Understanding timers in lpc2148


So today we are going to discuss about timers. LPC2148 have two 32bit timers. By 32 bit I mean it’s registers are 32bit wide. In other controllers it could be 8 or 16 bit also.

Timing is very important part of a microcontroller programming. We certainly can not depend on delay functions because no matter how precise they are, they are not accurate. For eg- DHT11, a temperature sensor, works only if we give it specific pulses for specific amount of time(in microseconds and milliseconds).We will learn about it later in the blog. But for now we will use timer for controlling blinking of LED with different time interval.
Before going to the program, let’s go through some important registers we are going to use in timer.


TCR- Timer Control Register. The TCR is used to control the Timer Counter functions. The Timer Counter can be disabled or reset through the TCR.

TC- Timer Counter. The 32-bit TC is incremented every PR+1 cycles of PCLK(Peripheral Clock). The TC is controlled through the TCR. We will set the delay required here.

PR- Prescale Register. The Prescale Counter (below) is equal to this value, the next clock increments the TC and clears the PC.

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 and the PC is cleared. The PC is observable and controllable through the bus interface.

CTCR- Count Control Register. The CTCR selects between Timer and Counter mode, and in Counter mode selects the signal and edge(s) for counting. We are mostly going to use timer mode.

That’s it regarding registers. One important thing before we go to main programming section, as I mentioned that I have 12 MHz oscillator providing clock input but LPC2148 have capability to reach upto 60MHz. So through some PLLs (Phase Locked Loops) setting, I am first setting my PCLK (Peripheral clock) to 60 MHz, and than divide it according to the need.

How To

1. Enable in timer mode by writing 0 (clearing bits) to bit 0 and 1 of TxTCR register.

2. Set the Prescale value in TxPR. This is very important as this will decide your delay interval.
formula used to get prescale value is

Let’s say I want the delay of 1ms and I have PCLK of 15MHz so according to the formula,

Note that as in the formula time is given in sec and we want the delay of millisecond so time will be 1/1000. Also we have to subtract 1 from final value because TC is incremented every PR+1 cycle.
3. Set the count in TxTC to 0.
4. Enable timer, wait for TC to reach the delay and disable timer.



#include <LPC214X.H>
#include <stdint.h>

void pll_init(void);
void timer_init (void);
void timer0 (uint32_t count);

int main ()
 PINSEL0=0;    // setting normal GPIO functions     
 IODIR0|=(1<<16);    // pin 0.16 as o/p
 pll_init ();    // initialise clocks
 VPBDIV=0x00;    // pclk=cclk/4=15MHz
 timer_init ();    //initialise timer 0
 while (1)
  IOSET0=(1<<16);    // led on
  timer0 (500);    //wait for 500ms
  IOCLR0=(1<<16);    //led off
  timer0 (500);    // wait for 500ms  


void timer_init (void)              // timer 0 initialisation
 T0CTCR=0x00;                  // start in timer mode 
 T0PR= ((15000000/1000)-1);             // formula is COUNT=(P.F X TIME DELAY IN SEC) - 1 and pclk=cclk/4 for us
 T0TCR=(1<<1);                 // reset timer

void timer0 (uint32_t count)
 T0TC=0;    // set 0 to TC counter
 T0TCR=(1<<1);    //reset timer
 T0TCR=(1<<0);    // enable timer
 while (T0TC<count);    // wait untill TC counter reaches our defined value
 T0TCR=0x00;    // disable timer

void pll_init(void)
 PLL0CON = 0x01;    //Enable PLL
 PLL0CFG = 0x24;    //Multiplier and divider setup  M is (5-1=4) because we need 60 Mhz and we have 12 so 5 is multiplier. P is 2. so 0x24.
 PLL0FEED = 0xAA;   //Feed sequence
 PLL0FEED = 0x55;   // feed sequence

 while(!(PLL0STAT & 0x00000400)); //is locked?

 PLL0CON = 0x03;   //Connect PLL after PLL is locked
 PLL0FEED = 0xAA;  //Feed sequence
 PLL0FEED = 0x55;  //Feed sequence

/*  Control Pclk using VPBDIV register
VPBDIV=0x00 APB bus clock (PCLK) is one fourth of the processor clock (CCLK)
VPBDIV=0x01 APB bus clock (PCLK) is the same as the processor clock (CCLK)
VPBDIV=0x02 APB bus clock (PCLK) is one half of the processor clock (CCLK)
VPBDIV=0x03 Reserved. If this value is written to the APBDIV register, it has no effect (the previous setting is retained). */

NOTE: Here PC will increment and it will reach PR in 1ms and  TC will increment by 1. This will keep going on for 500 times and we will get a delay of 500ms.

LED should be blinking every 500ms now.

In the same way if you want the delay of 1 second, formula will be COUNT=(15000000 *1)-1).



Check out this video for full tutorial and result



You can buy me a coffee sensor 🙂

download the CODE below

, ,

Leave a Reply

Notify of