AN879

AN879
Using the Microchip Ultra Low-Power Wake-Up Module
Authors:
current consumption. These types of applications
require a low-power periodic wake-up and can be
accomplished by activating a low-power timer prior to
placing the device in a Sleep mode. Upon rollover, the
timer interrupt can then wake-up the part after some
predefined period. A 32 kHz crystal timer used on one
of the secondary clock sources is very popular if
accuracy is required. Some parts also have dedicated
internal low-power, low-frequency oscillators that can
be used.
Ruan Lourens
Jose Benevides
Jonathan Dillon
Microchip Technology Inc.
INTRODUCTION
This application note describes ways to reduce system
current consumption with the use of the Ultra
Low-power Wake-up (ULPWU) module. The
PIC16F684 and PIC16F88X are examples of devices
with this feature.
One solution for a lower current periodic wake-up timer
is a simple RC timer that can be charged prior to Sleep
and left to slowly discharge. A change in state event
can be used to wake the part when the RC voltage
reaches the digital input threshold voltage. This sounds
ideal, but the problem is that a normal digital-input
structure consumes high-crowbar currents when a
slowly changing voltage is applied to it. The
digital-input structure will consume a few hundred
micro amps when driven by an analog voltage that is
not close to the rail voltages (VSS and VDD). To combat
these high-crowbar currents, Microchip has introduced
an ULPWU module, which provides an analog input
that can be used to implement a RC timer. The basic
module block diagram is shown in Figure 1.
The primary use of this module is as an ULPWU timer,
but its functionality can be expanded to function as a
temperature sensor and/or a low-voltage detector. The
main and expanded functions of this module are
explained in this document.
Many low-power applications require that the
microprocessor wake-up from a Sleep state on a
periodic basis to check the status of some signal. It can
then react based on a measurement of that signal and
go back to Sleep until the next timed wake-up. This is
a widely used method for reducing overall system
FIGURE 1:
ULTRA LOW-POWER WAKE-UP PIN DIAGRAM(1)
EXTERNAL
ANALOG
INTERNAL
TRIS
WRITE
C
Ultra Low-power Wake-up Module
VIL
EVENT
ISINK
Note 1:
ULPWUE
RA0 cannot be read as a digital pin when ULPWU is enabled.
© 2008 Microchip Technology Inc.
AN00879D-page 1
AN879
The module operates as a low-power analog
comparator that compares the voltage on the external
capacitor C to a reference VIL. The module generates
an event output when the analog comparator changes
state. The change in state event can generate an
interrupt-on-change. The module provides a very weak
current source to discharge the external capacitor in a
controlled manner. The code in Example 1 for
PIC16F684 initializes the module, charges the
capacitor, enables the module, and then goes to Sleep,
waiting for an interrupt-on-change.
The trip voltage VIL and the sink current ISINK are
basically independent of VDD, but are sensitive to
temperature and process variations. Data for the
module is given in Table 1.
EXAMPLE 1:
TABLE 1:
BANKSEL
BSF
MOVLW
MOVWF
BANKSEL
BCF
BCF
CALL
BSF
BSF
BSF
MOVLW
MOVWF
SLEEP
NOP
ULPWU CODE FOR THE
PIC16F684
PORTA
PORTA, 0
H’7’
CMCON0
ANSEL
ANSEL, 0
TRISA, 0
CapDelay
PCON, ULPWUE
IOCA, 0
TRISA, 0
B’10001000’
INTCON
;Bank 0
;Set RA0 data latch
;Turn off
;comparators
;Bank 1
;RA0 to digital I/O
;Output high to
;charge capacitor
;Enable ULP Wake-Up
;Select RA0 IOC
;RA0 to input
;Enable interrupt
;and clear flag
;Wait for IOC
;
From the data in Table 1, it becomes clear that the
variation in module parameters would limit the overall
accuracy of the timer, when used as in Figure 1. The
wake-up period can vary by as much as 30% between
modules. For a large number of applications, it is
acceptable to have a large variation in the wake-up
period and thus, the module’s accuracy is acceptable.
-40°C
25°C
85°C
125°C
The code in Example 2 for PIC16F88X devices
charges the external capacitor, sets up the module and
goes to Sleep, waiting for the ULPWU interrupt. The
interrupt is level triggered and, if global interrupts are
enabled, the Interrupt Service Routine (ISR) must disable either the ULPWU interrupt enable or the ULPWU
module to clear the ULPWU interrupt flag and charge
the external cap.
