Configuring and Using the Alarm Module of the MCP795WXX Real-Time Clock/Calendars

AN2140Configuring and Using the Alarm Module of the

MCP795WXX Real-Time Clock/Calendars

INTRODUCTION

An increasing number of applications that involve timemeasurement require a Real-Time Clock/Calendar(RTCC) device. The MCP795WXX is a feature-rich SPIRTCC that incorporates EEPROM, SRAM, unique ID,timestamp, Watchdog Timer and event-detect module.This application note describes how to build an alarmsystem based on the above mentioned SPI RTCC, usingthe related alarm and control registers. In addition, itindicates how to configure/prepare the SPI RTCC for aspecific alarm application. This document is applicable tothe use of the following devices: MCP795W10,MCP795W11, MCP795W12, MCP795W20,MCP795W21 and MCP795W22.

FEATURES OF THE SPI RTCC

• Real-Time Clock/Calendar:- Hours, minutes, seconds, hundredths of

seconds, day of week, month and year- Support for leap year

• Leap Year Calculation up to 2399• Timestamp Function• 2 Kbit (256 x 8) EEPROM Memory

• Low-Power CMOS Technology• 64-Byte x 8 Organization Battery-Backed SRAM• Input for External Battery Backup• On-Board Crystal Oscillator for RTCC Functions:

- Battery operated when VCC removed• Programmable Clock-out Function• Two Programmable Alarms• 64-Bit Unique ID in Protected Area:

- Support EUI-48/64• Programmable Watchdog Timer• On-Board Event Detection:

- High-speed digital event detection on the 1st, 4th, 16th or 32nd event (glitch filter)

- Low-speed detection with programmable debounce time

- Dual configurable inputs• On-Chip Digital Trimming/Calibration

The SPI RTCC has many other features that make thechip useful for applications such as:

• time keeping (calibrated or not)• alarm systems• event detection• Watchdog Timer

SCHEMATICThe schematic includes a PIC18 Explorer demo boardand the SPI RTCC PICtail™ daughter board as shown inFigure 1.

FIGURE 1: SCHEMATIC

Author: Alexandru ValeanuMicrochip Technology Inc.

RC5/SD01 RA2

RA4/T0 CKI

RC3/SCK1/SCL1

RA5

RB0

PIC18F87J11

VDD

VDD

S1

S2

MENU KEY

SCK

SDI

MCP23S17SPI Expander

CS

RS

E

DB7 - 0

LCDLUMEX

10K

10K

BAT

1K

R4

100 pF

BAT 85

Y

C4

C3

10 pF

32.768 kHz

X1

X2

VBAT

WD

IRQ

CS

VSS

VCC

CLKOUT

EVHS

EVLS

SCK

SDI

SDO

SPI RTCCMCP795W

3

7

13

14

12

11

10

9

8

1

2

RC5/SDO1

RC4/SDI1

C1 = 0.1 µF

VDD VDD

INC KEY

6

4

5

VDD VDD

RC2 RB1/INT1 RB2/INT2

WD IRQ

IRQ WD

12 pF

J1

J2

1K

VDD 10K

POT R3

RA0/AN0C2

Note: AC164147 PICtail™ Board with MCP795W20 is used for this application.

2016 Microchip Technology Inc. DS00002140A-page 1

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The hardware structures used on the demo board are:

• LCD module

• Two push buttons

• The on-board potentiometer related to RA0 input and internal 10-bit Analog-to-Digital Converter (ADC)

• AC164147 SPI RTCC PICtail™ daughter board

To access the LCD through a minimum of pins, the SPIon the MSSP1 module is used, in conjunction with a16-bit I/O expander with SPI interface (MCP23S17).

The two on-board push buttons are S1 and S2,connected to RB0, RA5 GPIOs. The SPI RTCC is partof the PICtail evaluation board and is directlyconnected to the MSSP1 module of the microcontrollerthrough the related SPI lines: SCK, SDI, SDO, CS.

Another necessary connection is between the IRQsignal of the SPI RTCC and the RB1/INT1 pin of themicrocontroller. The RTCC is programed to assert lowthe IRQ pin at the end of the delay.

