1. EE 319K Introduction to Embedded Systems
Lecture 10: Sampling, Analog-to-Digital Conversion
Bard, Gerstlauer, Valvano, Yerraballi

2. Agenda Recap Outline Local Variables Stack frames Recursion
Fixed-point numbers
LCD device driver (Lab 7)
Outline
Sampling, Nyquist theorem
Analog to Digital Conversion
Bard, Gerstlauer, Valvano, Yerraballi

3. Analog to Digital Converter (ADC)
Bard, Gerstlauer, Valvano, Yerraballi

4. Nyquist Theorem
A bandlimited analog signal that has been sampled can be perfectly reconstructed from an infinite sequence of samples if the sampling rate fs exceeds 2fmax samples per second, where fmax is the highest frequency in the original signal.
If the analog signal does contain frequency components larger than (1/2)fs, then there will be an aliasing error.
Aliasing is when the digital signal appears to have a different frequency than the original analog signal.
Valvano Postulate: If fmax is the largest frequency component of the analog signal, then you must sample more than ten times fmax in order for the reconstructed digital samples to look like the original signal when plotted on a voltage versus time graph.
Bard, Gerstlauer, Valvano, Yerraballi

5. Sampling (option 1) 200Hz signal sampled at 2000Hz
Bard, Gerstlauer, Valvano, Yerraballi

6. Sampling (option 1) 1000Hz signal sampled at 2000Hz
Bard, Gerstlauer, Valvano, Yerraballi

7. Sampling (option 1) 2200Hz signal sampled at 2000Hz This is aliasing
Bard, Gerstlauer, Valvano, Yerraballi

8. Sampling (option 2) 100Hz signal sampled at 1600Hz
Bard, Gerstlauer, Valvano, Yerraballi

9. Sampling (option 2)
A signal with DC, 100Hz and 400Hz sampled at 1600Hz
Bard, Gerstlauer, Valvano, Yerraballi

10. Sampling (option 2) 1500Hz signal sampled at 1600Hz This is aliasing
Bard, Gerstlauer, Valvano, Yerraballi

11. Analog to Digital Converter (ADC)
Successive approximation ADC
VIN is approximated as a static value in a sample and hold (S/H) circuit
the successive approximation register (SAR) is a counter that increments each clock as long as it is enabled by the comparator
the output of the SAR is fed to a DAC that generates a voltage for comparison with VIN
when the output of the DAC = VIN the value of SAR is the digital representation of VIN
end of conversion
Bard, Gerstlauer, Valvano, Yerraballi

12. Sample-And-Hold Circuit
S/H
Analog Input (AI) is sampled when the switch is closed and its value is held on the capacitor where it becomes the Analog Output (AO)
Bard, Gerstlauer, Valvano, Yerraballi

13. ADC on TM4C123 Sampling Range/Resolution
3.3V internal reference voltage
0x000 at 0 V input
0xFFF at 3.3 V
resolution = range/precision
= 3.3V/4096 alternatives < 1mV
Bard, Gerstlauer, Valvano, Yerraballi

14. ADC on TM4C123 PE2=Ain1 used for Lab 8, 9, 10
Twelve analog input channels
Single-ended and differential-input configurations
On-chip internal temperature sensor
Sample rate up to one million samples/second Hz
Flexible, configurable analog-to-digital conversion
Four programmable sample conversion sequences from one to eight entries long, with corresponding conversion result FIFOs
Flexible trigger control
Controller (software) We will use software initiated trigger
Timers
Analog Comparators
PWM
GPIO
Hardware averaging of up to 64 samples for improved accuracy
Converter uses an internal 3V reference
Bard, Gerstlauer, Valvano, Yerraballi

15. ADC on TM4C123 PE2=Ain1 used for Lab 8, 9, 10 Ain1PE2
Software initiated
Use sequencer 3
Bit 3 is done flag
Bard, Gerstlauer, Valvano, Yerraballi

16. ADC on TM4C123 PD4=Ain4 used for DLL testing
PE2=Ain1 used for Lab 8, 9, 10
PE4=Ain9 used in book and ADCSWTrigger_4F120.zip
Twelve different pins can be used to sample analog inputs.
Bard, Gerstlauer, Valvano, Yerraballi

