STMicroelectronics AN2841 This application note deals with the general principles of led dimming Datasheet

AN2841
Application note
LED dimming implemented on STM32™ microcontroller
1
Introduction
This application note deals with the general principles of LED dimming. This note also
focuses on the use of pulse width modulation (PWM) for digital dimming. Its implementation
on the STM32™ microcontroller in combination with the STP24DP05 LED driver is
described in detail and the most common design approaches are compared.
The conclusion helps developers choose the best approach for their LED dimming
application.
November 2008
Rev 1
1/31
www.st.com
Contents
AN2841
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Description of LED dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
2.1
Driving multiple LED applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2
Multi-channel dimming using serial-parallel conversion . . . . . . . . . . . . . . . 6
2.3
Specific dimming application using an STP24DP05 . . . . . . . . . . . . . . . . . 8
Dimming application using an STM32 microcontroller . . . . . . . . . . . . . 9
3.1
3.2
3.3
Data transfers and timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1
Dynamic data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2
Static data transfer using lookup tables . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.3
Single interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.4
Single interrupt with DMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.5
Interrupt + DMA + interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Data preparation: redundant and non redundant use of memory . . . . . . 19
3.2.1
Full lookup table (redundant) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.2
Reduced lookup table (non redundant) . . . . . . . . . . . . . . . . . . . . . . . . . 20
Timing: set up of the main dimming timer . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3.1
4
The main system timer, SysTick setting . . . . . . . . . . . . . . . . . . . . . . . . 22
Achieved values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1
Dimming approach comparison and conclusion . . . . . . . . . . . . . . . . . . . 29
4.1.1
Comparison between interrupt and interrupt + DMA approaches . . . . . 29
4.1.2
Comparison between dynamic preparation, static, and reduced static
lookup table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5
References documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2/31
AN2841
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
LEDmap - requested LED brightness configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Full lookup table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
LEDmap - requested LED brightness configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
LEDmap - reordered requested LED brightness configuration . . . . . . . . . . . . . . . . . . . . . . 22
Reduced lookup table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
TotalTime necessary for requested dimming parameters. . . . . . . . . . . . . . . . . . . . . . . . . . 23
Maximum refresh rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
SysTick-reload-time for 100 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Example of values used in Equations 3 and 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Lookup table size (full and reduced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Refresh frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Refresh frequencies for better optimized code (8 and 12-bit PWM) . . . . . . . . . . . . . . . . . . 28
Refresh frequencies for better optimized code (10 and 11-bit PWM) . . . . . . . . . . . . . . . . . 28
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3/31
List of figures
AN2841
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
4/31
PWM signal with 16 steps of possible voltage values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
PWM serial-parallel conversion for multiple LED applications . . . . . . . . . . . . . . . . . . . . . . . 7
Example of a specific lighting setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Simplified setup of an application using an STP24DP05 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
STM32 evaluation board setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
STM32 evaluation board block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Computation of LEDdriverDATAbuffer variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Conversion of LEDmap array to LEDdriverDATAbuffer . . . . . . . . . . . . . . . . . . . . . . . . 11
Dynamic data preparation - time consumption - block diagram . . . . . . . . . . . . . . . . . . . . . 12
Dynamic data preparation - time consumption - real signals . . . . . . . . . . . . . . . . . . . . . . . 12
Static lookup table time consumption with 166 kHz - 333 kHz LE signal . . . . . . . . . . . . . . 13
Timing diagram: Single interrupt method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Real timing signals for single interrupt method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Timing diagram: Single interrupt with DMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Timer interrupt + DMA transfer + DMA interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Real signals: timer interrupt + DMA transfer + DMA interrupt model . . . . . . . . . . . . . . . . . 18
AN2841
2
Description of LED dimming
Description of LED dimming
There are two categories of LED dimming:
●
Analog: Uses a linear device to set the current flow through the LED.
Advantages: Absence of digital noise; a simple implementation.
Limitations: Color shift, low dynamics.
●
Digital: Uses a switch to set nominal or zero current flow through the LED.
Advantages: Accurate and fast color control.
Limitations: The presence of digital noise and necessity to use a frequency high
enough to prevent flickering; a complex implementation.
This document focuses on digital dimming and the use of PWM (pulse width modulation) in
common applications.
When using PWM for lighting applications, the following rules must be kept in mind:
●
For the LED light to be seen as non-flickering by the human eye, in most cases the
PWM frequency must be higher than 100 Hz.
