This is the article to introduce the RTC (Real Time Clock) programming of ARM Cortex-M3 LPC1768 Microcontroller. Here we are going to set the recent time and date in the RTC registers and retrieve it to display on LCD. For setting up the Environment for the development of ARM cortex M3 refer this article.

The LPC 1768 is ARM Cortex- M3 based Microcontrollers for embedded application features in low power consumption and a high level of integration. The ARM Cortex M3 is designed in a such way to enhance debug features and a higher level of system integration. It clocks at a CPU frequency of 100 MHz, and incorporates a 3-stage pipeline and uses a Harvard architecture with separate local instruction and data buses for third bus peripherals. The ARM Cortex- M3 CPU have an internal pre-fetch unit to support speculative branching. The peripheral components include 512KB of flash memory, 64kb of data memory, Ethernet MAC, USB OTG, 4 UART’s, 8-channel general purpose DMA controller, 2 SSP Controllers, 10-bit DAC, Quadrature encoder interface, SPI interface, 3 I2C bus interface, 2 input plus 2 outputs I2S bus interface, 4 general purpose timers, ultra-low power Real-Time Clock (RTC) with separate battery supply, and up to 70 general purpose I/O pins, 6-output general purpose PWM. The LPC1768/66/65/64 are pin-compatible with the 100-pin LPC236x ARM7-based Microcontroller series.

Fig. 1: RTC Programming In LPC1768 Prototype

ARM Cortex M3 (LPC1768) has an inbuilt RTC-Real Time Clock. It includes 20 bytes of registers which are battery powered, which can also be used to store the system status when the power goes off. A 3 V lithium button cell is capable of running the RTC till the voltage drops as 2.1 V. An Interrupt from the RTC can wake up the CPU from any power saving mode.

POWERING THE TIMERS:

Conservation of power is the major concern in the LPC1768. Each peripheral has to turn ON before utilizing it. RESET makes some peripherals to enable while others are disabled. Here the RTC on RESET will be always enabled.

Fig. 2: Bit Values Of Power Control Registers In LPC1768
 

Code Snippet:

/* Enable CLOCK into RTC */

LPC_SC->PCONP |= (1 << 9);

SELECTING CLOCK FOR RTC:

The RTC uses the 1 Hz clock output from the RTC oscillator as the only clock source. The peripheral clock rate for accessing registers is CCLK/8.

RTC INTERRUPTS:

Interrupt Location Register (ILR), Counter Increment Interrupt Register (CIIR) and the Alarm Mask Register (AMR) controls the interrupt generation of RTC. Each bit in CIIR corresponds to one of the time counters. The alarm register allow the user to specify a date or a time at which the interrupt has to be generated, AMR also provides masking mechanism in their registers, this interrupt can wake the microcontroller out of Power-down mode when the RTC is operating from its own oscillator and Battery.

RTC REGISTERS:

The RTC block of LPC1768 has a number of registers, here we will concentrate and discuss about the important registers.

• 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 REGISTERS AND RTC OPERATION:

The section below will explain the Registers associated with RTC.

CCR ( Clock Control Register ):

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

Fig. 3: CCR Bits And Description In LPC1768 To Control Clock Divide Circuit
 

Time Counter Register:

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

Fig. 4: Time Counter Register In LPC1768 To Update RTC

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 )

Fig. 5: Bit Address Of Consolidated Time Register 0 In LPC1768
 

Consolidated Time Register 1 ( CTIME1 )

Fig. 6: Bit Address Of Consolidated Time Register 1 In LPC1768

Consolidated Time Register 2 ( CTIME2 )

Fig. 7: Bit Address Of Consolidated Time Register 2 In LPC1768

Create a project using Keil uvision4 for LPC1768 Microcontroller:

In this section, we will start creating a project in Keil MDK we have already installed Keil µVision and Co-MDK Plug-in + CoLinkEx Drivers required for the CoLinkEx programming adapter. You can start by downloading the project files and kick start your practical experiment.

Code.rar

Code Description:

Separate functions are used to set the time and read the time. The function to set time is

RTCSetTime( local_time );       where local_time is the structure consists of members holding date and time values.

To read the time from RTC the below function is used

              current_time = RTCGetTime();           /* Read Time */

Individual members can be accessed and stored in variables by the code below.

                        hour = current_time.RTC_Hour;

                        min  = current_time.RTC_Min;

                        sec  = current_time.RTC_Sec;

For Display in LCD the following lines are used

                        LCD_GoToLine(0);                             /* Go to First line of 2×16 LCD */

                        LCD_Printf(“Time: %2d:%2d:%2d”,hour,min,sec);

                        LCD_GoToLine(1);                             /* Go to Second line of 2×16 LCD */

                        LCD_Printf(“Date: %2d/%2d/%4u”,date,month,year);  

For more details on interfacing the LCD in 4 bit mode with LPC1768 refer this link.

Project Source Code

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 The codes are linked in Description ###

Circuit Diagrams

Circuit-Diagram-RTC-Programming-in-LPC1768

Project Components

Project Video

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Filed Under: ARM

 

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