EXAMPLE 2:
BANKSEL
BSF
BANKSEL
BCF
BANKSEL
BCF
CALL
BANKSEL
BCF
BANKSEL
BSF
BSF
BSF
MOVLW
MOVWF
SLEEP
NOP
ULPWU CODE FOR THE
PIC16F88X
PORTA
PORTA, 0
ANSEL
ANSEL, 0
TRISA
TRISA, 0
CapDelay
PIR2
PIR2, ULPWUIF
PCON
PCON, ULPWUE
TRISA, 0
PIE2, ULPWUIE
B’11000000’
INTCON
AN00879D-page 2
;
;Set RA0 data latch
;
;RA0 to digital I/O
;
;Output high to
;charge capacitor
;
;Clear flag
;
;Enable ULP Wake-up
;RAO to input
;Enable interrupt
;Enable peripheral
;interrupt
;
;Wait for interrupt
;
*
MODULE DATA*
VIL (VDC)
ISINK (nA)
Min
0.58
104
Typ
0.69
113
Max
0.81
131
Min
0.48
121
Typ
0.58
135
Max
0.69
158
Min
0.38
130
Typ
0.48
145
Max
0.58
169
Min
0.30
142
Typ
0.40
157
Max
0.49
183
Example data not characterized or tested
The module, when enabled, will add between 75 nA
and 160 nA to the microprocessor's Sleep current,
depending on process variations, temperature and
voltage. The total expected Sleep current with the
ULPWU module enabled should be only a few hundred
nA for the PIC16F684 and PIC16F88X devices, since
the Sleep current is typically 1 nA with all peripherals
disabled.
The average system current consumption will be higher
due to the energy required to charge the capacitor and
the energy consumed to execute code between Sleep
periods. The time between Sleep periods and active
duty cycle of use will largely dictate the overall current
consumption. A typical smoke detector or Tire
Pressure Monitoring (TPM) system with sub 1 μA
current consumption can be achieved.
© 2008 Microchip Technology Inc.
AN879
MODULE APPLICATIONS
The ULPWU module’s accuracy and functionality can
be improved by using it as a programmable timer or
using some additional external components. This
includes a programmable low-voltage detect and/or a
temperature sensor. The following sections will briefly
explain these functions.
Basic Timer
Although the operation of the basic wake-up timer has
been discussed, there are more aspects to consider.
Figure 2 shows the addition of a series resistor when
compared to Figure 1. The resistor R1 is added if C1 is
larger than 50 pF. This is done to reduce the peak
current drawn from RA0 while charging C1. For larger
capacitors, Equation 1 gives the peak charge current
drawn from RA0. The maximum allowable current
drawn from pin 1 is 25 mA. A resistor of 200 ohm is
sufficient for 5-volt supply voltages and large
capacitors.
FIGURE 2:
SERIAL RESISTOR
RA0
R1
C1
The discharge period is about 30 ms for a 1 nF
capacitor, a VO of 5 VDC with a current sink of 140 nA,
and VIL of 0.6 VDC. The internal current sink is fairly
constant with voltage, assuming the voltage on the
capacitor is VIL or more. This results in a near linear
voltage discharge of the capacitor over time. Keep in
mind that the weak current sink is equivalent to very
high-impedance of several tens of mega ohms. Such a
high-impedance discharge system is very sensitive.
Care must be given to layout, the influence of moisture,
and the capacitor’s self-discharge impedance.
To minimize noise and moisture effects, it is advisable
to keep trace lengths short by placing the discharge
capacitor close to the AN0 pin. Also, note that
capacitors have some internal leakage that will shorten
the discharge period. Different capacitors have
different self-discharge characteristics that will become
important, especially if long discharge periods are
required. Some electrolytic capacitors have fairly high
self-discharge rates that are temperature sensitive.
Use of External Components
For harsh noise and moisture conditions, the stability of
the ULPWU module can be improved by adding an
additional discharge resistor R2, as in Figure 3. The
voltage discharge on C1, due to R2, will follow
Equation 3, if the current through R2 is large compared
to the discharge current ISINK. Thus, the discharge
period can be derived as in Equation 4.
FIGURE 3:
R1
RA0
EQUATION 1:
IPEAK =
DISCHARGE RESISTOR
VDD
for C1 >> 50 pF
R1
R2
C1
IPEAK = peak charge current
Equation 2 gives the discharge period. VO is the initial
capacitor voltage and will be the same as VDD, if the
capacitor is allowed to fully charge prior to starting the
discharge process.