●
The LED light brightness depends on the PWM duty cycle. The brightness depends on
the resolution of the PWM duty cycle.
Figure 1.
PWM signal with 16 steps of possible voltage values
Step: 0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 0
1 ...
TPWM
AM00341
Using a 4-bit PWM resolution, there can be 16 different light intensity (energy) values.
Concerning color applications, 16 million (256 × 256 × 256) colors can be reached using
tricolor LEDs with an 8-bit dimming resolution for each single color LED.
5/31
Description of LED dimming
2.1
AN2841
Driving multiple LED applications
Displays, indicators and panels increasingly use more and more LEDs. There are three
approaches for driving higher numbers of LEDs:
1.
Dedicated pin
Each LED is driven by dedicated system (microcontroller, FPGA) pin.
Advantages: Simple implementation of dimming algorithm. No conversion.
Disadvantages: Unusable for high LED counts. Consumes pins of microcontroller.
2.
Serial-parallel conversion
Multiple LEDs are driven by a single system pin.
Advantages: Reduced number of system pins required. High LED count systems can
be developed.
Disadvantages: Additional components and external buffers are needed. The data rate
on a single pin must be n times greater than that for the single LED out of n, where n is
the number of LEDs.
3.
Matrix
LEDs are arranged in a square matrix and driven by a complex and sophisticated
algorithm that combines serial-parallel conversion with additional decomposition of the
serial data stream into the rows and columns by another active component.
This document focuses on serial-parallel conversion applications.
2.2
Multi-channel dimming using serial-parallel conversion
The serial-parallel conversion approach is in fact a parallel-serial-parallel conversion since
the data is stored in the system memory in parallel format and then is converted to serial
format before its transfer over the serial interface. The serial stream is then reconverted to
parallel format by the LED driver used to drive each LED. The serial data transfer speed
must be n times faster than driving a single LED, where n is the number of parallel driver
outputs.
Buffers (LED drivers) used for serial-parallel conversion come generally with the four most
important signals:
■
SDI:
■
SDO: Serial data out
■
CLK: Clock signal used for loading data in the driver buffer via the SDI (input) and
sending data from the buffer via the SDO (output). It also shifts all data actually
present (previously loaded) in the driver.
■
LE:
Serial data in
Copies (latch enable) the actual values loaded in the driver into its output cells.
The output cells drive parallel outputs.
The standard serial peripheral interface (SPI) contains SDO, SDI and CLK signals.
It is necessary to latch (LE signal pulse) internal driver data once per n CLK cycles, where
n is the total LED count. This latching rule ensures the correct mapping of the serialized
PWM data coming on SDI to the corresponding parallel output. Refer to Figure 2, Figure 3
and Figure 4.
6/31
AN2841
Description of LED dimming
Figure 2.
PWM serial-parallel conversion for multiple LED applications
Figure 3.
Example of a specific lighting setting
T PWM
SERIAL DATA 1111
1101 0101 0001
LEVELS
T SEND_DATA
0000 1111 or new
data
t
PARALLELDATA
4% duty cycle
Output 1- (LED1)
t
14% duty cycle
Output 2 - (LED2)
t
Output 3 - (LED3)
3% duty cycle
t
Output 4 - (LED4)
99% duty cycle
t
AM00343
The following equation is used to determine how many LEDs can used in an application with
specific maximum SPI speed and PWM resolution requirements:
Equation 1
N=
f SPI
2 f PWM
n
where;
N is the maximum number of LEDs
fSPI is the SPI communication speed
n is the PWM resolution
fPWM is the dimming frequency
7/31
Description of LED dimming
AN2841
The outcome of Equation 1 is valid only for the best-case scenario when the data are sent
to the LED driver without any delay between blocks of data. This example, of course, is not
a real one. The values for a real application can only get close to the value determined
above.
2.3
Specific dimming application using an STP24DP05
Figure 4 illustrates a simple dimming application using an STP24DP05 24-bit constant
current LED sink driver with output error detection.
Figure 4.
Simplified setup of an application using an STP24DP05
5V
3.3 V
RGB LED configuration
LED block
Temperature
sensor
R1...8 G1...8 B1...8
Driving
system
STP24DP05
STP24DP05
SPI
SPI
OE-R/G/B
LE
More drivers in cascade…
AM00344
8/31
AN2841
3
Dimming application using an STM32 microcontroller
Dimming application using an STM32 microcontroller
This section describes a dimming application that uses an STM32 microcontroller (MCU).
