IC: RTL8721Dx RTL8720E RTL8726E RTL8730E

Introduction

This chapter describes the usage of the 12-bit analog-to-digital converter (ADC).

ADC is a successive-approximation register (SAR) ADC and the conversion result of ADC sample is stored in the result register and can be read on demand.

ADC characteristics are as follows:

• Resolution: 12 bit

• ENOB: 10 bit

• Sample Frequency: 31.25 kHz ~ 250 kHz, 166.67 kHz is recommended (default)

• Sample Capacitance: 2.4 pF ~ 2.93 pF

• Reference Voltage: Fixed, cannot be externally supplied

• Internal Impedance: About 500 kΩ

• FIFO size: 64 x 16 bit

• Channel switch list length: 1 ~ 16

• Description of channels:

RTL8721DxRTL8720ERTL8726ERTL8730E

Channel	ADC channel ID	Pin name	Input voltage (V)
Normal channel	CH0~CH6	PB13~PB19	0~3.3
BAT_MEAS channel	CH7	BAT_MEAS	0 ~ 5

Note

ADC multi-channel sample is time-division multiplexing on the same circuit. The sample frequency value listed above is specific to a single channel. When multiple channels are set in the channel switch list, the sample frequency will decrease accordingly.

ADC Sample Mode

RTL8721DxRTL8720ERTL8726ERTL8730E

ADC only supports sample mode: Continuous Mode. In this mode, ADC samples the channels in the channel list continuously and sequentially. Users can start it or stop it at any time. It is suitable for continuous voltage sampling over short periods.

ADC supports multi-channel sample. Users can configure channel switch list, including channel list length and channel switch order.

ADC works as: each time a trigger signal is generated, ADC will sample all the channels in channel switch list in order.

To use ADC continuous mode, the following steps are mandatory.

1. Enable clock and function of ADC module.

   RCC_PeriphClockCmd(APBPeriph_ADC, APBPeriph_ADC_CLOCK, ENABLE);
   RCC_PeriphClockCmd(APBPeriph_CTC, APBPeriph_CTC_CLOCK, ENABLE);
   

2. Configure ADC pinmux according to the pinmux specification.

   For example, call the following functions to use ADC0.

   Pinmux_Config(ADC_CH0_PIN, PINMUX_FUNCTION_xxx); // Refer to pinmux table to get function ID
   PAD_InputCtrl(ADC_CH0_PIN, DISABLE);
   PAD_PullCtrl(ADC_CH0_PIN, GPIO_PuPd_NOPULL);
   PAD_SleepPullCtrl(ADC_CH0_PIN, GPIO_PuPd_NOPULL);
   

   Note

   Step2 could be skipped when user wants to use BAT_MEAS pin.

3. Initialize ADC parameters and modify ADC mode. ADC samples all the channels sequentially by default.

   ADC_StructInit(ADC_InitTypeDef *ADC_InitStruct);
   ADC_InitStruct->ADC_OpMode = ADC_AUTO_MODE; // Set auto mode
   

4. Channel list and other parameters can be modified in ADC_InitStruct if needed.

   For example, user sets channel switch list length to 3 and makes ADC sample ADC0/ADC2/ADC4 in order.

   ADC_InitStruct->ADC_CvlistLen = 3 - 1; // Set channel switch list length
   ADC_InitStruct->ADC_Cvlist[0] = ADC_CH0; // Set the 1st channel ID
   ADC_InitStruct->ADC_Cvlist[1] = ADC_CH2; // Set the 2nd channel ID
   ADC_InitStruct->ADC_Cvlist[2] = ADC_CH4; // Set the 3rd channel ID
   

5. Initialize ADC and enable ADC.

   ADC_Init(ADC_InitTypeDef *ADC_InitStruct);
   ADC_Cmd(ENABLE);
   

6. Read ADC sample data. Users can acquire ADC sample data in polling mode or interrupt mode.

   • Polling mode:

     ADC_ReceiveBuf(u16 *pBuf, u32 len);
     

   • Interrupt mode:

     ADC_INTConfig(ADC_BIT_IT_FIFO_OVER_EN | ADC_BIT_IT_FIFO_FULL_EN, ENABLE);
     InterruptRegister((IRQ_FUN)ADCIrqHandle, ADC_IRQ, NULL, INT_PRI_MIDDLE);
     InterruptEn(ADC_IRQ, INT_PRI_MIDDLE);
     ADC_AutoCSwCmd(ENABLE);
     

   Where ADCIrqHandle is defined as

   u32 ADCIrqHandle(void *para)
   {
      (void *)para;
      u32 status = ADC_GetISR();
   
      if (status & ADC_BIT_IT_FIFO_FULL_STS) {
         while (ADC_Readable()) {
            u32 sample_value = ADC_Read(); // Read out sample value
            if (sample_cnt ++ > MAX_SAMPLE_CNT) { // Get enough sample value
               ADC_AutoCSwCmd(DISABLE);
               ADC_INTConfig(ADC_BIT_IT_FIFO_OVER_EN | ADC_BIT_IT_FIFO_FULL_EN, DISABLE);
               InterruptDis(ADC_IRQ);
               break;
            }
         }
      }
   
      ADC_INTClearPendingBits(status);
      return 0;
   }
   

Note

ADC sample result consists of 4-bit Channel ID and 12-bit sample data. User could get them by the following macros:

• ADC_GET_CH_NUM_GLOBAL(sample_data): Get 4-bit channel ID

• ADC_GET_DATA_GLOBAL(sample_data): Get 12-bit sample value

ADC Calibration

ADC Voltage Calibration

• To improve the linearity of ADC input/output characteristics, the chip undergoes nonlinear calibration during factory programming. This calibration reduces ADC gain errors and offset errors.

