HT8 MCU Timer Module Application Note (2) Generating Infrared Remote Control Carrier Waves in the PWM Mode

Transcription

1 HT8 MCU Timer Module Application Note (2) Generating Infrared Remote Control Carrier Waves in the PWM Mode D/N: AN0445E Introduction The Holtek HT8 MCU series provide various types of timer modules, such as the Compact type TM, the Standard type TM and the Periodic type TM, which all have their own unique characteristics. In practical applications, PWM signals generated by the TM modules can be used in areas such as motor power control, heating control or illumination control. This application note will introduce another way of using PWM signals which is for infrared signal transmission applications. The application will show how to generate the desired 38kHz carrier wave signal used by infrared remote controls using the CTM PWM mode. The inclusion of a practical application will help users to understand and skillfully use the CTM function. Functional Description Infrared Transmission Introduction Infrared forms one of the invisible parts of sunlight, which was discovered in 1800 by German-British scientist William Herschel, who split sunlight using a prism. By placing thermometers under the different colours, in an attempt to measure the different colour heating effects, he discovered that just beyond the red end of the visible spectrum, a higher temperature was measured. Although invisible to the human eye but due to this effect being near the red end of the visible spectrum it was named infrared. The Infrared radiation wavelength is longer than that of visible light, therefore it is invisible and often used as a transmission media. The infrared wavelength extends from 0.75 to 1000 micrometers, which can be separated into three parts, namely near-infrared with a wavelength which extends from 0.75 to 1.50µm, mid-wavelength infrared with a wavelength which extends from 1.50 to 6.0µm and far-infrared whose wavelength extends from 6.0 to 1000µm. In actual circuit applications, an IR LED will be required if the data is transmitted by means of infrared. This component, which when supplied with a suitable forward bias AN0445E V / 10 February 20, 2017

2 voltage, will generate a current and stimulate infrared light radiation. The common infrared emission diode wavelengths are 850nm, 870nm, 880nm, 940nm 980nm, etc. Infrared Remote Control Signal Composition Carrier Wave Signals Carrier waves are waves which can be modulated in order to transmit information signals. Common infrared transmission carrier signal frequencies are normally within a range of 38kHz~56kHz. Data is transmitted after being modulated onto the transmitter. The receiver should use a frequency selective amplifier circuit which operates at the same frequency as the transmitter for optimal anti-interference performance. Infrared Remote Control Signal Encoding Format Presently, the most widely used infrared remote control encoding methods are NEC Protocol Pulse Width Modulation and the Philips RC-5 Protocol Pulse Position Modulation. In this application note the NEC protocol will be taken as an example to show how to generate a group of infrared remote control signals. The NEC infrared remote control signal encoding format characteristics are listed as follows: 1. Uses a 38kHz carrier wave frequency 2. The preamble code interval is 9ms + 4.5ms 3. Uses a 16-bit user code 4. Uses an 8-bit data code and an 8-bit data inverse code 5. 1-bit stop bit The remote control signal encoding format is composed of a preamble code, the user code, the data code as well as the data inverse code for data verification. Preamble code User code (High Byte) User code (Low Byte) Data code Data inverse code Fig 1. Signal Encoding Format Preamble code User code 8 bits (High Byte) User code 8 bits (Low Byte) Data code 8 bits Data inverse code 8 bits Stop bit 1 bit Fig 2. Encoding Time AN0445E V / 10 February 20, 2017

3 Carrier Wave Signal Format - H/L Ratio 60% (about 15.9µs) 40% (about 10.1µs) 38kHz (about 26µs) Fig 3. 38kHz Carrier Wave Signal DATA "0" and DATA "1" Formats Each bit in the user code or the data code can be either '0' or '1', which can be distinguished using the pulse time. Such an encoding method is called Pulse Position Modulation which is abbreviated to PPM. 0.56ms 0.56ms 1.125ms Bit ms Bit 1 Fig 4. DATA "0" and DATA "1" Formats Operational Principles Compact Type TM Basic Structure Description As the simplest form of the TM types, the Compact type TM can implement three operating modes, which are Compare Match Output, Timer/Event Counter and PWM Output modes, using an external input signal TCKn and an output signal TPn. The CTM core is composed of a count-up counter and two internal comparators with the names, Comparator A and Comparator P. The count-up counter can be driven by an internal clock or an external TCKn pin. The output signal TPn status is determined by comparing the count values in the Comparator A and Comparator P with the CCRA and CCRP register values. As the following figure shows, the counting values are compared with the 16-bit CCRA register value and the CCRP high byte register value. When the counter overflows, the TnON bit changes from low to high or a comparator match event occurs, which is determined by the operating mode, the counter will be automatically cleared. AN0445E V / 10 February 20, 2017

