Technical Bulletin - Sharp Electronics Corp.

Technical Bulletin 1

LH75400/01/10/11

Technical Bulletin System-on-Chip

PURPOSE

This document is a supplement to Product Change

Information containing Errata for some early SHARP

LH75400/01/10/11 series of System-on-Chip parts.

When the Errata have been corrected, the information

in this Technical Bulletin will no longer apply. These

parts may have a suseptibility to ground noise in the the

Real-Time Clock (32.768 KHz) oscillator. They there-

fore may require a software work around for the Real-

Time Clock (RTC), if the RTC was to be used as a time-

of-day clock.

This Technical Bulletin will use one of the chip’s tim-

ers to allow an application to keep track of the elapsed

time in seconds or to generate an interrupt when the

elapsed time matches a preset value in spite of these

difficulties. These methods will work with any revision of

the silicon, so the software will not need to be changed.

INTRODUCTION

If your application does not require the features that

the RTC provides, then this Technical Bulletin does not

apply to you, and you may saf ely ground the RTC crys-

tal input pin. If your application is always running in

Active or Standby mode, see the Active or Standby

mode section. If your application enters any other Low

Power mode, see th e Low Power Operation section.

The 32.768 kHz oscillator has a susceptibility to

ground noise induced by the on-chip System Clock

(HCLK) and Phase-Locked Loop (PLL). The suscepti-

bility manifests as extra pulses on the oscillator’s out-

put. If the SoC is in Active, Standby, or Sleep mode,

then the RTC Data Regist er 0 and RTC Data Register 1

will increment at too high a rate. How fast the clock

appears to run dep ends on the quality of the clock crys-

tal, the passive components, and the attention paid to

the circuit board layout.

As a consequence, if the RTC is used as a time-of-

day clock, the clock will run too fast.

The RTC is not recommended for use in the Sleep

or Stop1 modes.

ACTIVE OR STANDBY MODE

Unfortunately, in Active Mode and Standby Mode,

the LH75400/01/10/11 PLL and HCLK are always

active and therefore always producing the noise that

will corrupt the RTC. You should not use the RTC if

your application only uses Active or Standby Mode.

This part of the work around assumes that your appli-

cation has these properties:

• The application requires a way to track the elapsed

time in seconds (e.g., the time-of-day), and/or it

needs to generate an interrupt when the elapsed

time matches a pre-determined elaspsed time.

• The application is running an operating system that

supports software timers, or the application is not

using all three timers that are clocked using HCLK

(Timer 0, Timer 1, an d Timer 2) , or the applica tion is

using Timer 0, Timer 1, or Timer 2 to generate a peri-

odic waveform or periodi c interrupt.

• The application never sto ps the timer that is generat-

ing the operating system clock, or in a non-operat ing

system application, the application never stops the

timer that is generating a periodic waveform or peri-

odic interrupt.

Software Algorithm

Declare a 32-bit glo bal variable tha t your application

will use to keep track of the elapsed time in seconds:

rtc_elapsed_time;// UNS_32: 32-bit unsigned type

Initialize the 32-bit global variable with the same

value you would have used to initialize the RTC load

If your application requires the same feature pro-

vided by the RTC match register, (i.e., Match Register

0 and Match Register 1), declare a second 32-bit global

variable to store the match value:

UNS_32 rtc_match;

If your application uses an operating system with a

timer facility, use that facility to increment the 32-bit glo-

bal variable every second. For example, set up a high

priority task that wakes every second, increments the

variable, and then suspends.

For lower overhead if your application's operating

system provides hooks to the timer interrupt, or if your

application doesn't have an operating system, program

your application's periodic timer interrupt to increment

your elapse d time global v ariable after th e appr opriat e

number of timer interrupts have occurred.

If you require the interval match facility, use the soft-

Creating a Real-Time Clock (RTC) with an On-chip Timer and Software

Paul Kovitz, Staff Engineer

LH75400/01/10/11 Software R TC

2Technical Bulletin

ware interrupt feature of the Vectored Interrupt Control-

ler (VIC) to assert the RTC interrupt.