All the computations, results and tests were performed on an evaluation board (reference:
STEVAL-ILL015V1) containing an STM32 MCU running at a clock frequency of 48 MHz.
For more information about the use of this evaluation board and dimming applications,
please refer to user manuals UM0588 and UM0574 listed in Section 5: References
documents.
Figure 5 and Figure 6 provide a brief overview of the STM32 evaluation board.
Figure 5.
STM32 evaluation board setup
Figure 6.
STM32 evaluation board block diagram
USB
ESD for USB
USBUF01P6
SPI
LED driver
STP24DP05
SPI
Power supply
ST1S10
STM32
LED driver
STP24DP05
SPI
AM00345
3.1
Data transfers and timing
This section describes the two approaches used to transfer data (dynamic and static) as
well as timing considerations.
3.1.1
Dynamic data transfer
When using a dynamic data approach, all data is computed just before being transfered to
the LED drivers.
9/31
Dimming application using an STM32 microcontroller
AN2841
The LED brightness value is defined by the user in the LEDmap array. The LEDmap is an
array of 8-bit numbers (when using an 8-bit dimming resolution). Each value is dedicated
to one single LED (Figure 7).
The dimming algorithm converts the LEDmap array into a serial stream. The serial stream
in combination with the LE signal generates requested PWM outputs on the LED drivers
as shown in Figure 8.
Figure 7.
Computation of LEDdriverDATAbuffer variable
Begin
depth = 0
0 .. 255 for 8-bit PWM resolution
0 .. LEDcount / 8
DATAchar = 0
Detail in Figure 8:
LEDmap
LEDdriverDATAbuffer
SPI
LED
Driver
DATAchar++
–
DATAchar>3
+
depth++
–
depth>255
+
AM00346
10/31
AN2841
Dimming application using an STM32 microcontroller
Figure 8.
Conversion of LEDmap array to LEDdriverDATAbuffer
for one time stamp depth = 50
LEDmap= { 20, 60, 75, 22, 100, 255, 0, 5}
LEDmap[0] > depth = 0
for outputBIT from 0 to 7
LEDmap[1] > depth = 1
LEDmap[2] > depth = 1
LEDdriverDATAbuffer[0]
LEDmap[3] > depth = 0
LEDmap[4] > depth = 1
LEDmap[6] > depth = 1
LEDmap[7] > depth = 0
LEDmap[8] > depth = 0
SPI
LED driver
AM00347
Example using a 4×8 LED matrix with an 8-bit PWM resolution
while(1)
for(depth = 1; depth<=0xff; depth++){
for(DATAchar = 0; DATAchar < 4; DATAchar++){
for(outputBIT = 0; outputBIT < 8; outputBIT++){
LEDdriverDATAbuffer[DATAchar] <<= 1;
if(LEDmap[(DATAchar*8) + outputBIT] > depth)
LEDdriverDATAbuffer[DATAchar] |= 0x0001;
else
LEDdriverDATAbuffer[DATAchar] &= 0xFFFE;
}
SPI_SendData(SPI1, LEDdriverDATAbuffer[DATAchar]);
}
LE_LOW();
LE_HIGH();
LE_LOW();
}
Even though the dynamic data preparation consumes a minimum of system memory, it
takes a lot of computational time. This approach is only recommended for low LED count
applications. A dynamic data preparation approach can reach 100 Hz for 32 LEDs (with
maximum code optimization in C compiler on the STM32 evaluation board).
11/31
Dimming application using an STM32 microcontroller
Figure 9.
AN2841
Dynamic data preparation - time consumption - block diagram
Figure 10. Dynamic data preparation - time consumption - real signals
3.1.2
Static data transfer using lookup tables
When using static lookup tables, all data is computed before being sent to the LED drivers.
Static lookup tables are used to convert the brightness value from an 8-, 9-, 10-, 11- or
12-bit representation from the LEDmap array into the serial stream representing the PWM of
the LED driver outputs.
12/31
AN2841
Dimming application using an STM32 microcontroller
Once the brightness data is determined, the repetitive dimming procedure needs only to
take the prepared data and send it to the LED driver.
Figure 11 shows the calculation time of the algorithm.
Figure 11. Static lookup table time consumption with 166 kHz - 333 kHz LE signal
Accurate LE timing and the dimming part running in the application background are
necessary requirements for a successful application. These targets can be achieved using
the following methods:
3.1.3
●
Single interrupt
●
Single interrupt with DMA
●
Interrupt + DMA + interrupt
Single interrupt
The simplest approach is to use a timer interrupt approach where only a timer interrupt is
required for the data transfer timing. This approach which does not require direct memory
access (DMA) is designed for microcontrollers without DMA capability.