4 CCRP TCKn fsys/4 fsys fh/16 fh/64 f TB C fh/ TnCK2~TnCK0 TnON TnPAU 8-bit Comparator P b8~b15 16-bit Count-up Counter b0~b15 16-bit Comparator A Comparator P Match Counter Clear 0 1 TnCCLR Comparator A Match TnPF Interrupt TnOC Output Control Polarity Control TnM1, TnM0 TnPOL TnIO1, TnIO0 TnAF Interrupt Pin Control TnCP TPn CCRA Fig 5. TM-CTM Basic Structure Note:For the actual count-up counter bit size, Compare A and Compare P registers, refer to the selected MCU datasheet. PWM Output Mode The most common PWM functions are for motor control, heating control and illumination control applications. The CTM can be setup to operate in the PWM mode by configuring the TnM[1:0] bit field to 10, which will then allow a PWM signal to be generated on the TPn pin. In the PWM mode, the period and duty is adjustable. Taking the HT66F0185 as an example, it can be configured using the TnDPX bit. This bit allows the period to be determined by the 16-bit CCRA while the duty is determined by the CCRP high byte or vice versa. The TnOC bit in the TMnC1 register determines the PWM waveform polarity. The TnIO[1:0] bit field is used to enable the PWM output on the TPn pin or to setup the TPn to be either a fixed output logic high or low. The PWM output polarity will be inverted or non-inverted by configuring the TnPOL bit. Counter Value CCRP CCRA Counter cleared by CCRP Pause Resume TnDPX = 0; TnM [1:0] = 10 Counter Stop if TnON bit low Counter Reset when TnON returns high Time TnON TnPAU TnPOL CCRA Int. Flag TnAF CCRP Int. Flag TnPF TM O/P Pin (TnOC=1) TM O/P Pin (TnOC=0) PWM Duty Cycle set by CCRA PWM Period set by CCRP Output controlled by other pin-shared function PWM resumes operation Output Inverts when TnPOL = 1 Fig 6. CTM PWM Waveform AN0445E V / 10 February 20, 2017

5 Application Circuits U1 KEY PA PA VSS&AVSS VDD PC0/OSC1 PB0/INT0 PC1/OSC2 PB1/INT1 PC2 PB2 PA0/ICPDA TCK1 PC3/SDO PD3 PC4/SDI PD2/TX PC5/SCK PD1/RX PC6/SCS PD0 PA1 PA5/VREFI PA2/ICPCK PA6/VREF PA3 TP1 PB6 TP2 PB5 PB C KHz +5V R1 1K +5V R2 39R 39Ω LD Q1 C805J1 PA0 4 PA V 1 0ICP(OCDS) HT66F0185 Fig 7. HT66F0185 Infrared Transmission Circuit Taking the HT66F0185 as an example, the above figure shows a circuit that generates a 38kHz infrared carrier wave signal using the CTM PWM mode. Pin TP2 is the PWM signal output pin, which can be used to control the transistor Q1 through the transistor base current-limiting resistor, R1. Resistor R2 is the IR LED current-limiting resistor. The general IR LED forward voltage, V f, is about 1.3V and the C8050 transistor collector-emitter saturation voltage drop is about 0.3V. As a result the R2 voltage drop is (5V-1.3V-0.3V), namely 3.4V, resulting in a diode operating current of about 3.4V/39Ω=87mA. Note that the infrared emission diode wavelength λ is 940nm, while the operating current is 100mA. Software Design This application note takes the HT66F0185 as an example to implement a CTM timer module function which will transmit a group of infrared codes each time the key is pressed. Details are as follows. Infrared carrier wave signal frequencies generally range from 30kHz to 60kHz, where a 35kHz square wave is commonly used, determined by the transmitter 455kHz crystal. The transmitter crystal requires an integer frequency division, in this case a division factor of 12 is requird. The frequency is 455kHz kHz 38kHz. The HT66F0185 CTM is used to generate the 38kHz infrared carrier wave signal. The system frequency is defined as 16MHz which is also used as the CTM clock source. The CCRA register is selected to be used as the PWM period compared register with a register value of 416, thus the PWM period =16MHz/416=38kHz, namely about 26µs. The CCRP register is used to define the duty. As the CCRP available value comes from the highest 8 bits, when the CCRP register value is 01, the PWM duty = (1/16MHz) 15.9µs AN0445E V / 10 February 20, 2017