EQUATION 2:
(VO–VIL) . C
TDISCHARGE =
ISINK + ILEAKAGE
TDISCHARGE = discharge period
VO = initial capacitor voltage
VIL = trip voltage
ISINK = sink current
EQUATION 3:
(
–T
V(T) = VO . e C1R2
)
V(T) = voltage across capacitor
VO = initial capacitor voltage
T = time
ILEAKAGE = capacitors internal leakage current
© 2008 Microchip Technology Inc.
AN00879D-page 3
AN879
Temperature Sensor
EQUATION 4:
( )
VO
TDISCHARGE = C1R2 ln
VIL
TDISCHARGE = charge period
VO = initial capacitor voltage
VIL = trip voltage
Calibrated Timer
The following section explains how the accuracy of the
basic timer can be improved by controlling the charge
period. The discharge period for both implementations
shown in Figures 2 and 3 are dependent on VO, C1, VIL
and ISINK or R2. These parameters depend on process
variations, temperature effects, usage and more. A
software calibrated Sleep timer will compensate for
some of these variations by controlling VO. Timing the
charge period of C1 through R1 allows control over the
voltage on C1 at the start of the Sleep period VO. The
discharge period TDISCHARGE is timed against the main
clock source while the part is awake, then the charge
period can be adjusted based on the TDISCHARGE error.
This process is repeated until the desired accuracy is
obtained. Repeat the calibration process after a fixed
amount of normal Sleep periods, to maintain accuracy
over time.
Pay close attention to the residual charge across C1 at
the start of the charge period. There may be charge left
in C1, depending on VIL and whether or not the ULPWU
module was disabled, and whether RA0 turned into an
analog input, digital input or digital output. One
approach is to fully discharge C1 before starting the
charge process. This approach increases accuracy, but
will increase the overall current consumption.
The final capacitor voltage VO, when charging C1
through R1, is given by Equation 5. The residual
voltage across C1 at the beginning of the charge period
is represented by VRES and the charge period is
TCHARGE.
This section explains how to implement a temperature
sensor that gives a reading relative to the standard
temperature at which calibration was completed. The
module parameters VIL and ISINK are dependent on
temperature and process variations. The process
dependent component must be identified in order to
calculate the temperature from later measurements of
VIL and ISINK. The process variation can be measured
when the device is first turned on under controlled
conditions such as at final product testing. These
standard measured values can be stored in EEPROM
and used for future reference.
To measure VIL, sample the voltage across C1 with the
A/D converter after the output of the ULPWU module
changes the status of bit ‘0’ on PORTA. The sampled
voltage will be referenced to the A/D converter
reference, which can be VDD or an external voltage
reference. VIL has a negative temperature coefficient
and is approximately -1.25 mV/°C. VIL is calculated by
using the method described in Section “Use of External Components”.
The sink current ISINK is measured under standard
conditions by using Equation 2 and has a positive
temperature coefficient of approx. 140 pA/°C. The
discharge time TDISCHARGE is a function of VO, VIL and
temperature. Under standard conditions, VO and
temperature are controlled and VIL is measured. From
this, calculate the standard process dependent value
for ISINK.
Note:
The accuracy of the measurements is
dependent on VO, which can be VDD, and
the source for the A/D converter, which
may or may not be VDD. The method
described in Section “Use of External
Components” to calculate VIL without an
A/D is also dependent on a known value
for VO or VDD.
EQUATION 5:
–TCHARGE
C1R1
VO = VRES + (VDD – VRES) 1 – e
TCHARGE = charge period
VO = final capacitor voltage
VRES = residual voltage
AN00879D-page 4
© 2008 Microchip Technology Inc.
AN879
Equations 7 and 8 are used to calculate temperature
variation from the standard temperature using the
measured or calculated values for ISINK and VIL.
Note 1: The result is dependent on VDD or VO.
The temperature dependency of VIL is
linear with temperature, but ISINK has a
significant second order term that is not
shown. The second order term for ISINK
can be ignored if the temperature
deviation is relatively small.
2: The data is preliminary and will be
updated after full characterization is
completed. The values ISTANDARD and
VSTANDARD are the process dependent
values for ISINK and VIL, as measured
under standard conditions and stored in
EEPROM.
EQUATION 6:
VIL – VSTANDARD
ΔT ≈
–1.25 x
10–3
ΔT = temperature deviation
VSTANDARD = standard voltage
VIL = trip voltage
EQUATION 7:
ΔT ≈
ISINK – ISTANDARD
140 x 10–12
Programmable Low-voltage Detect
VDD can be calculated using the ULPWU module in two
basic ways; both methods are temperature dependent
and based on the standard values for VIL and ISINK, as
discussed in Section “Temperature Sensor”. The
method is fairly accurate for applications where the
system is subjected to small temperature variations.