This example genera tes a timer interrupt rate of 100

interrupts per second: (These C examples assume that the

interrupt handler functions are called by a wrapper function

that saves context on entr y and restores context on exit).

Note that the HCLK is still running in Standby mode,

so even if your application enters Standby mode, the

timers will still run and the timer interrupt will still move

the SoC from Standby to Active mode.

TIMER INTERRUPT EXAMPLE

/* Modifications to the periodic timer interrupt handler */

#define TIMER_INTS_PER_SECOND 100

#define VIC_SOFT_INT_PTR ((volatile UNS_32 *)0xFFFFF018)

#define VIC_SOFT_INT_CLR_PTR ((volatile UNS_32 *)0xFFFFF01C)

#define RTC_ALARM 15

void timer_int_handler(void)

{

static UNS_32 rtc_ints_remaining = TIMER_INTS_PER_SECOND;

/* other timer interrupt declarations go here */

if (0 == --rtc_ints_remaining)

{

rtc_elapsed_time++;

rtc_ints_remaining = TIMER_INTS_PER_SECOND;

if (rtc_elapsed_time == rtc_match)

{

*VIC_SOFT_INT_PTR = 1 << RTC_ALARM;

}

/* do what the timer interrupt normally does */

}

/* RTC Alarm Software Interrupt */

void rtc_soft_alarm(void)

{

/* clear the alarm interrupt */

*VIC_SOFT_CLR_PTR = 1 << RTC_ALARM;

/* do the processing for the alarm */

}

Software RTC LH75400/01/10/11

Technical Bulletin 3

LOW POWER OPERATION

The application that requires support from the RTC

more than an y other is the bat tery-powered a pplication

that needs to keep track of the time of day and date

even when every other chip function is off. The

LH75400/01/10/11 provides three very low power

modes: Standby, Stop1 and Stop2. Due to the RTC

oscillator errata, the only Low Power mode available to

use this function is Stop2. Fortunately for battery-pow-

ered applications, Stop2 is the lowest Power mode of

all these Soc’s op erating modes.

In Active or Standby mode, use the algorithims to

track time described previously in ‘Active or Standby

mode’. Use this algorithim with the RTC to count

elapsed seco nds in Low Power mode:

1. Copy the global var iable that counts elapsed time

into the RTC load register (i.e., copy the least sig-

nificant 16 bits into Load Register 0, and then cop y

the most significant 16 bits into Load Re gist er 1).

2. If your application needs to use the RTC Match

Interrupt to exit, copy the Global Elapsed Seconds

Match variable into the RTC match register (i.e.,

copy the least significant 16 bits into Match Reg-

ister 0, and then copy the most significant 16 bits

into Match Register 1).

3. P oll the RTC Data Register until it contains a value

that is 1 more than the value you loaded into the

RTC Match Register. This step ensures that the

value you wrote to the RTC Load Register has

been transferred from the HCLK domain into the

RT C clock domain.

4. Write 0b100 to the PWR DWN SEL bit field o f the

Reset, Clock and Power Controller (RC PC) Con-

trol Register to en ter Stop2 mode. Make sure th e

lock b it of the RCPC Control Register is set bef ore

trying to write to the RCPC or else writes to the

RCPC will be ignored.

RTC STOP2 MODE ENTRY EXAMPLE

#define RTC_DATA0_PTR ((volatile UNS_32 *)0xFFFE0000)

#define RTC_DATA1_PTR ((volatile UNS_32 *)0xFFFE0004)

#define RTC_LOAD0_PTR ((volatile UNS_32 *)0xFFFE0014)

#define RTC_LOAD1_PTR ((volatile UNS_32 *)0xFFFE0018)

#define RTC_MATCH0_PTR ((volatile UNS_32 *)0xFFFE0008)

#define RTC_MATCH1_PTR ((volatile UNS_32 *)0xFFFE000C)

#define RCPC_CTRL_PTR ((volatile UNS_32 *)0xFFFE2000)