Except for pattern loading, SPI data transfer and Latch Enable generation, the code must
also contain the TX buffer Empty Flag test. This test must be taken into account if you
compute the total time necessary for all dimming operations.
13/31
Dimming application using an STM32 microcontroller
AN2841
Figure 12 shows a timing diagram for a dimming application using this method.
Figure 12. Timing diagram: Single interrupt method
Figure 13 shows the waveforms of the signals output by the MCUs controlling the LED
drivers.
Figure 13. Real timing signals for single interrupt method
14/31
AN2841
Dimming application using an STM32 microcontroller
Example of C code implementation of the timer interrupt routine
void SysTickHandler(void)
{
LE_LOW(); (Step 1)
LE_HIGH();
depth+=4;
if(depth > 0x400) (Step 2)
depth = 0;
SPI_SendData(SPI1, LookUp[depth+0]); (Step 3)
while(!SPI_GetFlagStatus(SPI1, SPI_FLAG_TXE));
SPI_SendData(SPI1, LookUp[depth+1]);
while(!SPI_GetFlagStatus(SPI1, SPI_FLAG_TXE)); (Step 4)
SPI_SendData(SPI1, LookUp[depth+2]);
while(!SPI_GetFlagStatus(SPI1, SPI_FLAG_TXE));
SPI_SendData(SPI1, LookUp[depth+3]);
while(!SPI_GetFlagStatus(SPI1, SPI_FLAG_TXE));
LE_LOW(); (Step 5)
LE_HIGH();
}
In the above example, the waveform and program code are described in the following steps:
1.
The generation of the LE after entering the interrupt.
2.
Increment of the lookup table pointer.
3.
The transfer of the first 8 bits.
4.
Test of TX buffer Empty Flag.
5.
An additional edge on LE. This is for measurement purposes only.
6.
The time between the end of the current interrupt and the next one. This time can be
used for calculating certain foreground tasks in the application.
15/31
Dimming application using an STM32 microcontroller
3.1.4
AN2841
Single interrupt with DMA
The data is transferred from the microcontroller RAM into the LED driver using a single
interrupt with DMA approach. You cannot use a DMA approach without a timer, because you
cannot change the speed of the DMA and thus you cannot change PWM parameters and
the refresh rate in required steps. The benefit of using DMA is that the SPI buffer can be
automatically filled. The time saved is nearly in the range of the time needed for the DMA to
restart and re-configure itself for the next transfer (in comparison to previous approach). An
example of a timing diagram for this programming model is shown in Figure 14. The timing
can be compared in Figure 12 and Figure 14.
Figure 14. Timing diagram: Single interrupt with DMA
16/31
AN2841
3.1.5
Dimming application using an STM32 microcontroller
Interrupt + DMA + interrupt
Figure 14 shows that data from the previous transfer is latched during the current timer
interrupt which causes several problems. The algorithm is more complicated in this case.
A slightly different approach that has no impact on the system can be used: it is obtained by
generating the LE pulse when the DMA transfer is complete instead of generating it on
a timer interrupt. The LE pulse is generated at the DMA transfer complete interrupt as
shown in Figure 15.
Figure 15. Timer interrupt + DMA transfer + DMA interrupt
Figure 16 shows the waveform of the outbound signals from the microcontrollers controlling
the LED drivers. An example of the C code implementation of the timer routine is given
below. A detailed description follows the example.
17/31
Dimming application using an STM32 microcontroller
AN2841
Figure 16. Real signals: timer interrupt + DMA transfer + DMA interrupt model
Step "1" in Figure 16 is not necessary, but it is used for a better visualization of the tracking
of signals on the oscilloscope.
Example of C code of DMA and timer interrupt routine: interrupt + DMA +
interrupt approach
void SysTickHandler(void)
{
LE_LOW();(Step 1)
LE_HIGH();
depth+=4;
if(depth > 0x400)
depth = 0;
DMA_Channel3->CCR &= CCR_ENABLE_Reset;
DMA_Channel3->CNDTR = 0x04;
DMA_Channel3->CMAR = ((u32) (LookUp))+depth;
DMA_Channel3->CCR |= CCR_ENABLE_Set; (Step 2)
DMA_Channel3->CCR |= DMA_IT_TC;
}
LE_LOW();
void DMAChannel3_IRQHandler(void)(Step 5)
{
while(!SPI_GetFlagStatus(SPI1, SPI_FLAG_TXE));(Step 6)
}
18/31
LE_HIGH();(Step 7)
LE_LOW();
DMA_ClearITPendingBit(DMA_IT_GL3);
AN2841
Dimming application using an STM32 microcontroller
In the above example, the waveform and program code are described in the following steps:
1.
