AN-1111: Options for Minimizing Power Consumption When Using the...

AN-1111
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Options for Minimizing Power Consumption When Using the ADuCM360/ADuCM361
By Mike Looney
INTRODUCTION
The ADuCM360 is a 32-bit ARM® Cortex™-M3-based
microcontroller that integrates dual 24-bit sigma delta (Σ-Δ)
analog-to-digital converters (ADCs), each with a fully programmable instrumentation amplifier on the front end. The
ADuCM361 contains all of the features of the ADuCM360
except that it has a single 24-bit sigma delta ADC (ADC1).
This application note describes many of the operational modes
and clock options for these devices and details the power savings
for each configuration.
These microcontrollers target a wide range of applications
including industrial control and instrumentation applications.
In many of the target applications, reducing power consumption
in the application is of the utmost importance. For example, for
battery-powered applications, the lifetime of the battery can be
extended by using the many operating modes and clock options
on the ADuCM360/ADuCM361. In addition, in 4 mA to 20 mA
loop-based applications where ADC performance is important,
the ADuCM360/ADuCM361 ensure that the overall power
consumption of the sensor module remains below 3.2 mA.
For details regarding the specifications and operation of the
ADuCM360/ADuCM361, refer to the latest datasheet and the
UG-367 user guide.
By maximizing the use of clock and power mode options,
the average IDD consumed by the ADuCM360/ADuCM361
can be reduced to just 1 mA. This figure is achieved despite
both the ADCs and the PGAs on the front end being enabled,
the ARM Cortex-M3 processor being set to active mode, and
the SPI buses and all timers being enabled.
All IDD measurements in this document are typical values
measured at ambient temperature (25°C) using a supply voltage
of AVDD = IOVDD = 3.0 V.
Rev. 0 | Page 1 of 12
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Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
I2C ....................................................................................................7
Revision History ............................................................................... 2
PWM ...............................................................................................8
Clock Control Registers ................................................................... 3
Reducing Analog IDD .....................................................................8
CLKCON0 ..................................................................................... 4
Power-Down Modes .........................................................................9
CLKSYSDIV ................................................................................... 4
Mode 1: MCUHALT Mode ..........................................................9
CLKDIS .......................................................................................... 4
Mode 2: PERHALT Mode ............................................................9
CLKCON1 ..................................................................................... 6
Mode 3: SYSHALT Mode .............................................................9
UART ............................................................................................. 7
Mode 4 and Mode 5: TOTALHALT and Hibernate Modes ....9
SPI0/SPI1 ....................................................................................... 7
REVISION HISTORY
/12—Revision 0: Initial Version
Rev. 0 | Page 2 of 12
Application Note
AN-1111
CLOCK CONTROL REGISTERS
The ADuCM360/ADuCM361 contain four main clock control registers: CLKCON0, CLKCON1, CLKDIS, and CLKSYSDIV. Figure 1 provides
an overview of the ADuCM360/ADuCM361 clock structure.
FCLK
CORTEX
HCLK
FLASH/SRAM
CLKSYSDIV[0]
CLKCON0[2:0]
CLKDIS[8]
DMA
HFOSC
16MHz OSC
PCLK
DIV2EN
CLKDIS[7]
CD
DAC
LFOSC
CLKCON0[4:3]
LFXTAL
UCLK
CLKDIS[5]
CLKDIS[5]
TIMER0
TIMER0CLK
CLKDIS[6]
CLKDIS[6]
EXTCLK
WATCHDOG
TIMER
WAKE-UP
TIMER
CLKDIS[0]
SPI0CD
CLKDIS[1]
PWMCD
CLKDIS[3]
PCLK
UCLK
CLKDIS[1]
CLKDIS[4]
CLKDIS[3]
UART
UARTCLK
TIMER1CLK
CLKDIS[2]
PWM
PWMCLK
CLKCON1[11:9]
UARTCD
SPI1
SPI1CLK
CLKCON1[14:12]
TIMER0CLK
SPI0
SPI0CLK
CLKCON1[5:3]
SPI1CD
CLKDIS[4]
CLKDIS[0]
CLKCON1[2:0]
PCLK
PCLK
UCLK
TIMER1
TIMER1CLK
CLKDIS[2]
CLKCON1[8:6]
I2CCD
I2C
I2CCLK
CLKDIS[9]
ACLK
Figure 1. Clock Structure for the ADuCM360 and ADuCM361
Rev. 0 | Page 3 of 12
ADC
AN-1111
Application Note
CLKCON0
3.0
UCLK serves as the main system clock for both the ADuCM360
and the ADuCM361. The CLKCON0[2:0] bits in Register
CLKCON0 (Address 0x40002000) select the clock divide value
for UCLK. The clock divide setting is important because a lower
system clock setting reduces power consumption.