The PICtail daughter board has three other compo-nents:

• a 32,768 Hz crystal driving the internal clock of the RTCC

• a 3-volt battery sustaining the RTCC when VDD is not present on the demo board

• a SPI temperature sensor (TC72)

DETAILS ABOUT IMPLEMENTATION

The application is designed around the PIC18 Explorerboard, running on a PIC18F87J11 microcontroller. Thecode is written in C using MPLAB® X IDE and the XC8compiler. The firmware implements a delayed alarmsystem, based on the internal alarm registers forAlarm 0.

The value of the delay is imposed using the on-boardpotentiometer (R3) measured through the internal ADCand is enabled by the S1 push button.

The delay is then written to the Alarm 0 registers. Once this sequence is finished, the firmware displaysminutes and seconds (up to 17 minutes and3 seconds). After this time has been reached, an alarmmessage will be shown on the LCD. The applicationrestarts automatically after two seconds.

FUNCTIONAL DESCRIPTION

MCP795WXX works as an SPI slave, connected to theSPI bus of the PIC18 microcontroller (MSSP1 module).The Chip Select of the RTCC (CS = pin 6) is controlledby the RC2 GPO pin, defined in the spi_drivers.hheader as: #define NCS_SPI_RTCC PORTCbits.RC2.

As stated in the MCP795WXX data sheet (DS22280),for reads, the part is selected by pulling CS low, thenthe 8-bit READ instruction (13h) is transmitted to theMCP795WXX followed by the 8-bit address (A7through A0). After the correct READ instruction andaddress are sent, the data stored in the memory at theselected address is shifted out on the SO pin.

The data stored in the memory at the next address canbe read sequentially by continuing to provide clockpulses. The internal Address Pointer is automaticallyincremented to the next higher address after each byteof data is shifted out.

As the RTCC registers are separate from the SRAMarray, when reading the RTCC registers set, theaddress will wrap back to the start of the RTCCregisters.

Also when an address within the SRAM array is loaded,the internal Address Pointer will wrap back to the startof the SRAM array. The READ instruction can be usedto read the arrays indefinitely by continuing to clock thedevice. The read operation is terminated by raising theCS pin (Figure 2).

FIGURE 2: SPI READ SEQUENCE

SO

SI

SCK

CS

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 221

0 101000 1 A7 6 5 4 1 A0

7 6 5 4 3 2 1 0

Instruction Address Byte

Data OutHigh-Impedance

23

3 2 Don’t care

Note: The address will roll over to the start of either the RTCC or SRAM block.

DS00002140A-page 2 2016 Microchip Technology Inc.

AN2140

For writes, as the RTCC and SRAM registers do notrequire the WREN sequence like the EEPROM, theuser may proceed by setting the CS low, issuing theWRITE instruction (12h), followed by the address, andthen the data to be written. As no write cycle is requiredfor the RTCC and SRAM registers, the entire array canbe written in a single command.

For the data to be actually written to the array, the CSmust be brought high after the last byte has beenclocked in. If CS is brought high at any other time, thelast byte will not be written. Refer to Figure 3 for moredetailed illustrations on the write sequence.

FIGURE 3: SPI WRITE SEQUENCE

APPLICATION DESCRIPTION

This application performs a delayed alarm system. Thefirmware goes through three states:

• Setting the value of the delay through theon-board potentiometer. The LCD screen readsas indicated in Figure 4.

FIGURE 4: SETTING THE VALUES OF THE DELAY

Once the value is enabled (through the S1 pushbutton), it will be written in the related alarm registersand the RTCC will be initialized.

• Reading the current time count (minutes andseconds). The related LCD screen reads asshown in Figure 5.

FIGURE 5: READING THE CURRENT TIME COUNT

• Reaching the end of the alarm delay. When amatch occurs between the time count and thealarm registers, the IRQ is asserted low. The pinis polled through firmware and the code stopsdisplaying the time count. The LCD screen showsas indicated in Figure 6.

FIGURE 6: REACHING THE END OF THE ALARM DELAY

The application will restart automatically after twoseconds.

