Holtek HT67F86A Instruction Manual

HT67F86A Internal RTC Application Guidelines
AN0448E V1.00 1/7 January 16, 2017
HT67F86A Internal RTC Application Guidelines
D/N: AN0448E
Introduction
One special feature of the Holtek 8-bit Flash MCU, the HT67F86A, is that it includes an
ultra-low power consumption RTC oscillator and an LCD driver. To meet the demands for
low power consumption applications, the device’s internal RTC has a standby power
consumption of lower than 1µA at an operating voltage of 3V. The integrated LCD driver
can drive dot matrix LCD display applications of up to 1024 pixels, which can be used with
various display configurations and font settings. With the increasing trends of mobile
device applications such as cellphones and tablets, in addition to being utilised in
industrial control and consumer products, the ultra-low standby power consumption
performance is also an especially important consideration in battery-powered applications.
The MCU’s internal RTC oscillator forms an excellent solution for applications which
require a real time clock function as it reduces the need for an external timing IC with the
resulting reductions in cost and design simplification. This application note will introduce
the Holtek MCU internal RTC oscillator characteristics and application guidelines.
Functional Description
Oscillators
The HT67F86A provides 3 frequency oscillation sources, namely the HXT, HITC and LXT
oscillators. Users can select their desired system frequency, f
SYS,
using their application
program. The LXT provides an f
L
frequency of 32768Hz for various MCU clocks, but more
commonly for applications requiring an RTC real-time clock. Additionally, the MCU can
enter the IDLE2 or SLEEP power saving modes also using the application program to
implement different standby power consumption power down modes. The related
registers are listed in the following table, refer to the Oscillators section in the datasheet
for more detailed information.
Register
Name
Bit
7 6 5 4 3 2 1 0
SCC CKS2 CKS1 CKS0 — FHS — FHIDEN FSIDEN
HIRCC — — — — HIRC1 HIRC0 HIRCF HIRCEN
HXTC — — — — — HXTM HXTF HXTEN
LXTC — — — — — — LXTF LXTEN
System Operating Mode Control Registers List

HT67F86A Internal RTC Application Guidelines
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The clock frequency block diagram is shown as below.
HXT
Prescaler
f
H
LXT
High Speed
Oscillator
Low Speed
Oscillator
f
H
/2
f
H
/16
f
H
/64
f
H
/8
f
H
/4
f
H
/32
CKS2~CKS 0
f
SYS
f
SUB
f
SUB
HXTEN
f
LXT
HIRC
HIRCEN
f
H
FHS
IDLE2
SLEEP
LXT Oscillator
The LXT oscillation circuit consists of an external 32768Hz crystal and resistor as well as
capacitor components. The crystal is connected between pins XT1 and XT2. The
frequency accuracy adjustment is implemented by the R
P
resistor together with the C1
and C2 capacitors. Users should refer to the oscillator specification for the resistance and
capacitance configurations. After power on the LXTEN bit will be in a high state which
enable the LXT oscillator. By examining if the LXTF bit has been set high, it can be
determined if the LXT oscillator is powered up and ready for use. The LXT oscillator
circuit is shown as below.
Note: 1. R
P
, C1 and C 2 are required .
2. Although not shown pins have a parasitic capaci tance of around 7pF .
To internal
circuits
Internal
Oscillator
Circuit
C1
C2
XT1
XT2
R
P
32. 768kHz
Internal RC
Oscillator
External LXT Oscillator
Time Base Control Register
The HT67F86A provides a Real Time Clock function. A time counting function can be
implemented by configuring the Time Base 0 or Time Base 1 interrupt control bits, TB0E
or TB1E, together with the data memory. The time base function clock source, f
PSC,
comes
from the f
LXT
. The Time Base frequency division ratio is selected by configuring the
registers to choose a value within the range of f
PSC
/2
8
~ f
PSC
/2
15
. Refer to the datasheet for
the related registers and the TB0E and TB1E interrupt addresses.

HT67F86A Internal RTC Application Guidelines
AN0448E V1.00 3/7 January 16, 2017
Low Speed
Oscillator
f
SUB
f
SUB
LCD
WDT
f
LXT
Prescaler
TB0[2:0]
f
LXT
/4
Time Base 1
TB1[2:0]
LXT f
LXT
IDLE2
SLEEP
1/4 Time Base 0
f
SUB
LVR
1/8 f
LCD
Device Clock Configuration
f
LXT
/4 Prescaler 0
f
PSC
f
PSC
/2
8
~ f
PSC
/2
15
M
U
X
M
U
X
TB0[2: 0]
TB1[2: 0]
Time Base 0 Interrupt
Time Base 1 Interrupt
TB0ON
TB1ON
Prescaler 1 f
PSC
/2
8
~ f
PSC
/2
15
Time Base Interrupt
TB0C Register
Bit 7 6 5 4 3 2 1 0
Name TB0ON — — — — TB02 TB01 TB00
R/W R/W — — — — R/W R/W R/W
POR 0 — — — — 0 0 0
Bit 7 TB0ON: Time Base 0 Enable Control
0: Disable
1: Enable
Bit 6~3 Unimplemented, read as “0”
Bit 2~0 TB02~TB00: Time Base 0 time-out period selection
000: 2
8
/f
PSC
001: 2
9
/f
PSC
010: 2
10
/f
PSC
011: 2
11
/f
PSC
100: 2
12
/f
PSC
101: 2
13
/f
PSC
110: 2
14
/f
PSC
111: 2
15
/f
PSC
The f
PSC
is derived from the internal clock source f
LXT
/4.