17. ADC on TM4C123 TM4C ADC registers
Bard, Gerstlauer, Valvano, Yerraballi

18. ADC on TM4C123 TM4C123 ADC Operation select rate select sequencer
select trigger
select channel
select sample mode
0 not temperature
1 set completion flag
1 end sequence
0 not differential
Speed bits in ADC0_PC_R
EM3, EM2, EM1, and EM0 bits in ADC_EMUX_R
ADC0_SSCTL3_R = 0x06;
Bard, Gerstlauer, Valvano, Yerraballi

19. ADC on TM4C123 Initialization
Enable ADC clock: set bit 0 in SYSCTL_RCGCADC_R
Set 125kHz ADC conversion speed: write 0x01 to ADC0_PC_R
Set sequencer priority: 0,1,2,3 in ADC0_SSPRI_R
Disable selected sequence 3: zero bit 3 of ADC0_ACTSS_R
Set software start trigger event: zero bits of ADC0_EMUX_R
Set input source (0-11): write channel number in bits 3-0 of ADC0_SSMUX3_R (channel 9 is PE4, channel 1 is PE2)
Set sample control bits: write 0110 in bits 3-0 ADC0_SSCTL3_R to disable temp measurement, notify on sample complete, indicate single sample in sequence, and denote single-ended signal mode
Disable interrupts: zero bit 3 of ADC0_IM_R
Enable selected sequencer 3: set bit 3 of ADC0_ACTSS_R
Bard, Gerstlauer, Valvano, Yerraballi

20. ADC on TM4C123
Channel 9 is PE4
void ADC0_InitSWTriggerSeq3_Ch9(void){
SYSCTL_RCGCGPIO_R |= 0x10; // 1) activate clock for Port E
while((SYSCTL_PRGPIO_R&0x10) == 0){};
GPIO_PORTE_DIR_R &= ~0x10; // 2) make PE4 input
GPIO_PORTE_AFSEL_R |= 0x10; // 3) enable alternate fun on PE4
GPIO_PORTE_DEN_R &= ~0x10; // 4) disable digital I/O on PE4
GPIO_PORTE_AMSEL_R |= 0x10; // 5) enable analog fun on PE4
SYSCTL_RCGCADC_R |= 0x01; // 6) activate ADC0
ADC0_PC_R = 0x01; // 7) configure for 125K
ADC0_SSPRI_R = 0x0123; // 8) Seq 3 is highest priority
ADC0_ACTSS_R &= ~0x0008; // 9) disable sample sequencer 3
ADC0_EMUX_R &= ~0xF000; // 10) seq3 is software trigger
ADC0_SSMUX3_R = (ADC0_SSMUX3_R&0xFFFFFFF0)+9; // 11) Ain9 (PE4)
ADC0_SSCTL3_R = 0x0006; // 12) no TS0 D0, yes IE0 END0
ADC0_IM_R &= ~0x0008; // 13) disable SS3 interrupts
ADC0_ACTSS_R |= 0x0008; // 14) enable sample sequencer 3 }
Book shows Ain9=PE Lab 8, 9, 10 use Ain1=PE2
Bard, Gerstlauer, Valvano, Yerraballi

21. ADC on TM4C123 Analog to digital conversion Set software trigger
Write to PSSI bit 3
Busy-Wait
Raw Interrupt Status = RIS bit 3
Poll until sample complete
Read sample
Read from SSFIFO3
Clear sample complete flag
Write to ISC bit 3
Bard, Gerstlauer, Valvano, Yerraballi

22. ADC on TM4C123 //------------ADC_InSeq3------------
// Busy-wait analog to digital conversion
// Input: none
// Output: 12-bit result of ADC conversion
uint32_t ADC0_InSeq3(void){
uint32_t data;
ADC0_PSSI_R = 0x0008;
while((ADC0_RIS_R&0x08)==0){};
data = ADC0_SSFIFO3_R&0xFFF;
ADC0_ISC_R = 0x0008;
return data; }
Bard, Gerstlauer, Valvano, Yerraballi