Analog voltages with the 12-bit SAR ADC

Objectives

Read four potentiometers through the 12-bit SAR ADC and convert to volts and percent
Plot them as a scrolling scope and explain ADC resolution

Concepts

What the QWA309 base board gives you to practise with

The QWA309 base board of the TESAIoT Dev Kit gives you real hardware to practise with: push buttons, four potentiometers, a CAN transceiver and a header for external devices. This lesson uses Developer Hub exercises written specifically for this board. Both exercises in this lesson read the same four potentiometers (VR1–VR4 on P15.4–P15.7), but present them differently: the first as a numeric-plus-bar dashboard, the second as a scrolling oscilloscope-style chart.

What a 12-bit SAR ADC is, and what its numbers mean

The AUTANALOG SAR ADC is the PSoC Edge’s built-in successive-approximation analog-to-digital converter. It returns a 12-bit integer, 0–4095 (POT_ADC_FULL_SCALE = 4095), measured against a reference voltage (POT_ADC_VREF_MV = 1800) — that is 1.8 V, not 3.3 V. The resolution is therefore 1800 mV ÷ 4095 steps ≈ 0.44 mV per step, finer than the resolution at 3.3 V (≈ 0.81 mV) because the voltage range being divided is narrower. But finer resolution does not mean a steady reading — just a few millivolts of supply or reference noise already exceed 0.44 mV per step, so the raw value can be seen drifting by 1–3 counts even when the pot is untouched.

Initialising the ADC in the UI module’s own code, not the framework

Unlike the I2C bus, which the master template already opens (see lesson 1.1), reading the potentiometers requires initialising the SAR ADC entirely from scratch. pot_adc_init() first sets all four pins (P15.4–P15.7) to CY_GPIO_DM_ANALOG mode with Cy_GPIO_Pin_FastInit(), then calls Cy_AutAnalog_Init(&autonomous_analog_init) using the autonomous_analog_init config the BSP generates, followed by Cy_AutAnalog_Enable() and Cy_AutAnalog_StartAutonomousControl() to make the ADC convert continuously in the background. If Cy_AutAnalog_Init() fails (its return value does not equal CY_AUTANALOG_SUCCESS), neither exercise creates its polling timer, and the screen shows “ADC init failed” instead.

Converting the raw value to voltage and percentage with integer math

Pot Monitor’s update_channel() reads the result with Cy_AutAnalog_SAR_ReadResult(POT_ADC_INDEX, CY_AUTANALOG_SAR_INPUT_GPIO, channel->adc_channel), masks it with 0x0FFFU in case any stray bits arrive, then converts it with plain integer arithmetic (no float): millivolts = raw * 1800 / 4095 and percent_tenths = raw * 1000 / 4095 (in tenths of a percent, so one decimal digit can be printed without float). Example: raw = 2048 → 2048×1800/4095 = 900 mV (0.900 V) and 2048×1000/4095 = 500 (50.0 %). Integer division always truncates the remainder; it never rounds up.

What “Live” and “ADC settling” tell you — and what they do not

pot_timer_cb() checks Cy_AutAnalog_SAR_GetHSchanResultStatus(POT_ADC_INDEX) against POT_ADC_READY_MASK (the combined mask of all four GPIO0–GPIO3 channels). If all four channels’ bits are ready it shows “Live” in mint green; otherwise “ADC settling” in yellow. Notice that this function already called update_channel() to read and display all four channels in the loop above, without waiting for this status first. The “Live/settling” label is only a flag for whether the hardware conversion cycle has completed — not a gate on whether the on-screen numbers get updated.

A scrolling oscilloscope: point count × sample period = the time window shown

ADC Scope converts the raw value to percent more simply than Pot Monitor: pct = raw * 100 / SCOPE_FULL_SCALE (a plain integer 0–100, no decimal), then feeds it into the chart with lv_chart_set_next_value() on an lv_chart configured with lv_chart_set_update_mode(s_chart, LV_CHART_UPDATE_MODE_SHIFT) — this mode shifts old points off the left as new points arrive on the right. The chart holds SCOPE_POINTS = 100 points, sampled every SCOPE_PERIOD_MS = 60 ms, so the time window visible on the chart equals the point count times the sample period: 100 × 60 ms = 6 seconds. Halving the period to 30 ms with the same point count would only cover 3 seconds — more time detail, but a shorter visible history.

The VR1–VR4 names are not tied to the same index in both exercises

Pot Monitor’s pot_channels[] maps VR1 to result index 1 (P15.5) and VR2 to result index 0 (P15.4) — swapped from the order one would naturally expect. ADC Scope, on the other hand, defines s_ch[] = {0, 1, 2, 3} with names s_name[] = {"VR1","VR2","VR3","VR4"} in plain index order. The result: turning the very same pot on pin P15.4 moves the VR2 card in Pot Monitor but the VR1 trace in ADC Scope, even though both are reading SAR channel index 0. The name shown on screen, the physical pin on the board, and the result index in the code are three separate things that always need to be checked against each other — never trust one example’s on-screen name to match another’s directly.