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AN0448E V1.00 4/7 January 16, 2017
TB1C Register
Bit 7 6 5 4 3 2 1 0
Name TB1ON — — — — TB12 TB11 TB10
R/W R/W — — — — R/W R/W R/W
POR 0 — — — — 0 0 0
Bit 7 TB1ON: Time Base 1 Enable Control
0: Disable
1: Enable
Bit 6~3 Unimplemented, read as “0”
Bit 2~0 TB12~TB10: Time Base 1 time-out period selection
000: 2
8
/f
PSC
001: 2
9
/f
PSC
010: 2
10
/f
PSC
011: 2
11
/f
PSC
100: 2
12
/f
PSC
101: 2
13
/f
PSC
110: 2
14
/f
PSC
111: 2
15
/f
PSC
The f
PSC
is derived from the internal clock source f
LXT
/4.
Internal RTC Characteristics
Operating Voltage
The internal RTC operating voltage depends on the LXT oscillator which has a voltage
range of 2.2V~5.5V. Such applications are suitable for battery powered products which
require 2~3 batteries. Refer to the “Low Speed Crystal Oscillator Characteristics – LXT”
section in the datasheet or the following table for the related voltage characteristics.
Ta=25°C
Symbol Parameter Test Condition Min. Typ. Max. Unit
V
DD
Condition
f
LXT
Oscillator Frequency
2.2V~
5.5V f
SYS
=f
LXT
=32.768kHz — 32.76
8 — kHz
Duty
Cycle Duty Cycle — — 45 50 55 %
t
START
LXT Start-up Time — — — — 500 ms
R
NEG
Negative Resistance
Note
2.2V — 3×ESR — — Ω
Note: C1, C2 and R
P
are external components. C1=C2=15pF, R
P
=10MΩ, C
L
=7pF,
ESR=30kΩ.

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AN0448E V1.00 5/7 January 16, 2017
Negative Impedance
The LXT oscillator provides a negative impedance reference and for safety provides a value
which is at least 3 times that of the ESR to avoid oscillation problems in mass production.
Special attention should be made with regard to this when using the 32768Hz crystal.
Oscillation Frequency
The LXT oscillator oscillation frequency is mainly determined by the 32768Hz crystal
capacitance C
L.
For example, if the external capacitors C1 and C2 series equivalent
capacitance values together with the PCB parasitic capacitance is equal to the crystal
capacitance C
L
, then an accurate frequency of 32768Hz can be achieved. The user can
adjust the oscillation frequency to determine the appropriate C1 and C2 capacitance. It
should be noted that the oscillation frequency will be lower if the chosen C1 and C2
capacitors are greater than C
L
and vice versa. The crystal related characteristics and the
oscillation frequency curve used by the MCU should be taken into account for temperature
deviation. Note that checking the oscillation frequency by directly using an oscilloscope
probe is inadvisable. The best way to check the frequency is using the I/O pins.
Standby Power Consumption
When the LXT is in an oscillating state together and the Time Base clock circuit turned on to
reduce power, it can provides a low MCU standby power consumption of lower than 1µA at
an operating voltage of 3V, thus achieve longer service life for battery-powered products.
This means that the HT67F86A forms an excellent choice for power sensitive applications.
As well as considerations regarding the frequency characteristics, the user should also pay
attention to the standby power consumption when selecting the crystal CL and external C1
and C2 capacitors. The common CL capacitance is 12.5pF. When measuring the standby
current not that scope probes should not be placed on any MCU pins.
Symbol Standby Mode Test Condition Min. Typ. Max. Unit
V
DD
Condition
I
STB
SLEEP Mode
3V WDT off, Time Base on — 0.6 1.0
µA
5V — 0.8 1.5
3V WDT on — 0.7 1.5
µA
5V — 0.9 3.0
IDLE0 Mode 3V f
SUB
on — 1.0 3.0 µA
5V — 1.5 5.0
PCB Planning Considerations
During circuit design, designers should select an appropriate feedback resistor R
P
, with a
resistance range of between 5MΩand 10MΩ, and capacitors C1 and C2 for a matched
oscillation frequency after the frequency test. Any high frequency circuits located close to
the LXT circuit, will influence the oscillator characteristics, therefore special care must be
taken in such cases. The LXT oscillator circuit should be located as close to the XT1/XT2
pins as possible while the interconnecting lines between the C1/C2 capacitors and VSS
should be as short as possible. The reference circuit is shown below.

HT67F86A Internal RTC Application Guidelines
AN0448E V1.00 6/7 January 16, 2017
Application Range
Token Card Reader
Online banking, transactions using mobile devices, such as cellphones and tablets, can
be carried out using a Bluetooth connected smart card reader. As these devices are
normally battery powered, the HT67F86A is an excellent choice for these applications
due to its low power consumption and 1024 pixel LCD driver.
Consumer Products
The HT67F86A internal Real Time Clock function can be applied for use in a wide range
of battery powered consumer products such as electronic alarm clocks etc.
Conclusion
This application note summarises the internal RTC usage considerations providing
reference information for designers.
Reference Files
Reference file: HT67F86A Datasheet.
For more information, refer to the Holtek official website http://www.holtek.com/en.

HT67F86A Internal RTC Application Guidelines
AN0448E V1.00 7/7 January 16, 2017
Version and Modification Information
Date Author Issue
2016.12.20 黃啟德 First Version
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