SO

SI

SCK

CS

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 221

0 001000 A7 6 5 4 1 A0 7 6 5 4 3 2 1 0

Instruction Address Byte Data Byte

High-Impedance

23

3 21

ALM= xxmin yysecS1= ENABLE ALARM

ALM= xxmin yysecCNT= mm ss

END OF ALARM


AN2140

The application described in this application noterepresents an attempt of automation of a microwavebatch. It is important to know that an alarm systembased on a microcontroller and an RTCC cannot besustained on battery because when the VCC falls down,there is nothing to make the final decision at the end ofthe alarm. Moreover, without the VCC, IRQ is no longervalid and no interrupts appear, while SRAM andcounter of the RTCC are still alive.

FIRMWARE DESCRIPTION

The project follows the standard multi-file philosophy.All necessary drivers can be found in the relatedlibraries.

Delay Drivers (delay_drivers.h)

They have two applications in the project:

• LCD functions – Since the controller of the LCD needs some delays to process commands, few auxiliary delays were created, based on TMR1:

- dly39us() (39 µs)

- dly43us() (43 µs)

- dly1_5ms() (1.5 ms)

• Long delays – Used for the keyboard debounce or as general purpose. They are based on TMR3 and include:

- dly5ms() (5 ms)

- dly100ms() (100 ms)

- dly1s() (1s)


AN2140

LCD Drivers (lcd_drivers.h)

• Basic LCD functions – They handle data, com-mands and strings written into the LCD. The three drivers used are defined below:

Basic LCD Functions

• High-level LCD functions – They initialize or print date/time and long integers to the LCD.

The library also includes time and date globalvariables: sec, min, hr, day, dat, mon, yr.

High-Level LCD Functions

SPI Drivers (spi_drivers.h)

Represent the low-level communication between theMSSP1 module of the PIC18 and the SPI RTCC.

The related drivers will be detailed in the next para-graph, as called functions.

Software License AgreementThe software supplied herewith by Microchip Technology Incorporated (the “Company”) is intended and supplied to you, theCompany’s customer, for use solely and exclusively with products manufactured by the Company.The software is owned by the Company and/or its supplier, and is protected under applicable copyright laws. All rights are reserved.Any use in violation of the foregoing restrictions may subject the user to criminal sanctions under applicable laws, as well as to civilliability for the breach of the terms and conditions of this license.THIS SOFTWARE IS PROVIDED IN AN “AS IS” CONDITION. NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATU-TORY, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICU-LAR PURPOSE APPLY TO THIS SOFTWARE. THE COMPANY SHALL NOT, IN ANY CIRCUMSTANCES, BE LIABLE FORSPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES, FOR ANY REASON WHATSOEVER.

void wrcmnd_lcd (unsigned char cmnd_lcd) ; // writes a command in the LCD

void wrdata_lcd (unsigned char data_lcd) ; // writes a data byte in the LCD

void wrstr_lcd (const unsigned char *str_lcd) ; // writes a string

// the string is stored in the Flash

void clr_lcd(void) ; void ini_lcd(void) ; // initialization of the LCD

void sec_to_lcd(void) ; void min_to_lcd(void) ;

void hr_to_lcd(void) ; // time printed to the LCD

void dat_to_lcd(void) ; void mon_to_lcd(void) ;

void yr_to_lcd(void) ; // date printed to the LCD

void back_lcd(unsigned char pos); // turns back cursor with 'pos' positions

void del_lcd(unsigned char pos) ; // deletes back 'pos' characters

void incr_yr(void) ; // increments YEARS value (2 digits), used in an electronicwatch

void incr_mon(void) ; // increments MONTHS value (2 digits), used in an electronicwatch

void incr_dat(void) ; // increments DATE value (2 digits), used in an electronicwatch

void incr_hr(void) ; // increments HOURS value (2 digits), used in an electronicwatch

void incr_min(void) ; // increments MINUTES ('minutes' have no aux flags)

void incr_sec(void) ; // increments SECONDS ; bit7 = ST = 1, used in an electronicwatch


AN2140

RTCC Drivers (rtcc_drivers.h)

Represent the medium-level communication betweenthe MSSP1 module of the PIC18 and the SPI RTCC.

The related functions call the SPI drivers, as describedbelow. Moreover, the library defines all necessaryconstants as: register’s addresses and masks.

Writing a Byte to the SPI RTCC

The firmware for writes to the RTCC follows in the chartbelow.

