Transcript Embedded Systems - Ulster University
Embedded Systems
Lecture 5: PIC32 development board, Timers ADC and interrupts,
Ian McCrum Room 5B18, Tel: 90 366364 voice mail on 6 th Email: [email protected]
Web site: ring http://www.eej.ulst.ac.uk
Timers
PIC32MX1xx/2xx family have 5 (6 counting the internal core timer) Core Timer you can just read the 32 bit number that increments automatically There is a Plib function ReadCoreTimer(void); that returns an unsigned int.
It runs at half the system frequency. Note figure 8-1 from the datasheet
Timers type A and type B;
TIMER1: Type A a 16 bit counter that has a PR1 register allowing generating a trigger, either when overflow occurs or when the count equals the value held in PR1. It can use the system clock (actually the Peripheral clock PBCLK) either direct or divided by 1,8,16 or 64. Or it can use an external clock, either an input or an external crystal Relevant register is T1CON. The trigger condition raises the T1IF Flag
Timers type B; (Timers 2,3 and Timers 4,5)
TMRx is even – timer 2 or 4 TMRy is odd – timer 3 or 5 X is the master timer, y is slave
Code for type A – 16 bit Timer1 - see next slide for T1CON again…
Using Interrupts with Timer 1 (plib calls)
Try this code and then do the exercises at the end…
Using the Analogue to Digital Convertor (ADC)
The 10-bit Analog-to-Digital Converter (ADC) includes the following features: • Successive Approximation Register (SAR) conversion • Up to 1 Msps conversion speed • Up to 13 analog input pins • External voltage reference input pins • One unipolar, differential Sample and Hold Amplifier (SHA) • Automatic Channel Scan mode • Selectable conversion trigger source • 16-word conversion result buffer • Selectable buffer fill modes • Eight conversion result format options • Operation during Sleep and Idle modes 11.1.2 CONFIGURING ANALOG AND DIGITAL PORT PINS The ANSELx register controls the operation of the analog port pins. The port pins that are to function as analog inputs must have their corresponding ANSEL and TRIS bits set.
ANSELA on Our PIC
Overview of Operation
Analog sampling consists of two steps: acquisition and conversion Sample start time can be controlled in software by setting the SAMP bit (AD1CON1<1>) or controlled automatically by the ASAM bit (AD1CON1<2>) Conversion time is the time required for the ADC to convert the voltage held by the SHA. The ADC requires one ADC clock cycle (TAD) to convert each bit of the result, plus two additional clock cycles. Therefore, a total of 12 TAD cycles are required to perform the complete conversion. When the conversion time is complete, the result is written into one of the 16 ADC result registers (ADC1BUF0 through ADC1BUFF). The sum of the acquisition time and the analog-to-digital conversion time provides the total sample time The conversion trigger source SSRC<2:0> (AD1CON1<7:5>) ends the sampling time and begins an ADC conversion. Taken from a variety of hardware sources, or can be controlled manually in software by clearing the SAMP bit.
The Auto-Sample mode and auto-conversion trigger can be used together to provide endless automatic conversions without software intervention.
The ADC can optionally switch between MUX A and MUX B configurations between conversions. The ADC can also optionally scan through a series of analog inputs using a single MUX.
Note, inputs can be single or differential, we can use external Vref + and The DP32 board has AN9 wired to a Variable Resistor AN9 is RB15/RPB15, pin 26 on the 28 pin device Very powerful buffer system
Note, inputs can be single or differential, we can use external Vref + and The DP32 board has AN9 wired to a Variable Resistor AN9 is RB15/RPB15, pin 26 on the 28 pin device
So you can repetitively sample a selected range of channels, the values are stored in the buffer registers An interrupt may be generated at the end of each sample sequence or multiple sample sequences as determined by the value of the SMPI<3:0> bits (AD1CON2<5:2>). The number of sample sequences between interrupts can vary between 1 and 16. The user should note that the analog-to-digital conversion buffer holds the results of a single conversion sequence.