2.5
IDD (mA)
2.0
Figure 2 shows the IDD of the ADuCM360/ADuCM361 for different UCLK rates. In Figure 2, the processor is enabled but the
ADCs are turned off.
1.5
6
0.5
5
0
0
2
4
6
FREQUENCY (MHz)
IDD (mA)
4
Figure 3. Total IDD Using a 3.0 V Supply with the Processor Running,
CLKSYSDIV = 0x1
3
Table 1. CLKSYSDIV Register Bit Descriptions
Bit
[7:1]
0
2
0
4
8
12
16
FREQUENCY (MHz)
Name
Reserved
DIV2EN
09746-002
1
0
Figure 2. Total IDD Using a 3.0 V Supply with the Processor Running at
Different Frequencies
The CLKCON0[4:3] bits select the source of UCLK. The
options include the following:
•
•
•
•
8
09746-003
1.0
Description
These bits are reserved and are cleared to 0.
Divide-by-two enable bit. By default, this bit
is 1, meaning that the system clock is 8 MHz.
Enable this bit in low power systems.
1: enable the system clock divider; the
system clock is 8 MHz.
0: disable the system clock divider; the
system clock is 16 MHz.
CLKDIS
The CLKDIS register (Address 0x4000202C) enables and disables the system clock to 10 different peripherals as shown in
Figure 1. By default, all CLKDIS bits are set to 1 except Bit 9, the
ADC system clock enable bit. This disables the system clock to 9
of the 10 of these peripherals after a reset. To use any one of
these 10 peripherals, the user must clear the appropriate bit in
the CLKDIS register to enable the peripheral system clock.
Internal 16 MHz oscillator (default), HFOSC
Internal 32 kHz oscillator, LFOSC
External 32 kHz oscillator, LFXTAL
External clock from P1.0, EXTCLK
The 16 MHz internal oscillator (HFOSC), by default, uses
170 µA.
If an application does not use any of the 10 peripherals, to
minimize power, set the CLKDIS register bit for each unused
peripheral to 1.
CLKSYSDIV
The CLKSYSDIV register (Address 0x40002444) enables and
disables a divide-by-two (DIV2EN) option on the output of the
16 MHz oscillator.
When CLKSYSDIV = 0x1, the system clock (UCLK) becomes
8 MHz; therefore, the entire chip is clocked from an 8 MHz clock
source instead of 16 MHz. This has the effect of not only halving
the dynamic current shown in Figure 3 but also reducing the
background (static) current by 90 µA typically.
Table 2 lists the IDD savings by disabling the clock to each peripheral by setting CLKCON1 = 0x00, assuming that CLKSYSDIV =
0x01 (system clock = 8 MHz). In most cases, by default, the
peripheral is clocked by the system clock although the peripheral
remains inactive. For more information about the CLKCON1
register, see the CLKCON1 section.
Rev. 0 | Page 4 of 12
Application Note
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Table 2. CLKDIS Register Bit Descriptions
Bit
[15:10]
9
Name
Reserved
DISADCCLK
8
DISDMACLK
7
DISDACCLK
6
DIST1CLK
5
DIST0CLK
4
DISPWMCLK
3
DISUARTCLK
2
DISI2CCLK
1
DISSPI1CLK
0
DISSPI0CLK
Description
These bits are reserved and cleared to 0
1: disable ADC system clock
0: enable ADC system clock
1: disable DMA system clock
0: enable DMA system clock
1: disable DAC system clock
0: enable DAC system clock
1: disable Timer1 system clock
0: enable Timer1 system clock
1: disable Timer0 system clock
0: enable Timer0 system clock
1: disable PWM system clock
0: enable PWM system clock
1: disable UART clock
0: enable UART system clock
1: disable I2C system clock
0: enable I2C system clock
1: disable SPI1 system clock
0: enable SPI1 system clock
1: disable SPI0 system clock
0: enable SPI0 system clock
IDD Reduction
75 µA reduction when set to 1. Note that ADC0 and ADC1 are
in idle mode by default, ADCxMDE register = 0x0003.
40 µA reduction when set to 1. Note that by default, all DMA
channels are disabled.
12 µA reduction when set to 1. Note that the DAC, by default, is
in power-down mode, DACCON register = 0x200.
20 µA reduction when set to 1.
15 µA reduction when set to 1.
95 µA reduction when set to 1.
135 µA reduction when set to 1.
70 µA reduction when set to 1.
