Experiment 4: Hardware Timers

1. Objectives

Using hardware timers
Using the LPC1769 manual to figure out how to use a given register
Identifying how to access a given register by referring to the LPC17xx.h file

2. Parts List

LPC1769 LPCXpresso board
USB A-Type to Mini-B cable
Breadboard
LEDs
330-Ohm Resistors
Jumper wires

3. Background

There are four hardware timers in LPC1769: Timer 0, Timer 1, Timer 2, and Timer 3. They are identical and can work independently with different settings.

Throughout this document, timer or TIMERx refer to one of these LPC1769 timers. Every one of the timer-related registers discussed henceforth applies to all of these four timers, and cannot be used without specifying the targeted timer.

3.1. Timer Basic Operation: Timer Counter

The basic function of any timer is to have a counter running. In LPC1769, this counter is called Timer Counter (TC).

In this section, we will learn how to enable TC to start ticking, and will find out how fast it can run.

3.1.1. Controlling the Counting Speed

Peripherals in LPC1769 are fed with an input clock called the peripheral clock (PCLK). By default, the Timer Counter (TC) register is incremented every PCLK cycle.

There are two ways to change that:

Divide PCLK by a factor other than the default. This will change the input clock frequency. Since this method is applicable for all peripherals we will discuss it in a separate section at the end of this document.
Using an intermediate counter called the prescale counter.

The prescale counter is always incremented every PCLK pulse. This continues till the prescale counter = the prescale register. When that happens, two events take place in the next PCLK pulse:

Increment TC by 1
Reset the prescale counter and continue counting

If the prescale register is not set to any value (the default is 0), TC to be incremented every PCLK.

Although it was claimed ealier that TC is by default incremented every PCLK, you now know that this is only true when the prescale register = 0.

Example 1. TC and the Prescale Register

If you set the prescale register to 5, TC will be incremented every 6 PCLK pulses.

3.1.2. Enabling the Counter

To start using a timer, you first must enable counting! In LPC1769, the Timer Control Register (TCR) is the register that allows you to do that.

As should be clear from previous experiments, you interact with peripherals through registers. In the case of timers, to enable a counter and have it start counting, you need to write to the TCR register. To do that, you must:

Identify the relevant bit(s) of the register. For that, refer to table 428 in chapter 21 of the LPC1769 manual.
Find out how to access this register using CMSIS. One easy way to do that is to check the lpc17xx.h file to find the structure containing the TCR field.

Exercise

How can you enable Timer 0 counter?
How can you enable Timer 3 counter?

You can write a single line of code that would enable the counter, and then use printf() to see whether TC is counting.

3.2. Timer Counter (`TC`) is Ticking; Now What?

There are two main ways to use a ticking timer:

Load a match register (MR) with some value and then wait till TC=MR to trigger some action.
Capture the time in a capture register (CR), i.e. set CR= TC, whenever an event takes place on a given pin. The event is simply any change of the pin state (HIGH → LOW or LOW → HIGH, i.e. a positive edge or a negative edge).

In this section, we will discuss these two options.

3.2.1. Timing Using a Match Register

For each LPC1769 timer, there are four match registers: MR0, MR1, MR2, and MR3.

Timer Registers

Hereafter, MR or MRx refers to one of MR0, MR1, MR2, and MR3 match registers of a specific LPC1769 timer.

When the value of TC reaches the value in the match register (MR), an action is triggered. Therefore, setting MR specifies the timer’s period. The action triggered every time TC reaches MR0 can be set using the Match Control Register (MCR) to one (or more) of the following:

Generate an interrupt
Reset TC
Stop TC

You can enable or disable the above actions by setting or clearing the three least significant bits of the MCR register.

Table 1. Setting Timer Actions Using the `MCR` Register
`MCR` bit	Bit value = 1	Bit value = 0
0	Enable timer interrupt	Disable timer interrupt
1	Reset `TC`	Disable this feature
2	Stop `TC`	Disable this feature

External Match Action

You can also trigger a different action when TC=MR, which is, to set, reset, or toggle a specific bit. This bit can be pinned out to an external output pin, hence the name: External Match bit (EMx).