The generation of the LE after entering the interrupt.
2.
Setting the DMA: source address, data length, and Start flag.
3.
Data is sent automatically by DMA.
4.
Data is sent automatically by DMA.
5.
DMA interrupt routine is called when the last byte is sent by DMA.
6.
Wait until the data from the SPI TX buffer is delivered to the driver.
(Step 6 can be exchanged with calculations that take the amount of time necessary for
the SPI transfer to be finished).
7.
LE generation.
3.2
Data preparation: redundant and non redundant use of
memory
3.2.1
Full lookup table (redundant)
The static lookup table generation is mentioned in Section 3.1.2. The lookup table
generation procedure is described in more detail in this section.
The lookup table row is computed for every PWM step (8-bit PWM means 256 rows for 256
steps). The C code of the full lookup table generation is given in the example below.
Example using a 4×8 LED matrix with an 8-bit PWM resolution
void generateLookUpTable(unsigned char *LEDmapIN){
unsigned int tmpdepth;
for(tmpdepth = 0; tmpdepth < 0xff; tmpdepth++){
for(DATAchar = 0; DATAchar < 4; DATAchar++){
for(outputBIT = 0; outputBIT < 8; outputBIT++){
LookUp[tmpdepth*4 + DATAchar] <<= 1;
if(LEDmapIN[(DATAchar*8) + outputBIT] > tmpdepth)
LookUp[tmpdepth*4 + DATAchar] |= 0x01;
else
LookUp[tmpdepth*4 + DATAchar] &= 0xFE;
}
}
}
}
Table 1.
LEDmap - requested LED brightness configuration
Output
Duty cycle (brightness)
LED 1
LED 2
LED 3
2n
2n
2n
2/
6/
4/
…
…
LED i
100%
19/31
Dimming application using an STM32 microcontroller
Table 2.
AN2841
Full lookup table
Output
Step
3.2.2
LED 1
LED 2
LED 3
…
…
LED i
0
1
1
1
1
1
1
1
1
1
2
1
1
1
1
3
0
1
1
1
4
0
1
1
1
5
0
1
0
1
6
0
1
0
1
…
0
0
0
1
…
0
0
0
1
2n
0
0
0
1
Reduced lookup table (non redundant)
Only the data that changes the LED driver output state is generated. A LED can change the
state only once per complete PWM cycle: from one to zero. If there are i number of LEDs,
there are i new data items for the LED drivers. For example, if i = 32 LEDs when using an
8-bit PWM, you need a cycle with i × i = 32 × 32 = 1024 loops.
The inside of the loop is the same for full lookup table generation as described in
Section 3.2.1. The lookup table row is computed only in case you need to change the LED
intensity pattern (specific value of a parallel output configuration). Table 3, Table 4 and
Table 5 bring better understanding of the lookup table reduction. The C code of the reduced
lookup table generation is shown in the example below.
20/31
AN2841
Dimming application using an STM32 microcontroller
Example of reduced lookup table generation
void performGenerateReducedLookUpTable(unsigned char *LEDmapIN,
int LEDcount, int PWMdepth, unsigned char **tableIN, unsigned
char **tableKeyIN, int *table_size){
int streamSize = LEDcount/8;
*table_size = differentValuesCount(LEDmapIN, LEDcount);
*tableKeyIN = SortAndReduce(LEDmapIN, LEDcount, *table_size);
*tableIN = malloc(streamSize * (*table_size)
* sizeof(unsigned char));
for(tmpdepth = 0; tmpdepth < (*table_size); tmpdepth++){
for(DATAchar = 0; DATAchar < streamSize; DATAchar++){
for(outputBIT = 0; outputBIT < 8; outputBIT++){
(*tableIN)[tmpdepth*streamSize + DATAchar] <<= 1;
if(LEDmapIN[(DATAchar*8) + outputBIT] >
(*tableKeyIN)[tmpdepth])
(*tableIN)[tmpdepth*streamSize + DATAchar] |= 0x01;
else
(*tableIN)[tmpdepth*streamSize + DATAchar] &= 0xFE;
}
}
}
}
.
