incremented. |
| MCR | Match Control Register: The MCR is used to control the reseting of TC and generating of interrupt whenever a Match occurs. |
| MR0 | Match Register: This register hold the max cycle Time(Ton+Toff). |
| MR1-MR6 | Match Registers: These registers holds the Match value(PWM Duty) for corresponding PWM channels(PWM1-PWM6). |
| PCR | PWM Control Register: PWM Control Register. Enables PWM outputs and selects PWM channel types as either single edge or double edge controlled. |
| LER | Load Enable Register: Enables use of new PWM values once the match occurs. |
Register Configuration
The below table shows the registers associated with LPC1768 PWM.
| TCR | ||||
|---|---|---|---|---|
| 31:4 | 3 | 2 | 1 | 0 |
| Reserved | PWM Enable | Reserved | Counter Reset | Counter Enable |
Bit 0 – Counter Enable
This bit is used to Enable or Disable the PWM Timer and PWM Prescalar Counters
0- Disable the Counters
1- Enable the Counter incrementing.
Bit 1 – Counter reset
This bit is used to clear the PWM Timer and PWM Prescalar Counter values.
0- Do not Clear.
1- The PWM Timer Counter and the PWM Prescale Counter are synchronously reset on the next positive edge of PCLK.
Bit 3 – PWM Enable
Used to Eable or Disable the PWM Block.
0- PWM Disabled
1- PWM Enabled
As all the LPC1768 SFRs(Special Function Registers) are defined in lpc17xx.h, this has to be included at the beginning of our project/code.
LPC1768 has its GPIOs divided into five ports PORT0 - PORT4, although many of them are not physically 32bit wide. Refer the data sheet for more info. The Below registers will be used for Configuring and using the GPIOs registers for sending and receiving the Digital signals. A structure LPC_GPIOn(n= 0,1,2,3) contains all the registers for required for GPIO operation. Refer lpc17xx.h file for more info on the registers.
PINSEL: GPIO Pins Select Register
Almost all the LPC1768 pins are multiplexed to support more than 1 function. Every GPIO pin has a minimum of one function and max of four functions. The required function can be selected by configuring the PINSEL register. As there can be up to 4 functions associated with a GPIO pin, two bits for each pin are available to select the function. This implies that we need two PINSEL registers to configure a PORT pins.
By this the first 16(P0.0-P0.16) pin functions of PORT0 can be selected by 32 bits of PINSELO register. The remaining 16 bits(P0.16-P0.32) are configured using 32bits of PINSEL1 register.
As mentioned earlier every pin has max of four functions. Below table shows how to select the function for a particular pin using two bits of the PINSEL register.
| Value | Function | Enumeration |
| 00 | Primary (default) function, typically GPIO port | PINSEL_FUNC_0 |
| 01 | First alternate function | PINSEL_FUNC_1 |
| 10 | Second alternate function | PINSEL_FUNC_2 |
| 11 | Third alternate function | PINSEL_FUNC_3 |
FIODIR:Fast GPIO Direction Control Register.
This register individually controls the direction of each port pin.
| Values | Direction |
| 0 | Input |
| 1 | Output |
FIOSET:Fast Port Output Set Register.
This register controls the state of output pins. Writing 1s produces highs at the corresponding port pins. Writing 0s has no effect. Reading this register returns the current contents of the port output register not the physical port value.
| Values | FIOSET |
| 0 | No Effect |
| 1 | Sets High on Pin |
FIOCLR:Fast Port Output Clear Register.
This register controls the state of output pins. Writing 1s produces lows at the corresponding port pins. Writing 0s has no effect.
| Values | FIOCLR |
| 0 | No Effect |
| 1 | Sets Low on Pin |
FIOPIN:Fast Port Pin Value Register.
This register is used for both reading and writing data from/to the PORT.
Output: Writing to this register places corresponding values in all bits of the particular PORT pins.
Input: The current state of digital port pins can be read from this register, regardless of pin direction or alternate function selection (as long as pins are not configured as an input to ADC).
Note:It is recommended to configure the PORT direction and pin function before using it.
Examples
Example 1
Program to demonstrate the LED blinking.
Here first the PORT2 pins are selected for GPIO using PINSEL register then they are configured as Output using the FIODIR register.
LEDs are turned ON by sending a high pulse using FIOSET register.
After some time the LEDs are turned OFF by sending the low pulse using FIOCLR register.
Example 2
This is second approach in which FIOPIN register is used for both setting and clearing the PORT pins.
Writing Logic 1 will set the PORT pin and writing 0 will Clear the particular PORT bit.
Using Explore Embedded Libraries :
In the above tutorial we just discussed how to configure the PORTS for GPIO for blinking the Leds.
Once you know the GPIO configurations, you can directly use the ExploreEmbedded libraries to play around with LEDs.
For that you need to include the gpio.c/gpio.h and the associated files(delay/stdutils).
The below sample code shows how to use the GPIO functions.
Refer this link for more info on GPIO libraries.
Downloads
Download the complete project folder from the below link:
https://codeload.github.com/ExploreEmbedded/Explore-Cortex-M3-LPC1768-Stick-DVB-14001/zip/master
Have a opinion, suggestion , question or feedback about the article let it out here!