#define RCPC_PWR_DWN_SEL_BIT 2

#define RCPC_STOP2_VAL 4

#define RCPC_LOCK_BIT 9

#define HALFWORD_MASK 0xFFFF

#define HALFWORD_SIZE 16

void enter_stop2_mode(void)

{

/* make sure the RCPC is unlocked */

*RCPC_CTRL_PTR |= (1 << RCPC_LOCK_BIT);

/* prepare the RTC registers */

*RTC_LOAD0_PTR = rtc_elapsed_time & HALFWORD_MASK;

*RTC_LOAD1_PTR = rtc_elapsed_time >> HALFWORD_SIZE;

*RTC_MATCH0_PTR = rtc_match & HALFWORD_MASK;

*RTC_MATCH1_PTR = rtc_match >> HALFWORD_SIZE;

/* make sure the load value has transferred to the RTC Data Register */

while (*RTC_DATA0_PTR < rtc_elapsed_timer + 1)

; /* wait */

/* enter Stop2 mode */

*RCPC_CTRL_PTR |= RCPC_STOP2VAL << RCPC_PWR_DWN_SEL_BIT;

}

LH75400/01/10/11 Software R TC

4Technical Bulletin

To exit Stop2 mode, use either an external interrupt

(one of the INT0 -INT6 p ins) or th e RTC_ALARM inter-

rupt. The interrupt handler that exits Stop2 mode

should do the following:

• Clear the interrupt that caused the exit from Stop2

mode.

• Read the RTC Data Register until the value read

from the RTC Data Re gister is greater than th e value

that was written prior to entering Stop2 mode.

• Write this value into the global variable that counts

Elapsed Time.

• Perform any other processing the interrupt service

routine need s to do.

• Exit the interrupt. At this point, keep track of the

elapsed time using the procedure described in

‘Active or Standby mode’.

RTC STOP2 MODE EXIT EXAMPLE

This example exits Stop2 mode on RTC_ALARM

interrupt: (This assumes the interrupt handler is called

by a wrapper that saves the registers on entry and

restores them on exit.)

#define RTC_EOI_PTR ((volatile UNS_32 *)0xFFFE0010)

void rtc_alarm_handler(void)

{

UNS_32 new_time;

/* clear the RTC_ALARM interrupt */

*RTC_EOI_PTR = 1;

/* get the time from the RTC */

{

new_time = *RTC_DATA0_PTR;

new_time |= *RTC_DATA1_PTR << HALFWORD_SIZE;

} while (new_time < rtc_elapsed_time + 1);

rtc_elapsed_time = new_time;

/* do processing required by the alarm */

}

Software RTC LH75400/01/10/11

Technical Bulletin 5

LIMITATIONS

For most applications that require a real-time clock,

the software methods described here will allow accept-

able performa nce.

Applications that are using all three on-chip timers

for non-per iodic t imin g functi ons (e. g., are onl y use d to

time brief intervals or are used with an aper iodic exter -

nal clock) cannot use this software method.

Your application cannot use the RTC ALARM func-

tion in Active mode or Standby mode.

Your application cannot use the RTC with Sleep or

Stop1 mode.

Your application will take as long as 1 second to

enter and exit Sto p2 mode due to RTC synchronization .

Your application could gain as much as 1 seco nd of

error in the elapsed time every time it enters or exits

Stop2 mode.

Software RTC LH75400/01/10/11

SPECIFICATIONS ARE SUBJECT TO CHANGE WITHOUT NOTICE.

Suggested applications (if any) are for standard use; See Important Restrictions for limitations on special applications. See Limited

Warranty for SHARP’s product warranty. The Limited Warranty is in lieu, and exclusive of, all other warranties, express or implied.

ALL EXPRESS AND IMPLIED WARRANTIES, INCLUDING THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR USE AND

FITNESS FOR A PARTICULAR PURPOSE, ARE SPECIFICALLY EXCLUDED. In no event will SHARP be liable, or in any way responsible,

for any incidental or consequential economic or property damage.

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