80 µA reduction when set to 1.
85 µA reduction when set to 1.
Rev. 0 | Page 5 of 12
AN-1111
Application Note
CLKCON1
The CLKCON1 register (Address 0x40002004) scales the clock
to the main communications peripherals: SPI0, SPI1, UART, I2C,
and PWM. By default, the CLKCON1 register is 0x0000.
peripheral and set the clock frequency to its minimum value. For
example, if the PWM is not used, set CLKCON1, Bits[14:12] to
111. Note that the peripheral clock must be equal to or faster
than the processor clock speed. If the peripheral clock is slower
than the processor clock speed, the peripheral is disabled.
To minimize IDD when any of the SPI, I2C, UART, or PWM
peripherals are unused in an application, disable the clock to that
Table 3. CLKCON1 Register Bit Descriptions1
Bit
15
[14:12]
Name
Reserved
PWMCD
[11:9]
UARTCD
[8:6]
I2CCD
[5:3]
SPI1CD
[2:0]
SPI0CD
1
Description
Clock divide bits for PWM system clock
000: UCLK/1 = 16 MHz
001: UCLK/2 = 8 MHz
010: UCLK/4 = 4 MHz
011: UCLK/8 = 2 MHz
100: UCLK/16 = 1 MHz
101: UCLK/32 = 500 kHz
110: UCLK/64 = 250 kHz
111: UCLK/128 = 125 kHz
Clock divide bits for UART system clock
000: UCLK/1 = 16 MHz
001: UCLK/2 = 8 MHz
010: UCLK/4 = 4 MHz
011: UCLK/8 = 2 MHz
100: UCLK/16 = 1 MHz
101: UCLK/32 = 500 kHz
110: UCLK/64 = 250 kHz
111: UCLK/128 = 125 kHz
Clock divide bits for I2C system clock
000: UCLK/1 = 16 MHz
001: UCLK/2 = 8 MHz (the minimum value to support a 400 kHz I2C baud rate)
010: UCLK/4 = 4 MHz
011: UCLK/8 = 2 MHz (the minimum value to support a 100 kHz I2C baud rate)
100: UCLK/16 = 1 MHz
101: UCLK/32 = 500 kHz
110: UCLK/64 = 250 kHz
111: UCLK/128 = 125 kHz
Clock divide bits for SPI1 system clock
000: UCLK/1 = 16 MHz
001: UCLK/2 = 8 MHz
010: UCLK/4 = 4 MHz
011: UCLK/8 = 2 MHz
100: UCLK/16 = 1 MHz
101: UCLK/32 = 500 kHz
110: UCLK/64 = 250 kHz
111: UCLK/128 = 125 kHz
Clock divide bits for SPI0 system clock
000: UCLK/1 = 16 MHz
001: UCLK/2 = 8 MHz
010: UCLK/4 = 4 MHz
011: UCLK/8 = 2 MHz
100: UCLK/16 = 1 MHz
101: UCLK/32 = 500 kHz
110: UCLK/64 = 250 kHz
111: UCLK/128 = 125 kHz
Calculations are for UCLK = 16 MHz with CLKSYSDIV[0] = 0; an additional divide-by-two is required when CLKSYSDIV[0] is set to 1.
Rev. 0 | Page 6 of 12
Application Note
AN-1111
Master Mode
UART
To minimize the current drawn by the UART, use the lowest
possible clock setting that allows the application to comply with
the required UART baud rate setting. Changing the clock frequency
to the UART likewise requires recalculating the correct values
that are entered into the COMDIV register, which is the register
that controls the baud rate.
SPI Clock Rate = SPI Clock/(2 × (1 + SPIxDIV))
where SPI Clock is the divided system clock to the SPI set up by
the CLKSYSDIV and CLKCON1 registers.
To calculate the baud rate, use the following equation:
Baud Rate =
UARTCLK ÷ (2 × 16 × COMDIV) ÷ (M + N ÷ 2048)
Slave Mode
In slave mode, the master on the SPI bus controls the baud rate.
However, the internal ADuCM360/ADuCM361 SPI clock rate,
set by the CLKCON1 register, must be at least four times faster
than the master SPI output clock (host clock).
where:
COMDIV = 1 to 65,536.
M = 1 to 3.
N = 0 to 2047.
UARTCLK is the divided system clock to the UART set up by
the CLKSYSDIV and CLKCON1 registers. Figure 4 shows the
possible IDD savings when the UART clock is reduced.