• The ADC calibration employs a 5-point curve fitting method, calculating calibration parameters A/B/C based on the principle of minimum variance. After calibration, each chip stores its unique parameters A/B/C in OTP memory.

• After obtaining ADC sampled values, calculate the current voltage (in mV) using the following formula, where \(x\) represents the ADC sampled value:

  \[\begin{split}\begin{aligned} y &= ax^2 + bx + c \\ &= \frac{A}{2^{26}}x^2 + \frac{B}{2^{15}}x + \frac{C}{2^{6}} \end{aligned}\end{split}\]

• Normal channels and BAT_MEAS channel have different calibration parameters. Users can directly retrieve voltages via the following APIs:

  • ADC_GetVoltage(u32 chan_data): Get normal voltage value.

  • ADC_GetVBATVoltage(u32 vbat_data): Get battery voltage value.

ADC Internal Resistor Calibration

• Variations in the ADC internal resistor R among chips affect measurement accuracy:

  

  ADC internal resistor

  • Ideal voltage: \(V_{ideal} = \frac{V_{cc} \times R_{2}}{R_{1} + R_{2}}\)

  • Actual voltage: \(V_{actual} = \frac{V_{cc} \times R_{2}//R}{R_{1} + R_{2}//R}\)

  • Relationship: Due to voltage division of internal resistor, \(V_{actual} < V_{ideal}\)

  • Conclusion: Larger \(\frac{R_{1} \times R_{2}}{R}\) leads to greater deviation between actual and ideal voltages.

• Users can directly retrieve internal resistor value via ADC_GetInterR()

ADC Digital Comparator

The ADC includes a digital comparator that generates an interrupt when the sampled value of a specified channel falls within a configured range. Users can perform specific operations (e.g., reading the current voltage) in the interrupt handler.

Digital comparator can work well under all the ADC sample modes.

Digital comparator supports the following comparison modes:

• Below Low Threshold

• Above High Threshold

• Within Range: Greater than or equal to low threshold AND less than or equal to high threshold

• Outside Range: Less than low threshold OR greater than high threshold

Note

Low threshold and high threshold values must be in the range 0 to 4095. Low threshold should not be greater than high threshold.

For example, if user wants to use ADC digital comparator under continuous sample mode, the following steps are mandatory.

1. Enable clock and function of ADC module as shown above.

2. Configure ADC pinmux according to the pinmux specification as shown above.

3. Assign a channel and configure the low/high thresholds and comparison mode of digital comparator.

   For example, call the function below to set comparison mode of ADC0. An interrupt will arise when the sampled value is between 2000 and 3000.

   ADC_SetComp(ADC_CH0, 3000, 2000, ADC_COMP_WITHIN_THL_AND_THH);
   InterruptRegister((IRQ_FUN)ADCIrqHandle, ADC_IRQ, NULL, INT_PRI_MIDDLE);
   InterruptEn(ADC_IRQ, INT_PRI_MIDDLE);
   

   Where ADCIrqHandle is defined as

   u32 ADCIrqHandle(void *para)
   {
      (void)para;
      u32 status = ADC_GetISR();
      u32 value;
   
      if (status & ADC_BIT_IT_COMP_ALL_STS) {
         printf("ISR:0x%x, Comp status:0x%x\n", status, ADC->ADC_COMP_STS); // ADC sample data is in predefined range
         while (ADC_Readable() == 0);
         value = ADC_Read();
      }
      ADC_INTClearPendingBits(status);
      return 0;
   }
   

4. Initialize ADC parameters and modify ADC mode as shown above.

5. Channel list and other parameters can be modified in ADC_InitStruct if needed.

6. Initialize ADC and enable ADC as shown above.

7. It is suggested to read ADC sample result by polling mode.

ADC Troubleshooting

Sample Offset

Phenomenon	Sample offset
Cause	Calibration parameters are not applicable. Mismatched input impedance.
Solution	Calibrate again. Adjust the input impedance, making sure internal resistance of the signal source is much lower than that of ADC.

Note

• Calibrate the whole application circuit together if possible.

• Refer to the hardware design specification to obtain ADC internal impedance.

Channel Interference

Phenomenon	Channel interference
Cause	Mismatched input impedance leads to poor driving capability of input source. Too high sample frequency.
Solution	Reduce the internal resistance of the signal source. Reduce sample frequency.

Sampling Voltage Interference

Phenomenon	Sampling voltage interference
Cause	Unreasonable PCB layout
Solution	Check PCB layout, such as location of power supplies and high-speed signals, etc. Taking mean value of multi-point samples is suggested.