Refer to Section “Temperature Sensing and Programmable Low-voltage Detect” for applications
where both VDD and temperature need to be measured
across a large range.
INTERNAL CURRENT SINK DISCHARGE
METHOD
This method uses the setup as in Figure 2 by measuring TDISCHARGE, while keeping the part active and
measuring it against the main clock source. Before
measuring TDISCHARGE, make sure that C1 is fully
charged to VDD by allowing a long enough charge
period. Then, use Equation 9 to calculate VO or VDD.
EQUATION 9:
.
Vo = (Tdischarge •
VO
VIL
TDISCHARGE
ISINK
Isink
C1
) + Vil
= Total Capacitor Voltage
= Trip Voltage
= Discharge Period
= Sink Current
The accuracy of the calculated VDD is dependent on
VIL, TDISCHARGE, C1 and ISINK. Interestingly, VIL has a
negative temperature coefficient while ISINK has a
positive temperature coefficient, which reduces the
temperature dependency.
ΔT
It is still possible to use this method if R2 is required, as
shown in Figure 3. VO or VDD is now calculated using
Equation 10, as most of the dicharge is through R2.
Using this method, R2 is more accurate and, for the
most part, independent of temperature and process
variations.
EQUATION 8:
Connecting R2 through an I/O controlled MOSFET provides a means for disconnecting R2 from ground, as
shown in Figure 4. The additional I/O enables the
MOSFET when R2 is needed.
= temperature deviation
ISINK = sink current
VSTANDARD = standard voltage
VDD
≈
.
VO = VIL + TDISCHARGE ISINK
C1
VO = total capacitor voltage
VIL = trip voltage
ISINK = sink current
TDISCHARGE = discharge period
© 2008 Microchip Technology Inc.
EQUATION 10:
VO = VIL • e
-TDISCHARGE
C1 R2
AN00879D-page 5
AN879
FIGURE 4:
RA0
R2 TO I/O
R1
C1
Calculate VDDn with Equation 8, using the
discharge period TDISCHARGE from the Step 1,
or use Equation 9.
8. Use VO-n to measure VIL-n, with the A/D
converter, or calculate VIL using Equations 2
or 3.
9. With VIL-n, use Equation 6 to calculate the
temperature ΔTn.
10. Store the values for VDDn and ΔTn.
11. Increment n and go to Step 5, until desired n is
reached.
7.
R2
RA1
VIL CHARGE METHOD
This method uses the same setup as illustrated in
Figure 3. This method is applicable if R1 is much
smaller than R2. Again, the capacitor fully charges to
VDD and the TDISCHARGE is measured while the part is
still active. Equation 9 can be used to calculate VO or
VDD, but note that the result is a multiple of VIL, which
is temperature sensitive.
TEMPERATURE SENSING AND
PROGRAMMABLE LOW-VOLTAGE DETECT
Section “Temperature Sensor” of this application
note explains a simple method to measure temperature. Clearly, the accuracy of the result is dependent on
knowing the VDD and the process dependent variation
of the variable. Similarly, Section “Programmable
Low-voltage Detect” explains how to calculate VDD,
but the result depends on temperature and the process
variation.
The accuracy of the process can be evaluated by using
the alternative methods for specific iterations. In
addition, use the EEPROM write time as a temperature
sensor for improving the accuracy of Step 1. The
EEPROM write time is dependent on temperature and
the variation from a standard-measured time can be
used to calculate temperature.
CONCLUSION
The ULPWU module is a flexible module with
unmatched current consumption that enables the
designer to implement not only a wake-up timer, but
also a low-cost PLVD (Programmable Low-voltage
Detect) and temperature sensing functions. The module is especially attractive in lithium and other battery
applications where very low Sleep currents are
required.
The accuracy of measuring the interdependent values
VDD and temperature is greatly improved by knowing
the standard values ISTANDARD and VSTANDARD, as
explained in Section “Temperature Sensor”. The
deviation of the measured unit from the standard value
can then be used in an iterative process to calculate
VDD and temperature. The following sequence can be
followed (see Figure 5):
1.
2.
3.
4.
5.
6.
Calculate VO or VDD using Equation 8,
assuming standard temperature, VIL =
VSTANDARD and ISINK = ISTANDARD. The
discharge period TDISCHARGE is measured
against the main clock source, while the device
is still active. Alternatively, using Equation 9 is
less accurate.
Use the resulting VDD to measure VIL with the
A/D converter, as explained in Section “Temperature Sensor”. Alternatively, VIL can be
calculated using Equations 2 or 3.