Table 3.
LEDmap - requested LED brightness configuration
Outputs
Duty cycle (brightness)
LED 1
LED 2
LED 3
n
2n
2n
2/2
6/
4/
…
…
LED i
100%
21/31
Dimming application using an STM32 microcontroller
AN2841
LEDmap - reordered requested LED brightness configuration
Table 4.
Output
LED 1
LED 3
LED 2
Duty cycle (brightness)
2 / 2n
4 / 2n
6 / 2n
Step of change (time of
change)
2
4
6
100%
Table row
1
2
3
i
Table 5.
…
LED x
LED i
× / 2n
100%
Reduced lookup table
Output
Row
Level
LED 1
LED 2
LED 3
…
…
LED i
0
0
1
1
1
1
1
1
1
2
1
1
1
1
1
1
2
4
0
1
1
1
1
1
3
6
0
1
0
1
1
1
…
…
0
0
0
1
1
1
…
…
0
0
0
0
1
1
m(1)
x+1
0
0
0
0
0
1
1. "m" is less or equal to number of LEDs = i
3.3
Timing: set up of the main dimming timer
3.3.1
The main system timer, SysTick setting
The approach with interrupt + DMA + interrupt is considered the most effective.
The microcontroller can reach the maximum frame rate based on the minimum time
required for the all the necessary operations. A time reserve of 6% was used:
●
tDMAen re-enabling the DMA (~ 1.25 µs)
●
toneSPI time necessary for transfer of the data over SPI (~ LEDs × 55 ns)
●
tLEgen LE generation and new data pointer generation (~ 1 µs)
The total time needed for correct processing is the sum of the times above:
Equation 2
TotalTime = tDMAen + toneSPI × LEDs + tLEgen
= 1.25 µs + 55 ns × LEDs + 1 µs
= 2.25 µs + 55 ns × LEDs
The number of LEDs must be rounded up to nearest number commensurable by the length
of an SPI buffer. The SPI buffer length is 8 either or 16 bits in most cases.
22/31
AN2841
Dimming application using an STM32 microcontroller
More examples of TotalTime for different number of LEDs and PWM resolution can be seen
in Table 6.
Table 6.
TotalTime necessary for requested dimming parameters
LEDs
PWM
TotalTime
Max. LE frequency
8
8-bit
2.25 + 0.055 × 8 = 2.69 µs
371.7 kHz
8
12-bit
2.25 + 0.055 × 8 = 2.69 µs
371.7 kHz
672
8-bit
2.25 + 0.055 × 672 = 39.21 µs
25.5 kHz
Table 6 shows the time required for processing one PWM step, one buffer load and one
buffer latch. With an 8-bit PWM resolution, 256 buffer latches are needed; the maximum LE
(Latch Enable) frequency must be divided by 256. These results are shown in Table 7.
.
Table 7.
Maximum refresh rate
LEDs
PWM
Max. LE frequency
Max. refresh rate
8
8-bit
371.7 kHz
1452 Hz
8
12-bit
371.7 kHz
90.7 Hz
672
8-bit
25.5 kHz
99.6 Hz
Table 7 shows the maximum refresh frequencies for controlled LEDs. But the inverse
computation must be used to solve our task. Table 8 shows the inversion of the Table 7
values and gives the required refresh frequency. The LE frequency must be derived and the
SysTick timer preload value (SysTick-preload-value) set to generate the LE pulse by
generating an interrupt at every counter reload event.
Table 8.
SysTick-reload-time for 100 Hz
LEDs
PWM
Requested refresh rate
SysTick-reload-time
8
8-bit
100 Hz
39 µs (25.6 kHz)
8
12-bit
90.7 Hz
2.7 µs (371.7 kHz)
672
8-bit
100 Hz
39 µs (25.6 kHz)
32
8-bit
100 Hz
39 µs (25.6 kHz)
Table 7 and Table 8 show that SysTick-reload-frequency will be set between 25.6 kHz and
371.7 kHz according to requested PWM resolution and LED count.
The STM32 SysTick timer uses a reference clock of 72 MHz when set to the maximum
value. The interrupt period for this timer is the SysTick-preload-value multiplied by 1 / (72
000 000).
The reference clock of STM32 evaluation board is set to 48 MHz for the USB to operate
correctly. This value should be taken into the account while making measurements on this
evaluation board as their computation will influence Equation 3 .
