In this tutorial, we will be discussing the internal RTC of LPC2148 . We will see what are the different registers associated with the RTC but we will mainly focus on basics registers required to use RTC. Later we will see how to use Explore Embedded library for RTC.

RTC Registers

The RTC includes a number of registers. We will concentrate on some of the basic required registers.

Register Description
CCR Clock Control Register
Consolidated Time Registers
CTIME 0 Consolidated Time Register 0
CTIME 1 Consolidated Time Register 1
CTIME 2 Consolidated Time Register 2
Time Counter Registers
SEC Seconds Counter
MIN Minutes Register
HOUR Hours Register
DOM Day of Month Register
DOW Day of Week Register
DOY Day of Year Register
MONTH Months Register
YEAR Years Register
CALIBRATION Calibration value Register

RTC Register Configuration

Lets see how to configure these registers.

31:5 4 3:2 1 0
CCR ( Clock Control Register )

The Clock Control Register register controls the operation of the clock divide circuit.

Bit 0 – CLKEN : Clock Enable
This bit is written with one to enable time counters and written with zero to disable time counters.

Bit 1 - CTCRST : CTC Reset
When this bit is set to one, the internal oscillator divider are reset.

Bit 3:2 - CTTEST
Test Enable. These bits should always be zero during normal operation.

Bit 4 - CLKSRC: Clock Source
0: Clock Tick Counter takes its clock from the Prescaler.
1: If this bit is 1, the CTC takes its clock from the 32 kHz oscillator that’s connected to the RTCX1 and RTCX2 pins.

Bit 31:5 - Reserved

Time Counter Register

The time value consists of the eight counters and there is separate register associated with each counter. To update the RTC, new values should be written to these registers.

Time Counter Registers
Register Description Minimum Value Maximum Value
SEC Seconds value 0 59
MIN Minutes value 0 59
HOUR Hours value 0 23
DOM Day of Month value 1 28,29,30 or 31(depending on the month)
DOW Day of Week value 0 6
DOY Day of Year value 1 365 (366 for Leap Year)
MONTH Month value 1 12
YEAR Year value 0 4095

Consolidated Time Registers

The values of the Time Counters can optionally be read in a consolidated format which allows the programmer to read all time counters with only three read operations.

There are 3 Consolidated Time Registers. The minimum and maximum values of the various fields in these registers are same as the respective Time Counter Register.

These registers are read only. New values should be written to Time Counter Registers.

Consolidated Time Register 0 ( CTIME0 )
31:27 26:24 23:21 20:16 15:14 13:8 7:6 5:0
Reserved Day of Week Reserved Hours Reserved Minute Reserved Second

Consolidated Time Register 1 ( CTIME1 )
31:27 27:16 15:12 11:8 7:5 4:0
Reserved Year Reserved Month Reserved Day of Month

Consolidated Time Register 2 ( CTIME2 )
31:12 11:0
Reserved Day of Year

Steps for Using RTC

Initialize RTC

  • Select the Clock source Internal/external(32k).
  • Enable the clock for RTC

Set Date and Time

As discussed, there are separate Time Counter registers for each time parameter(hour, min, sec and date). We just need to copy the required values to these registers.

Read Date and Time

The values of date and time can be read from associated Time Counter registers. Alternately, date and time can also be read from Consolidated Time registers.

We will use Time Counter registers for both reading and writing.

Code Example

Example 1

In this example we will see how to use lpc1768 inbuilt RTC. The time and date will be displayed continuously on 2x16 LCD.

Using Explore Embedded Libraries

Now we will discuss how to use Explore Embedded Libraries for LPC2148 inbuilt RTC.


Do the connections as shown the below image and flash the compiled code. LPC2148 RTC.png


Download the complete project folder from this link