Use the resulting VIL to calculate the temperature with Equation 6.
Save the Step 1 iteration values for VDD and
temperature in VDDn and ΔTn, where n is the
iteration step number.
Calculate ISINKn using Equation 7 with ΔTn.
Calculate VIL-n using Equation 6 with ΔTn.
AN00879D-page 6
© 2008 Microchip Technology Inc.
AN879
FIGURE 5:
CALCULATING VDD AND TEMPERATURE BLOCK DIAGRAM
Start
Step 1
Find VO (VDD)(1,2)
Step 2
Find VIL w/A/D converter
(3)
Step 3
Find Temperature(4)
ΔT
measurement
VDD
measurement
VDD
measurement
VIL
measurement
Step 4
Iteration Values/Step Numbers
(VDDn and ΔTn)(5)
Step 5
Find ISINKn(6)
Step 6
Find VIL-n(7)
Step 7
Find VDDn(8)
Step 8
Find
VIL-n(9)
TDISCHARGE
measurement
Step 9
Find ΔTn(10)
Step 10
Store Values VDDn and ΔTn
No
Step 11
Increment n/repeat cycle(11)
Note
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
Use Equation 8, assuming standard temperature; VIL = VSTANDARD; ISINK = ISTANDARD.
Measure TDISHARGE against the main clock source while device is still active. Using
Equation 9 as an alternative is less accurate.
See Section “Temperature Sensor”, using VDD from Step 1 to measure VIL with A/D
converter. Alternatively, Equations 2 and 3 can be used.
Use Equation 6 w/VIL from Step 2.
Use iteration values for VDD and temperature from Step 1 as VDDn and ΔTn, where n is the
iteration step number.
Use Equation 7 with ΔTn.
Calculate VIL-n using Equation 6 with ΔTn.
Use Equation 8, using TDISHARGE from Step 1 or use Equation 9.
Use VO-n to measure VIL-n, with the A/D converter or calculate VIL using Equations 2 or 3.
With VIL-n, use Equation 6 to calculate the ΔTn.
Increment n and go to Step 5, until desired n is reached.
© 2008 Microchip Technology Inc.
Desired n reached?
Yes
Done
AN00879D-page 7
AN879
NOTES:
AN00879D-page 8
© 2008 Microchip Technology Inc.
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•
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DS00879D-page 9
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China - Hong Kong SAR
Tel: 852-2401-1200
Fax: 852-2401-3431
Korea - Seoul
Tel: 82-2-554-7200
Fax: 82-2-558-5932 or
82-2-558-5934
China - Nanjing
Tel: 86-25-8473-2460
Fax: 86-25-8473-2470
Malaysia - Kuala Lumpur
Tel: 60-3-6201-9857
Fax: 60-3-6201-9859
China - Qingdao
Tel: 86-532-8502-7355
Fax: 86-532-8502-7205
Malaysia - Penang
Tel: 60-4-227-8870
Fax: 60-4-227-4068
China - Shanghai
Tel: 86-21-5407-5533
Fax: 86-21-5407-5066
Philippines - Manila
Tel: 63-2-634-9065
Fax: 63-2-634-9069
China - Shenyang
Tel: 86-24-2334-2829
Fax: 86-24-2334-2393
Singapore
Tel: 65-6334-8870
Fax: 65-6334-8850
China - Shenzhen
Tel: 86-755-8203-2660
Fax: 86-755-8203-1760
Taiwan - Hsin Chu
Tel: 886-3-572-9526
Fax: 886-3-572-6459
China - Wuhan
Tel: 86-27-5980-5300
Fax: 86-27-5980-5118
Taiwan - Kaohsiung
Tel: 886-7-536-4818
Fax: 886-7-536-4803
China - Xiamen
Tel: 86-592-2388138
Fax: 86-592-2388130
Taiwan - Taipei
Tel: 886-2-2500-6610
Fax: 886-2-2508-0102
China - Xian
Tel: 86-29-8833-7252
Fax: 86-29-8833-7256
Thailand - Bangkok
Tel: 66-2-694-1351
Fax: 66-2-694-1350
Italy - Milan
Tel: 39-0331-742611
Fax: 39-0331-466781
Netherlands - Drunen
Tel: 31-416-690399
Fax: 31-416-690340
Spain - Madrid
Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
UK - Wokingham
Tel: 44-118-921-5869
Fax: 44-118-921-5820
China - Zhuhai
Tel: 86-756-3210040
Fax: 86-756-3210049
01/02/08
DS00879D-page 10
© 2008 Microchip Technology Inc.
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