How to use PWT module on Kinetis E and EA MCUs

How to use PWT module on Kinetis E and EA MCUs
Freescale Semiconductor, Inc.
Application Note
Document Number: AN4839
Rev. 1, 05/2014
How to Use PWT Module on Kinetis E
and Kinetis EA Series
1
Overview
The Pulse Width Timer (PWT) is a versatile module
of the KE04, KE06, KEA128, and KEA8 devices. It
functions as a 16-bit pulse width measurement unit
or a general purpose timer. The PWT allows users to
find the width of input pulse and its frequency
immediately. The module can also measure positive
and negative pulse width separately with 16-bit
resolution and the measurement can be triggered by
rising-edge, falling-edge, or both. The user can
decide to start measuring the first rising-edge or first
falling-edge. The PWT module supports an external
input clock source, called ALTCLK. PWT has four
input channels, programmable timer overflow, and
pulse width read-ready interrupt. In addition, it
includes a flexible mechanism to reset or restart
operation.
© 2014 Freescale Semiconductor, Inc.
Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . .
2. PWT introduction . . . . . . . . . . . . . . . . . . .
2.1.Pulse input pins . . . . . . . . . . . . . . . . .
2.2.PWT clock sources . . . . . . . . . . . . . . .
2.3.Soft reset . . . . . . . . . . . . . . . . . . . . . . .
2.4.Input edge capture . . . . . . . . . . . . . . .
2.5.Measurement error . . . . . . . . . . . . . . .
3. Measurement method . . . . . . . . . . . . . . . .
4. Demo code . . . . . . . . . . . . . . . . . . . . . . . .
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . .
6. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . .
7. Revision history . . . . . . . . . . . . . . . . . . . .
1
2
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3
4
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5
5
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PWT introduction
2
PWT introduction
The PWT includes two external clock source input selects via the PWT_R1[PCLKS] bit, TIMER_CLK
and ALTCLK. The valid clock source is divided by a clock prescaler which is controlled by the
PWT_R1[PRE] bit. The maximum presacler is 128.
Input pulse is connected via PWT input pin from PWTIN[0] to PWTIN[3]. The PWT_R1[EDGE] bit is
used to select the trigger and capture mode.
When the PWT is enabled, it will start capturing the input pulse. The Positive Pulse Width (PPW) and
Negative Pulse Width (NPW) bit is available once the PWT_R1[PWTRDY] bit is set. That means the data
is ready. If PWT_R1[PWTIE] and PWT_R1[PRDYIE] are enabled, the pulse width data ready interrupt
will be produced when the PWT_R1[PWTRDY] bit is set.
2.1
Pulse input pins
The PWT module includes four input channels. The following table shows the connections of KE04,
KEA128, and KEA8 devices:
Table 1. PWT input connection
2.2
PWT input channel
KE04 connection
KEA128 connection
KEA8 connetction
0
PTC4
PTD5/PTE2
PTC4
1
PTB0
PTB0/PTH7
PTB0
2
ACMP0 output
ACMP0 output or ACMP1 output
ACMP0 output
3
ACMP1 output
UART0_RX, UART1_RX, or UART2_RX
ACMP1 output
PWT clock sources
Figure 1 shows the PWT clock source. One PWT clock source is from DIV3 and the other is from an
external TCLK input pin. The PWT_R1[PCLKS] bit selects the clock.
How to Use PWT Module on Kinetis E and Kinetis EA Series, Application Note, Rev. 1, 05/2014
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Freescale Semiconductor Inc.
PWT introduction
Figure 1. PWT clock source
2.3
Soft reset
The soft reset function is used to restart a capture of the input pulse at any time. PWT performs a soft reset
by writing 1 to the PWT_R1[PWTSR] bit.
For example, PWT_R1[EDGE] = 0x01. That is, it is configured to start a measurement from the first
rising-edge, and capture all rising-edge and falling-edge pulses. If the soft reset bit is set, the PWT counter
PWT_R2 [PWTC] is reset to 0, PWT_R1[PPW] and PWT_R2[NPW] are also reset to 0, and
PWT_R1[PWTOV] and PWT_R1[PWTRDY] are cleared. The 16-bit buffer of PWT counter is reset, the
PWT clock prescaler output is reset, the edge detection logic is reset, the capture logic is reset and the
latching mechanism of pulse width registers is also restarted. The PWT will be held until next rising-edge
of input pulse is coming, and capture all the falling-edge and rising-edge continuously. See Figure 2.