The reference clock can be also divided by 8 by using the hardware divider. The divided
reference clock is then 9 MHz. In this case, the interrupt period for this timer is equal to
a preload value multiplied by 1 / (9 000 000).
Preload value = requested SysTick interrupt time / reference period
Preload value = reference frequency / requested SysTick interrupt frequency.
23/31
Dimming application using an STM32 microcontroller
Table 9.
AN2841
Example of values used in Equations 3 and 4
LEDs
PWM
Requested refresh rate
SysTick-reload-time
32
8-bit
100 Hz
39 µs (25.6 kHz)
Equation 3
SysTick-preload-value = 72 000 000 / 25 600 = 2812
The frequency ranges 25.6 kHz and 371.7 kHz required for all possible applications ranges
can be achieved using preload values: 2812 and 194, or with divider by 8: 351 or 24.
SysTick timer setting with no divider on the reference clock:
SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK);
SysTick_SetReload(900);
generates pulses with the period of 12 µs.
SysTick timer setting with the divider by 8 on the reference clock:
SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8);
SysTick_SetReload(900);
generates pulses with the period of 100 µs.
The setup algorithm for the SysTick timer configuration has to check if it is possible to set
preload value to keep enough time for processing (TotalTime) together with reaching the
requested refresh frequency:
TotalTime = 2.25 + 0.055 × LEDs
SysTick-reload-time = 1 / SysTick-reload-frequency
TotalTime is the time slot, during which the SPI transfer and all the arbitrations needs to be
achieved. The SysTick time is the time slot which must be respected to reach the required
refresh rate.
Equation 4
SysTick-reload-time > TotalTime
To reach the requested refresh rate and set up the SysTick timer preload value:
SysTick-reload-frequency = requested-refresh-rate × PWMdepth (100 Hz × 256 steps).
24/31
AN2841
4
Achieved values
Achieved values
Dynamic computing takes a lot of time and 120 Hz can be reached only for 32 LEDs. In
addition, no other foreground or background tasks can be performed during computation.
As a consequence, this solution is recommended only for simple applications that use only
a few diodes. For other more complicated applications, it is recommended to use the lookup
table approach using a single interrupt only or interrupt + DMA (if DMA is available).
The LED brightness resolution is limited by the size of the lookup table (Table 10). For every
level of brightness, you need a single string of zeros and ones for all the LEDs.
Table 10.
Lookup table size (full and reduced)
Bit depth
LEDs
Lookup table size: i × 2n
Reduced table size(1): i × n
8
32
256 × 4 = 1 kB
32 × 4 = 128 B
12
32
4096 × 4 = 16 kB
32 × 4 = 128 B
8
128
256 × 16 = 4 kB
128 × 16 = 2 kB
12
128
4096 × 16 = 64 kB
128 × 16 = 2 kB
8
512
256 × 64 = 16 kB
256 × 64 = 16 kB
12
512
4096 × 64 = 256 kB
512 × 64 = 32 kB
1. Values indicates the maximum size.
From Figure 13, the maximum refresh frequency can be computed. The time between two
interrupts is 6 µs. This is a refresh rate for one brightness level, for 32 LEDs, 166 kHz.
Table 11. shows the possible refresh rates for whole brightness depth and different LED
count.
25/31
Achieved values
AN2841
Table 11.
Refresh frequencies
8-bit [Hz]
9-bit [Hz]
10-bit [Hz]
11-bit [Hz]
12-bit [Hz]
256
512
1024
2048
4096
8
1514
757
379
189
95(1)
16
1293
647
323
162
81
24
1129
564
282
141
71
32
1002
501
250
125
63
40
900
450
225
113
56
48
817
409
204
102
51
56
748
374
187
94
47
64
690
345
173
86
43
72
640
320
160
80
40
80
597
299
149
75
37
88
560
280
140
70
35
96
526
263
132
66
33
104
497
248
124
62
31
112
471
235
118
59
29
120
447
223
112
56
28
128
426
213
106
53
27
136
406
203
102
51
25
144
388
194
97
49
24
152
372
186
93
47
23
160
357
179
89
45
22
168
343
172
86
43
21
176
330
165
83
41
21
184
319
159
80
40
20
192
308
154
77
38
19
7200
297
149
74
37
19
208
288
144
72
36
18
216
279
139
70
35
17
224
270
135
68
34
17
232
262
131
66
33
16
240
255
127
64
32
16
248
248
124
62
31
15
256
241
120
60
30
15
264
234
117
59
29
15
LEDs/PWM
26/31
AN2841
Achieved values
Table 11.