Flowchart for Writes

Reading a byte from the SPI RTCC

void spi_rtcc_wr (unsigned char rtcc_reg, unsigned char time_var) {

// SPI write to the SPI RTCC

spi_rtcc_start() ; // start SPI comm with the SPI RTCC

spi_wrbyte(SPI_RTCC_WRITE) ; // send the SPI WRITE command

spi_wrbyte(rtcc_reg) ; // send the register's address

spi_wrbyte(time_var) ; // send the SPI data byte

spi_rtcc_stop() ;} // stop SPI comm

Assert low CS

Write the SPI WRITE command

Write the register’s address

Write data byte

Assert high CS

unsigned char spi_rtcc_rd(unsigned char rtcc_reg){

// SPI read from the SPI RTCC

spi_rtcc_start() ; // start the SPI comm with the SPI RTCC

spi_wrbyte(SPI_RTCC_READ) ; // send the SPI READ command

spi_wrbyte(rtcc_reg) ; // send the register's address

rtcc_buf = spi_rdbyte() ; // read the result and store it

spi_rtcc_stop() ; // stop the SPI comm with the SPI RTCC

return rtcc_buf ;} // return the read result


AN2140

The firmware for reads from the RTCC is shown in theflowchart below.

Flowchart for Reads

CONFIGURING/PREPARING THE SPI RTCC FOR ALARM APPLICATIONS

The function responsible for this important action is:void ini_spi_rtcc(void) and it is included in thespi_rtcc_drivers.h header.

The corresponding code is indicated below:

Assert low CS

Write the SPI REcommand

Write the register’s address

e data byte

Assert high CS

void ini_spi_rtcc(void) // initialization of the SPI RTCC

{

spi_rtcc_wr(ADDR_MNTH, 0x01) ; // month = 01

spi_rtcc_wr(ADDR_ALM0MTH, 0x01) ; // alarm month = 01

spi_rtcc_wr(ADDR_DAY, 0x01) ; // day of week = 01

// since an alarm system doesn’t work on back-up,

// the VBATEN bit was not set

spi_rtcc_wr(ADDR_DATE, 0x01) ; // date of month = 01

spi_rtcc_wr(ADDR_ALM0DAT, 0x01) ; // alarm date = 01

spi_rtcc_wr(ADDR_HOUR, 0x00) ; // hour = 00

spi_rtcc_wr(ADDR_ALM0HR, 0x00) ; // alarm hour = 00

spi_rtcc_wr(ADDR_MIN, 0x00 ; // minutes = 00

spi_rtcc_wr(ADDR_SEC, 0x00) ; // seconds = 00, stop oscillator

spi_rtcc_wr(ADDR_ALM0MIN, alm_min) ; // alarm min = alm_min(imposed by the pot)

spi_rtcc_wr(ADDR_ALM0SEC, alm_sec) ; // alarm sec = alm_sec(imposed by the pot)

spi_rtcc_wr(ADDR_CTRL, ALM_0) ; // enable ALARM0, OUT=0, no SQWAVE

spi_rtcc_wr(ADDR_ALM0CTL, ALMxC_ALL+0x01) ; // match on all variables,

} // alarm day of week = 1


AN2140

This function initializes the SPI RTCC for a particularalarm application: the Alarm 0 module is activatedwhen all variables match. The below pairs of variablesare set identically, in order to offer a match from thebeginning:

• month & alarm0_month

• date & alarm0_date

• hour & alarm0_hour

The initialization is performed by the below codestatements:

The only exceptions are represented by the two pairsof variables which are active in this application:

• minutes & alarm0_minutes

• seconds & alarm0_seconds

Minutes and seconds are set to zero, but the relatedalarm variables are set to the values obtained from theinternal ADC: alm_min (alarm0_minutes), alm_sec(alarm0_seconds). As can be easily seen, the two pairswill match when the SPI RTCC will reach the final countup.

In addition, keep in mind that the related controlregisters are initialized in order to give an alarm (theIRQ pin will be asserted low) when all variables match.

The most important registers are:

• Control register (ADDR_CTRL) = address 08h

• Alarm 0 Control register (ADDR_ALM0CTL) = address 0fh

The general control register is set to ALM0 = 0x10.This means that only the Alarm 0 module is activated.