250
200
UART IDD (µA)
Changing the clock frequency to the SPI while in SPI master
mode requires recalculating the correct values to be entered into
the SPIxDIV register (where x is 0 for SPI0 and 1 for SPI1),
which is the register that controls the SPI clock rate. Calculate
the baud rate as follows:
I2C
To minimize the current drawn by the I2C block, use the lowest
possible clock setting that allows the application to comply with
the required I2C clock rate. Figure 6 shows the possible IDD savings
when the I2C clock is reduced. Note that these figures do not
include the IDD from the external pull-up resistors on the SDA
and SCL pins.
The minimum value to support a 400 kHz I2C baud rate is an
I2C system clock of 8 MHz. Whereas, the minimum value to
support a 100 kHz I2C baud rate is an I2C system clock of 2 MHz.
150
100
90
80
50
70
2
4
6
8
10
12
14
16
FREQUENCY (MHz)
I2C IDD (µA)
Figure 4. UART IDD vs. UART Clock Frequency
SPI0/SPI1
50
40
30
To minimize the current drawn by the SPI block, use the lowest
possible clock setting that allows the application to comply with
the required SPI clock rate. Figure 5 shows the IDD savings from
each SPI block when a reduced clock is selected.
20
10
0
0
2
6
8
10
12
14
16
Figure 6. IDD vs. I2C System Clock Frequency
80
Master Mode
70
Changing the clock frequency to the I2C block while in I2C master
mode requires recalculating the correct values for entering data
to the I2CDIV register, which is the register that controls the
I2C clock rate. The I2CDIV is a 16-bit register containing two
8-bit values, high and low. This is set up according to the formula:
60
50
40
30
fI2CSCL = fPERIPH/(Low + High + 3)
20
10
0
0
2
4
6
8
10
12
14
FREQUENCY (MHz)
Figure 5. IDD vs. SPI Clock Frequency for Each SPI Port
16
09746-005
SPI IDD (µA)
4
FREQUENCY (MHz)
90
09746-006
0
09746-004
60
0
where:
fPERIPH = is the I2C peripheral clock.
fPERIPH = fUCLK/(CLKSYSDIV × I2CCD) where UCLK is the
system clock, 16 MHz;. CLKSYSDIV is 1 or 2, depending on the
CLKSYSDIV[0] bit setting; and I2CCD is the clock divide value
Rev. 0 | Page 7 of 12
AN-1111
Application Note
It is important to consider the following information when
reducing the analog IDD:
and is set by the CLKCON1[8:6] bits from 1 to 7.
Low = the low period of the clock, I2CDIV[7:0] =
(REQD_LOW_TIME/UCLK_PERIOD) − 1.
High = the high period of the clock, I2CDIV[15:8] =
(REQD_HIGH_TIME/UCLK_PERIOD) – 2.
•
Thus, for 100 kHz operation with an I2C peripheral clock of
16 MHz, the low and high bit values are as follows:
•
Low = 0x4F
High = 0x4E
For 400 kHz operation, the low and high bit values are as
follows:
•
Low = 0x13
High = 0x12
•
Slave Mode
In slave mode, the master on the I2C bus controls the baud rate.
•
PWM
To minimize the current drawn by the PWM block, use the
lowest possible clock setting that allows the application to meet
the required PWM duty cycle and output frequency. Figure 7
shows the possible IDD savings when reducing the PWM clock.
•
120
Table 4. Breakdown of Analog Peripherals IDD
100
80
PWM IDD (µA)
The ADC update rate selected by the ADCxFLT registers
does not affect the IDD consumption. Regardless of the ADC
filter update rate, the ADC modulator always works with a
500 kHz clock source.
When the PGA is enabled and the gain is greater than or
equal to 2, the ADC input buffers are not required; therefore,
Register ADCxCON, Bits[17:14] can be set to 1111. This
setting saves 70 µA per ADC.
When the PGA is set to a gain greater than or equal to 32,
an additional 60 µA is consumed by the PGA compared to
gains of less than 32.
When the PGA is disabled (gain = 1), enable the ADC
input buffers, unless an external buffer is provided.
The external reference input buffers consume 60 µA each.
If the external reference negative voltage is connected to
AGND, the negative input buffer can be bypassed and
powered by setting Register ADCxCFG, Bits[1:0] = 11.
Register IEXCCON, Bit 2 = 0 disables the Excitation
Current Source 0. Similarly, Register IEXCCON, Bit 5 = 0
disables Excitation Current Source 1. When only one excitation current is used, disable the other one to save power.