For each timer, there are 4 EM bits, namely EM0, EM1, EM2, and EM3. Each EMx bit can be controlled when TC equals the corresponding MRx. These four bits belong to the EMR register.

Study the EMR register tables (432 and 433) in chapter 21 of the LPC1769 manual to understand the following examples.

Example 2. External Match Actions

Assigning 0 to bit 6, and 1 to bit 7 in EMR will force bit 1 in EMR to be HIGH when TC = MR1.
Assigning 1 to both bits 10 and 11 in EMR will toggle bit 3 in EMR when TC = MR3.

Theoretically, any EM bit can be pinned out to a pin that is named MATx.y, where x is the timer number and y is the match register number.

Example 3. Pinned Out External Match Actions

When using MR3 with Timer 2, the EM3 bit of the EMR register of Timer 2 can be pinned out to MAT2.3.
When using MR1 with Timer 0, the EM1 bit of the EMR register of Timer 0 can be pinned out to MAT0.1.

Practically, however, only MATx.0 and MATx.1 are available in LPC1769 for Timer 0, Timer 1, and Timer 3, whereas Timer 2 can use all four MAT2.y pins.

You need to change a pin’s function to use it as MATx.y. Refer to the PINSEL section in experiment 3 and chapter 8 of the LPC1769 manual for more details.

Exercise

One of the tasks in this experiment is about external match actions. To be able to complete that task, you need to find a suitable MATx.y pin.

So, refer to chapter 8 of the LPC1769 manual and list all the MATx.y pins and find out which of them is physically available and accessible on your LPCXpresso board.

3.2.2. Capturing an Event (Event Timers)

Instead of using a match register, you can capture the time in a capture register (CR) when a pin’s state changes. In other words, you can take a snapshot of the timer value when an input signal changes.

This happens by loading the TC value into a CR (CR ⇐ TC) when an input pin has a positive edge and/or a negative edge.

For each timer, there are two capture registers: CR0 and CR1. A pin that can be used with a CR is named CAPx.y, where x is the timer number and y is capture register number.

Example 4. Capture Registers

By using CAP1.0, you will be loading TC into CR0 of Timer 1.
By using CAP0.1, you will be loading TC into CR1 of Timer 0.

To enable this feature, you need to use the CCR register. In addition to capturing the time, you can use the CCR register to enable generating an interrupt when the state of CAPx.y changes.

Study the CCR register table (431) in chapter 21 of the LPC1769 manual to understand the following examples.

Example 5. Using the CCR Register

Assign 15 (1111 in binary) to the CCR register of Timer 0 will:

Load TC to CR0 on both the positive and negative edges of CAP0.0
Generate a Timer 0 interrupt request
Load TC to CR1 only on the positive edges of CAP0.0, without generating interrupt requests.

You need to change a pin’s function to use it as CAPx.y. Refer to the PINSEL section in experiment 3 and chapter 8 of the LPC1769 manual for more details.

Exercise

One of the tasks in this experiment is about capturing event times. To be able to complete that task, you need to find a suitable CAPx.y pin.

So, refer to chapter 8 of the LPC1769 manual and list all the CAPx.y pins and find out which of them is physically available and accessible on your LPCXpresso board.

3.3. Important Notes

If you choose to enable the timer interrupt, remember to enable the the NVIC and to clear the interrupt bit in the ISR. To clear the MR0 interrupt flag, set the least significant bit in the Interrupt Register (IR).
A common misconception is to assume that register MR0 can be used with timer 0 only, register MR1 with timer 1 only, and so on. Each timer has its own 4 match registers.
As usual, all the registers in this experiment are fields of some structures. Refer to the LPC17xx.h header file to find the required name and field to access the required register.

Exercise

In this exercise, we will use a hardware timer and timer interrupts to blink an LED.