ANSELB=0x0080 The DP32 board has AN9 wired to a Variable Resistor AN9 is RB15/RPB15, pin 26 on the 28 pin device Redo this code for AN9
ANSELB = 0x0080 The DP32 board has AN9 wired to a Variable Resistor AN9 is RB15/RPB15, pin 26 on the 28 pin device Redo this code for AN9
Bit 15 ON 1 = ADC is operating Bit 13 SIDL only relevant in idle mode Bit 10-8 FORM<2:0> 000 = 16 bit Data out – bottom 10 bits are the reading Bit 7-5 SSRC<2:0> 000 = clearing SAMP bit ends sampling and starts conversion Bit 4 CLRASAM 0 = normal, buffer get overwritten by next conversion sequence Bit 2 ASAM 1 = sampling begins as soon as last conversion complete 0 = Sampling begins when SAMP bit is set Bit 1 SAMP 1 = ADC sample and hold Amp is sampling 0 = ADC sample and hold Amp is holding When ASAM = 0, writing ‘1’ to this bit starts sampling When SSRC = 000, put ‘0’ in SAMP -ends sampling & starts conversion Bit 0 DONE 1 = ADC conversion is done
Bit 15-13 VCFG<2:0> 000 = Vrefh = AVDD and Vrefl = Avss (3.3V and 0V) Bit 12 OFFCAL 0 = disable input offset calibration Bit 10 CSNA 0 = do not scan inputs {1 = scan inputs} Bit 7 BUFS only valid if BUFM is ‘1’; 1 = ADC is filling buffer 8-F, you read 0-7 0 = ADC is filling buffer 0-7, you read 8-F Bit 5-2 SMPI<3:0> 0000 = Interrupt at completion of conversion for each sample Bit 1 BUFM 0 = Buffer configures as one 16 word buffer ADC1BUFF to ADC1BUF0 1 = Buffer configures as two 8 word buffers Bit 0 ALTS 0 = Use Sample A input multiplexor settings {1=>alternate sample A/B)
bit 15 ADRC: ADC Conversion Clock Source bit 1 = Clock derived from FRC 0 = Clock derived from Peripheral Bus Clock (PBCLK) bit 12-8 SAMC<4:0>: Auto-Sample Time bits
bit 31 CH0NB 1 = Channel 0 negative input is AN1 0 = Channel 0 negative input is VREFL bit 27-24 CH0SB<3:0>: Positive Input Select bits for Sample B 1111 = Channel 0 positive input is Open 1100 = Channel 0 positive input is AN12 bit 23 • • • 0001 = Channel 0 positive input is AN1 0000 = Channel 0 positive input is AN0 CH0NA: Negative Input Select bit for Sample A Multiplexer Setting 1 = Channel 0 negative input is AN1 0 = Channel 0 negative input is VREFL bit 19-16 CH0SA<3:0>: Positive Input Select bits for Sample A Mux Setting 1111 = Channel 0 positive input is Open 1110 = Channel 0 positive input is IVREF 1101 = Channel 0 positive input is CTMU temperature (CTMUT) 1100 = Channel 0 positive input is AN12 • • • 0001 = Channel 0 positive input is AN1 0000 = Channel 0 positive input is AN0
http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/adc The 10-bit ADC can be split into two sections:
Acquisition
and
Conversion
.
Manual Mode Configuration
In manual mode, the programmer must make an individual request every time the ADC starts a sample. In the version presented here, the acquisition is started manually by setting AD1CON1bits.SAMP HIGH .
Conversion will then start automatically after an amount of time specified in the AD1CON3 SFR. When acquisition is finished, the SAMP bit will go LOW. When conversion is done, the AD1CON1bits.DONE
bit will go HIGH. The result will be stored in one of the
ADC1BUFx
registers.
Since we’re only requesting the analog value of one pin at a time in this manual mode, the result is always stored in the first buffer register or ADC1BUF0.
The AD1CHS is the channel select SFR. Of importance to us is that
AD1CHS<16:19>
control which pin is being input to the ADC.
With this information, we can make a simple function that takes as input an analog pin number and returns the 10-bit analog voltage as an int from 0-1023 as follows.
To configure for this mode, we set the conversion to trigger automatically after acquisition is done by setting the in this register.
SSRC bits found at AD1CON1<5:7> . Choosing manual mode is also found In AD1CON3 , we will set the source of the ADC clock, how long a period (TAD) is, and how many periods of this clock per acquisition. Conversion is always 12 TAD cycles long. We’ll set the analog clock period to be four times the peripheral bus clock period which will be equal to SYSCLK. TAD = 4*TPB. At FPB = 50MHz, TAD = 4*TPB = 80ns. To be on the safe side, we’ll configure the acquisition period as 15*TAD = 1.2us. Thus, the entire analog-to-digital conversion takes 27*TAD = 2.16us. The configuration is shown below.
Automatic Scan Configuration
In the automatic scan mode we’ll be using, the ADC peripheral will be sampling and converting a specified number of analog pins as long as the ADC has power. Whenever the processor then requests the 10-bit voltage representation of some of these pins, the ADC peripheral will give the most recent completed conversion.
With this method, ADC call latency is severely reduced! Once the ADC is configured and running, every additional pin request is as simple as a single assignment line. However, this method uses slightly more power as the ADC is constantly sampling and converting voltages. Also, while the ADC call time is very short, there is a minimum time between calls to make sure new analog voltages have been sampled.
We’re only talking a few microseconds and even this can be reduced by careful planning of the ADC clock (TAD) and how many cycles are necessary for sampling in your particular application!
Please read the link below if you wish to understand this further http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/adc
Working ADC Code on the DP32; using AN9(RB15) and LEDs 3-0(RB0-3)
Modified from http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/adc
Modified from http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/adc
Modified from http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/adc
Exercises
1. Add a timedelay so that a reading is taken every 1/100 th second.
2. Add a buffer so that data is continually filling a 1024 ring buffer 3. Output on the LEDs how full the buffer is 4. Convert the ADC routine so that it fills the buffer under interrupts and the only code is main loops continually checking how full the buffer is, These exercises prepare you for the mini-project.