The related mask is defined in the “spi_rtcc_driv-ers.h” header as #define ALM0 0x10.

The Alarm 0 Control register (or day register) isprogrammed with the ALMxC_ALL mask, offering analarm interrupt when all variables match: month, date,hour, minutes, seconds. This mask is defined in the“spi_rtcc_drivers.h” header as#define ALMxC_ALL 0x70.

“0x01” must be added to this constant, in order toprogram the three Least Significant bits for the day ofthe week.

spi_rtcc_wr(ADDR_MNTH, 0x01) ; // month = 01

spi_rtcc_wr(ADDR_ALM0MTH, 0x01) ; // alarm month = 01

spi_rtcc_wr(ADDR_DATE, 0x01) ; // date of month = 01

spi_rtcc_wr(ADDR_ALM0DAT, 0x01) ; // alarm date = 01

spi_rtcc_wr(ADDR_HOUR, 0x00) ; // hour = 00

spi_rtcc_wr(ADDR_ALM0HR, 0x00) ; // alarm hour = 00


AN2140

ACCESSING THE RTCC REGISTERS

There are two basic functions for accessing the RTCCregisters: one for writes and one for reads. Each ofthem is fully described in the above paragraphs. Bothuse register’s addresses, inside the SRAM zone of theSPI RTCC.

As described in the MCP795WXX data sheet, theaddresses of the RTCC registers are shown in Table 1.

TABLE 1: RTCC REGISTER ADDRESS

According to these addresses, in the basic read/writefunctions, only the register’s address will differ. Readsare used in the RTCC count up until reaching the end.Writes are used in the initialization function.

An SPI access to the RTCC needs two SPI commandsas:

CONCLUSION

This application note presents how to control an alarmsystem, using the related alarm and control registers ofMicrochip’s SPI RTCC, MCP795WXX. The project isperformed on a PIC18 Explorer demo board, using theon-board resources: LCD (accessed through the SPIbus), push buttons and the on-board R3 potentiometerin conjunction with the related 10-bit ADC. The code(drivers and main function) is written in C, usingMPLAB® X IDE and the XC8 compiler. The preferredmicrocontroller is the PIC18F87J11.

Address BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE

Time and Configuration Registers

00h Tenth Seconds Hundredths of Seconds Hundredths of Seconds

00 - 99

01h ST (CT) 10 Seconds Seconds Seconds 00 - 59

02h 10 Minutes Minutes Minutes 00 - 59

03h CALSGN 12/24 10 HourAM/PM

10 Hour Hour Hours 1-12 + AM/PM00 - 23

04h OSCON VBAT VBATEN Day Day 1 - 7

05h 10 Date Date Date 01 - 31

06h LP 10 Month Month Month 01 - 12

07h 10 Year Year Year 00 - 99

08h OUT SQWE ALM1 ALM0 EXTOSC RS2 RS1 RS0 Control Reg.

09h CALIBRATION Calibration

0Ah WDTEN WDTIF WDDEL WDTPLS WD3 WD2 WD1 WD0 Watchdog

0Bh EVHIF EVLIF EVEN1 EVEN0 EVWDT EVDLB EVHS1 EVHS0 Event Detect

Alarm 0 Registers

0Ch 10 Seconds Seconds Seconds 00 - 59

0Dh 10 Minutes Minutes Minutes 00 - 59

0Eh 12/24 10 HourAM/PM

10 Hours Hour Hours 1-12 + AM/PM00 - 23

0Fh ALM0PIN ALM0C2 ALM0C0 ALM0C1 ALM0IF Day Day 1 - 7

10h 10 Date Date Date 01 - 31

11h 10 Month Month Month 01 - 12

READ 0001 0011 Read RTCC/SRAM arraybeginning at selected address

WRITE 0001 0010 Write RTCC/SRAM data tomemory array beginning atselected address


AN2140

APPENDIX A: REVISION HISTORY

Revision A (May 2016)

• Initial release of this document.


AN2140

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2016 Microchip Technology Inc.

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ISBN: 978-1-5224-0573-3

DS00002140A-page 11


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Configuring and Using the Alarm Module of the MCP795WXX Real-Time Clock/Calendars

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Transcript of Configuring and Using the Alarm Module of the MCP795WXX Real-Time Clock/Calendars