6 The control register configurations for the HT66F0185 Compact type timer module which generates the PWM carrier wave signal are shown below: Register Operating Bit Function Setup Description Bit 7: T2PAU: Counter pause control bit When set to 0, the counter is in 0: Run the running state 1: Pause TM2C0 TM2C1 Bit 6~4: T2CK2~T2CK0: Select TM1 counter clock bit 000: f SYS /4 001: f SYS 010: f H /16 011: f H /64 100: f TBC 101: f H /8 110: TCKn rising edge clock 111: TCKn falling edge clock Bit 3: T2ON: TM2 counter On/Off control bit 0: Off 1: On Bit 2~0: Unimplemented, read as "0" Bit 7~ 6: T2M1~T2M0: Select TM2 operating mode 00: Compare Match Output Mode 01: Undefined 10: PWM output mode 11: Timer/counter mode Bit 5~4: T2IO1~T2IO0: Select TP2 output function PWM mode 00: PWM output inactive state 01: PWM output active state 10: PWM output 11: Undefined Bit 3: T2OC: TM2 TP2 output control bit PWM mode 0: Active low 1: Active high Bit 2: T2POL: TM2 TP2 output polarity control bit 0: Non-invert 1: Invert Bit 1: T2DPX: TM2 PWM period/duty control bit 0: CCRP period; CCRA - duty 1: CCRP duty; CCRA - period Bit 0: T2CCLR: Select TM1 counter clear condition 0: Comparator P match 1: Comparator A match Set this field to 001 to select the f SYS as the TM1 counter clock When set to 1, the counter is on Set to "10" to select the PWM output mode Set to "10" to select PWM output When set to "1", the PWM signal is active high Set to "0" Set to "0" Set to "1" for Comparator A match Software Design Flowchart Start Initialisation (I/O port, PTM, CTM, variables) No Check if key is pressed? Yes Transmit a group of infrared data Fig 8. System Flowchart AN0445E V / 10 February 20, 2017

7 CTM Intialisation Set the bit 2 in the TMPC to 1, select the PB3 as TP2 output Setup 38kHz Duty by configuring the TM2RP register value Setup 38kHz period by configuring the TM2RAL/TM2RAH register value Setup the TM2C0 and TM2C1 control registers Turn on the TM, the 38kHz PWM will be output on the TP2(PB3) pin END Fig 9. CTM PWM Generation Software Flowchart Transmit a group of infrared data Transmit preamble code (Carrier signal 9ms, low level signal 4.5ms) Transmit user code 16 bits (The actual value is 0x5555 ) Transmit data code 8 bits (The actual value is 0xaa) Transmit data inverse code 8 bits (The actual value is 0x55) Transmit stop bit (1 bit stop bit) End Fig 10. Transmitting a group of NEC Codes - Software Flowchart AN0445E V / 10 February 20, 2017

8 Start No PA.1==0? Yes Delay for 20ms No Yes PA.1==0? Yes If Key Hold Pressed? No No key pressed Set Key Hold Press flag, Set Key OK flag Start Fig 11. Key Scanning Flowchart Remote Control Signal Output Waveform Preamble code User code 16 bits 0x5555 Data code 8 bits 0xaa Data inverse code 8 bits 0x55 Stop bit Fig 12. Infrared Transmission Waveform AN0445E V / 10 February 20, 2017

9 38kHz Carrier Wave Signal Fig kHz Carrier Wave Signal Waveform Program Examples Test-38KHz with KEY-asm.rar Test-38KHz with KEY-c.rar Conclusion This application note has shown how to configure the counter, output pin, PWM period and duty in the PWM operating mode of the CTM to help users understand and utilise the specific TM functions. Reference Files Reference file: HT66F0175/185 Datasheet For more information, refer to the Holtek official website Version and Modification Information Date Author Issue 王贊臣 (Jasenwang) First Version AN0445E V / 10 February 20, 2017

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