How to Use PWT Module on Kinetis E and Kinetis EA Series, Application Note, Rev. 1, 05/2014
Freescale Semiconductor Inc.
3
PWT introduction
Figure 2. PWT soft reset (PWT_R1[EDGE] = 0x01)
When the PWT module is working rapidly, the soft reset operation can be used to refresh PWT. Once the
soft reset is triggered, the user can configure a new PWT clock divider and input edge sensitivity bit. The
example of soft reset code is as follows:
PWT->R1 |= PWT_R1_PWTSR_MASK; /*trigger once soft reset*/
/* start measurement from first falling-edge, capture all falling-edge and rising-edge pulse */
PWT->R1 &= ~( PWT_R1_EDGE_MASK | PWT_R1_PRE_MASK;
PWT->R1 |= PWT_R1_EDGE(0x02) | PWT_R1_PRE(0x06); /*configure new clock prescaler and edge*/
2.4
Input edge capture
The PWT has the ability to decide to select first rising-edge or falling-edge to trigger PWT, and can capture
each of the edges using a corresponding control PWT_R1[EDGE] bit.
The PWT_R1[PWTRDY] bit is read ready flag. If the PWT_R1[PWTRDY] bit is set, the PPW and NPW
is updated and it is available. PWT_R1[PWTOV] bit is timer overflow flag.
2.5
Measurement error
Normally the accuracy increases following the bigger radio of PWT clock to input pulse. The total
measurement error is less than 1 PWT clock plus 1 bus clock.
If the user wants to test low-frequency input pulse, but PWT is only at 16-bit resolution, it is difficult to
increase the PWT clock infinitely. If the positive or negative level of input pulse is more than 0xFFFF PWT
clocks, the corresponding bit will overflow to 0. Therefore, the user has to compromise PWT clock select.
How to Use PWT Module on Kinetis E and Kinetis EA Series, Application Note, Rev. 1, 05/2014
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Measurement method
3
Measurement method
Normally the user can calculate the frequency of input pulse by reading PPW and NPW.
Figure 3. Input pulse capture
The frequency (Hz) is PWT Clock / (PPW ? NPW), PWT Clock = PWT clock source / presacler.
4
Demo code
The KE04_PWT_demo is developed in FRDM-KE04Z hardware platform under IAR EWARM V6.60. It
explains how to sample input pulse and calculate its frequency, the input pulse is on PTC4. The PWT clock
source is from TIMER_CLK, the user can short PTC4 and PTC5, or connect external input pulse to PTC4.
The PTC5 is 1 KHz toggle output using RTC interrupt.
The TRK-KEA128_PWT is developed in TRK-KEA128 hardware platform under Codewarrior 10.6. It
explains the procedure to sample input pulse and calculate its frequency, the input pulse is on PTD5. The
PWT clock source is from the bus clock, the user can short PTC5 and PTD5, or connect external input pulse
to PTD5. The PTC5 generates a 5 Hz toggle output.
5
Conclusion
The PWT module can be used to conveniently measure the width of input pulse including positive and
negative pulse, and can obtain instantaneous frequency of input pulse via a simple calculation.
6
Glossary
PWT Pulse Width Timer
PPW Positive pulse width
NPW Negative pulse width
How to Use PWT Module on Kinetis E and Kinetis EA Series, Application Note, Rev. 1, 05/2014
Freescale Semiconductor Inc.
5
Revision history
7
Revision history
Table 2. Revision history
Revision number
Date
Substantial changes
0
03/2014
Initial release
1
05/2014
Updated with Kinetis EA series
How to Use PWT Module on Kinetis E and Kinetis EA Series, Application Note, Rev. 1, 05/2014
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Freescale Semiconductor Inc.
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© 2014 Freescale Semiconductor, Inc.
Document Number: AN4839
Rev. 1
05/2014
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