60
40
0
0
2
4
6
8
10
12
FREQUENCY (MHz)
14
16
09746-007
20
Figure 7. IDD vs. PWM Clock Frequency
REDUCING ANALOG IDD
Table 4 lists some of the options when configuring ADC0 and
ADC1 on the ADuCM360/ADuCM361, and the associated IDD
values for each option. As Table 4 shows, the analog IDD can be
minimized by carefully configuring the ADCs and the DAC.
Peripheral Name
Modulator
Gain = 2, 4, 8, or 16
(PGA Total)
Gain = 32, 64, or 128
(PGA Total)
ADC Positive Input
Buffer
ADC Negative Input
Buffer
External Reference
Buffer
Positive
Negative
Excitation Current
(Excluding Output
Current)
Source 0
Source 1
DAC
Rev. 0 | Page 8 of 12
ADC0
70 µA
130 µA
ADC1
70 µA
130 µA
190 µA
190 µA
35 µA
35 µA
35 µA
35 µA
Common
to Both
ADCs
Other
60 µA
60 µA
25 µA
25 µA
50 µA
Application Note
AN-1111
POWER-DOWN MODES
The ADuCM360/ADuCM361 provide five power-down levels.
When a user is deciding on which power-down mode best suits an
application, there is a trade-off among the IDD savings, the wake-up
time, and which of the peripherals needs to be active.
The IDD savings between Mode 3 and Mode 1 or between
Mode 3 and Mode 2 are minimal at FCLK rates ≤ 1 MHz.
1600
In Mode 1, Mode 2, and Mode 3, it is possible for peripherals to
continue operating by using the DMA operation while the CPU
is powered down. To achieve this, either ADC1 or ADC0 must
be enabled for DMA operation. Use a DMA complete interrupt
to awaken the device from Mode 1, Mode 2, or Mode 3.
1200
IDD (µA)
MODE 2
The IDD figures in the following sections assume that the ADC
DMA mode is not enabled.
MODE 1
800
MODE 3
400
In Mode 1, HCLK is off and the ARM Cortex-M3 processor is
in sleep mode. The wake-up time is three to five times FCLK,
where FCLK is the clock selected by the CLKCON0[2:0] bits.
0
0
4
8
12
16
FREQUENCY (MHz)
09746-008
MODE 1: MCUHALT MODE
Figure 8. Power-Down IDD for Mode 1, Mode 2, and Mode 3 vs. FCLK,
CLKSYSDIV = 0x0
Calculate the expected IDD in Mode 1 as follows:
When Register CLKSYSDIV = 0x0
IDD [µA] = 50 × FCLK + 355
900
When Register CLKSYSDIV = 0x1
IDD [µA] = 50 × FCLK + 435
600
MODE 2
MODE1
IDD (µA)
MODE 2: PERHALT MODE
In Mode 2, PCLK is off and the ARM Cortex-M3 processor is in
sleep mode. The wake-up time is three to five times FCLK,
where FCLK is the clock selected by the CLKCON0[2:0] bits.
MODE 3
300
When Register CLKSYSDIV = 0x0
IDD [µA] = 60 × FCLK + 345
0
0
When Register CLKSYSDIV = 0x1
IDD [µA] = 60 × FCLK + 425
2
4
6
FREQUENCY (MHz)
8
09746-009
Calculate the expected IDD in Mode 2 as follows:
Figure 9. Power-Down IDD for Mode 1, Mode 2, and Mode 3 vs. FCLK,
CLKSYSDIV = 0x1
The IDD savings between Mode 1 and Mode 2 are minimal.
MODE 3: SYSHALT MODE
MODE 4 AND MODE 5: TOTALHALT AND
HIBERNATE MODES
In Mode 3, HCLK, ACLK, and PCLK are off and the ARM
Cortex-M3 processor is in sleep mode. The wake-up time is three
to five times FCLK, where FCLK is the clock selected by the
CLKCON0[2:0] bits.
In Mode 4 and Mode 5, HCLK, ACLK, and PCLK are off
and the ARM Cortex-M3 processor is in DEEPSLEEP mode.
The wake-up time is ~30.8 µs. The typical IDD in these modes is
2 µA to 4 µA.
Calculate the expected IDD in Mode 3 as follows:
When Register CLKSYSDIV = 0x0
IDD [µA] = 16 × FCLK + 345
When Register CLKSYSDIV = 0x1
IDD [µA] = 16 × FCLK + 420
Rev. 0 | Page 9 of 12
AN-1111
Application Note
NOTES
Rev. 0 | Page 10 of 12
Application Note
AN-1111
NOTES
Rev. 0 | Page 11 of 12
AN-1111
Application Note
NOTES
I2C refers to a communications protocol originally developed by Philips Semiconductors (now NXP Semiconductors).
©2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN09746-0-10/12(0)
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