// "x" is a placeholder. Replace x with an appropriate value.

int main(void) {

    // Try to find out the IRQ number. Why is this step important?
    NVIC_EnableIRQ(x);
    // Answer:

    // What does register TCR do?
    LPC_TIMx->TCR |= x;
    // Answer:

    // What does register MRx do?
    LPC_TIMx->MRx = x;
    // Answer:

    // What does register MCR do?
    LPC_TIMx->MCR = x;
    // Answer:

    LPC_GPIOx->FIODIR = 1 << x ;

    // Can we remove this while loop? Why?
    while(1);
    // Answer:

    return 0 ;
}


// When will the following function be executed? Who is going to call it?
// Answer:

void TIMERx_IRQHandler() {

    LPC_GPIOx->FIOPIN ?? (1 << x);
    // Replace "??" with the appropriate operator

    // What does register IR do?
    LPC_TIMx->IR |= (1 << x);

}

3.4. Peripheral Clock (`PCLK`)

Timers, among other devices, rely on peripheral clocks (PCLK), which in turn are derived from the core clock (CCLK).

There are four possible frequency configurations for the peripheral clock (PCLK), which are set using a pair of bits.

Table 2. Peripheral Clock (`PCLK`) Frequency Configurations
Bit Values	Frequency Configuration
`01`	`PCLK` = `CCLK`
`10`	`PCLK` = `CCLK` / 2
`00`	`PCLK` = `CCLK` / 4
`11`	`PCLK` = `CCLK` / 8

These pairs of bits belong to the PCLKSEL0 and PCLKSEL1 registers, which control the PCLK frequency for all peripherals.

The PCLKSEL0 and PCLKSEL1 Register Fields figure illustrates some of the fields of the PCLKSEL0 and PCLKSEL1 registers. Every two bits control the PCLK frequency for a specific peripheral.

Figure 1. PCLKSEL0 and PCLKSEL1 Register Fields

Question

Can you ignore this step? What would happen if we skip it?

For the full list of peripherals and their corresponding two bits in PCLKSEL0 or PCLKSEL1, you can refer to Chapter 4 (section 4.7.3) in the LPC1769 manual.

This section is not specific to timers. It is about configuring the frequency of PCLK, which is required for timers.

You may want to refer back to this section whenever you want to use a peripheral that requires PCLK.

3.5. Power Up

All microcontroller peripherals must be powered up before they can be used. This was not a concern in earlier experiments because we were using peripherals that are powered up by default.

Powering peripherals up and down is controlled through the Power Control for Peripherals Register (PCONP).

By referring to table 46 in Chapter 4 of the LPC1769 manual, you can see that the reset value (default value) is 1 for some peripherals, meaning that they are powered on by default, whereas it is 0 (OFF by default) for others.

Timer 0 and Timer 1 are powered up by default. However, if you use Timer 2 or Timer 3, your experiment will not work without powering up the timer in your program.

To save power, you can turn the power OFF for any unused peripherals that are ON by default.

4. Tasks

Complete the LED blinking exercise above. Note that a for loop is not needed to implement the delay.
Blink an LED without using timer interrupts.
Connect an output pin to two capture pins, say CAP2.0 and CAP2.1. Enable one of them to capture the time with the rising edge and the other one with falling edge.

Now, set the output pin high then clear it immediately. Calculate the difference between CR0 and CR2 and use printf() to display this difference.

Can you explain the result?

Try using FIOPIN instead of FIOSET and FIOCLR to control the output pin. Can you explain the different result?

Use external match actions for task 2.

5. Resources

[lpc1769-manual]: NXP Semiconductors. 'UM10360 LPC176x/5x User manual'. Rev. 3.1. 2 April 2014.
http://www.nxp.com/documents/user_manual/UM10360.pdf

6. Grading Sheet

Task	Points
Use hardware timers and timer interrupts	5
Identifying required register settings for alternative configurations	3
Discussion	2

Experiment 4: Hardware Timers

1. Objectives

2. Parts List

3. Background

3.1. Timer Basic Operation: Timer Counter

3.1.1. Controlling the Counting Speed

3.1.2. Enabling the Counter

3.2. Timer Counter (TC) is Ticking; Now What?

3.2.1. Timing Using a Match Register

External Match Action

3.2.2. Capturing an Event (Event Timers)

3.3. Important Notes

3.4. Peripheral Clock (PCLK)

3.5. Power Up

4. Tasks

5. Resources

6. Grading Sheet

3.2. Timer Counter (`TC`) is Ticking; Now What?

3.4. Peripheral Clock (`PCLK`)