Refresh frequencies (continued)
8-bit [Hz]
9-bit [Hz]
10-bit [Hz]
11-bit [Hz]
12-bit [Hz]
256
512
1024
2048
4096
272
228
114
57
29
14
280
223
111
56
28
14
288
217
109
54
27
14
296
212
106
53
27
13
304
207
104
52
26
13
312
202
101
51
25
13
320
198
99
49
25
12
328
194
97
48
24
12
336
189
95
47
24
12
344
185
93
46
23
12
352
182
91
45
23
11
360
178
89
45
22
11
368
175
87
44
22
11
376
171
86
43
21
11
384
168
84
42
21
10
392
165
82
41
21
10
400
162
81
40
20
10
408
159
79
40
20
10
416
156
78
39
20
10
424
153
77
38
19
10
432
151
75
38
19
9
440
148
74
37
19
9
448
146
73
36
18
9
456
144
72
36
18
9
464
141
71
35
18
9
472
139
70
35
17
9
480
137
68
34
17
9
488
135
67
34
17
8
496
133
66
33
17
8
504
131
65
33
16
8
512
129
64
32
16
8
520
127
64
32
16
8
528
125
63
31
16
8
LEDs/PWM
27/31
Achieved values
AN2841
Table 11.
Refresh frequencies (continued)
8-bit [Hz]
9-bit [Hz]
10-bit [Hz]
11-bit [Hz]
12-bit [Hz]
256
512
1024
2048
4096
536
124
62
31
15
8
544
122
61
30
15
8
552
120
60
30
15
8
560
119
59
30
15
7
568
117
59
29
15
7
576
116
58
29
14
7
584
114
57
29
14
7
592
113
56
28
14
7
600
111
56
28
14
7
608
110
55
27
14
7
616
110
55
27
14
7
624
110
55
27
14
7
632
106
53
26
13
7
640
105
52
26
13
7
648
103
52
26
13
6
656
102
51
26
13
6
664
101
51
25
13
6
672
100
50
25
12
6
680
99
49
25
12
6
688
98
49
24
12
6
LEDs/PWM
1. 100 Hz can be reached with very high level of optimization.
Table 12.
LEDs
8-bit PWM freq. [Hz]
12-bit PWM freq. [Hz]
32
250 000 / 256 = 1000
250 000 / 4096 = 61
128
(250 000 / 256) / 4 = 244
(250 000 / 4096) / 4 = 15
512
(250 000 / 256) / 16 = 61
(250 000 / 4096) / 16 = 4
Table 13.
28/31
Refresh frequencies for better optimized code (8- and 12-bit PWM)
Refresh frequencies for better optimized code (10- and 11-bit PWM)
LEDs
10-bit PWM freq. [Hz]
11-bit PWM freq. [Hz]
32
250 000 / 1024 = 244
250 000 / 2048 = 122
128
(250 000 / 1024) / 4 = 64
(250 000 / 2048) / 4 = 30
512
(250 000 / 1024) /16 = 15
(250 000 / 2048) / 16 = 7
AN2841
Achieved values
Table 12 and Table 13 show the results in case you remove the additional LE edge used for
the measurement of the interrupt end and optimize time between two interrupts (used for
SPI in 16-bit mode).
4.1
Dimming approach comparison and conclusion
4.1.1
Comparison between interrupt and interrupt + DMA approaches
When using the single interrupt approach described in Figure 12 and Figure 13, the time
spent by the MCU to drive the LEDs is long and little time is left to execute other tasks.
The combination of the single interrupt + DMA approach frees more computing time for
other tasks. This approach does not improve the refresh rate.
4.1.2
Comparison between dynamic preparation, static, and reduced static
lookup table
The dynamic data preparation shown in Figure 9 allows to save memory but requires
additional computing time.
The static lookup table method saves computing time. However, the computing must be
performed at the beginning of the dimming.
The reduced static lookup table offers the same functionalities as a full lookup table, while
saving memory space.
29/31
References documents
5
6
References documents
1.
User manual UM0588 - Multilayer C library for LED dimming used on systems with SPI
and DMA capabilities
2.
User manual UM0574 - STEVAL-ILL015V1 - LED dimmer demonstration board based
on the STP24DP05 and STM32™.
Revision history
Table 14.
30/31
AN2841
Document revision history
Date
Revision
10-Nov-2008
1
Changes
Initial release.
AN2841
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