Worked example

The QWA309 exercise set on the Developer Hub (pinned to commit e5c7722) runs only on the TESAIoT Dev Kit, because it uses hardware on the base board.

QWA309 — Potentiometer Monitor — reads four potentiometers (P15.4–P15.7) through the AUTANALOG 12-bit SAR ADC (Vref 1.8 V), showing each as a bar plus voltage and percentage in real time — the first exercise to use a real ADC on the TESAIoT Dev Kit README · code · Developer Hub
QWA309 — 4-Channel ADC Scope — plots all four potentiometers (P15.4–7, 12-bit SAR) as scrolling traces on a 0–100% LVGL chart — an analogue oscilloscope README · code · Developer Hub

The excerpts below are copied from the actual files at the same commit (Apache-2.0, tesaiot/developer-hub).

pot_monitor_ui.c — sets the pins to analog input, then starts the AUTANALOG SAR ADC:

static bool pot_adc_init(void)
{
    uint32_t init_status;

    pot_adc_init_pin(P15_4_PORT, P15_4_PIN);
    pot_adc_init_pin(P15_5_PORT, P15_5_PIN);
    pot_adc_init_pin(P15_6_PORT, P15_6_PIN);
    pot_adc_init_pin(P15_7_PORT, P15_7_PIN);

    init_status = Cy_AutAnalog_Init(&autonomous_analog_init);
    if (CY_AUTANALOG_SUCCESS != init_status)
    {
        return false;
    }

    Cy_AutAnalog_Enable();
    Cy_AutAnalog_StartAutonomousControl();

    return true;
}

pot_monitor_ui.c — converts the raw value to voltage and percentage with integer math:

raw = (uint16_t)Cy_AutAnalog_SAR_ReadResult(POT_ADC_INDEX,
                                            CY_AUTANALOG_SAR_INPUT_GPIO,
                                            channel->adc_channel);
raw &= 0x0FFFU;

millivolts = ((uint32_t)raw * POT_ADC_VREF_MV) / POT_ADC_FULL_SCALE;
percent_tenths = ((uint32_t)raw * 1000U) / POT_ADC_FULL_SCALE;
bar_value = ((uint32_t)raw * 1000U) / POT_ADC_FULL_SCALE;

pot_monitor_ui.c — VR1/VR2 swapped against the index order one would expect:

static pot_channel_t pot_channels[POT_COUNT] =
{
    { "VR1", "P15.5  ADC5", 1U, 0x14B8A6, NULL, NULL, NULL, NULL },
    { "VR2", "P15.4  ADC4", 0U, 0x22C55E, NULL, NULL, NULL, NULL },
    { "VR3", "P15.6  ADC6", 2U, 0xF59E0B, NULL, NULL, NULL, NULL },
    { "VR4", "P15.7  ADC7", 3U, 0xF43F5E, NULL, NULL, NULL, NULL },
};

adc_scope_ui.c — reads every channel and feeds it into the SHIFT-mode scrolling chart:

static void scope_timer_cb(lv_timer_t *timer)
{
    (void)timer;
    if (!s_adc_ok) { return; }
    for (uint8_t i = 0U; i < SCOPE_CH; i++) {
        uint16_t raw = (uint16_t)Cy_AutAnalog_SAR_ReadResult(SCOPE_ADC_INDEX,
                          CY_AUTANALOG_SAR_INPUT_GPIO, s_ch[i]) & 0x0FFFU;
        uint32_t pct = ((uint32_t)raw * 100U) / SCOPE_FULL_SCALE;
        lv_chart_set_next_value(s_chart, s_series[i], (int32_t)pct);
        lv_label_set_text_fmt(s_val[i], "%s %lu%%", s_name[i], (unsigned long)pct);
    }
}

Common mistakes

Assuming the reference voltage is 3.3 V — this SAR ADC uses POT_ADC_VREF_MV = 1800 (1.8 V). Assuming 3.3 V instead breaks every voltage conversion from the raw value.
Treating a 1–3 count jitter as a broken ADC — at a resolution of roughly 0.44 mV per step, just a few millivolts of noise is enough to make the reading drift; that is normal, and is reduced with a moving average, not a symptom of damage.
Comparing the VR1–VR4 names across exercises without checking the real index — Pot Monitor and ADC Scope map the names to SAR indices differently (VR1/VR2 are swapped); always check pot_channels[]/s_ch[] in the code, or the board’s schematic.
Waiting for the “Live” label before trusting the on-screen numbers — the Live/ADC settling label only reports whether the hardware conversion cycle has completed; the code already updates the on-screen numbers every tick regardless of what the label says.

Build and flash

# In the master template folder (see lesson 1.1)
# 1) Delete the old episode's files in proj_cm55/apps/
# 2) Copy all of this episode's files into proj_cm55/apps/
make build
make program     # flash through KitProg3

Try a change

Guess before you change anything: pick one value the example’s README explains in the How section, and write down what you expect to change on the screen or in the log.
Change and run: build + flash, then compare against your guess. If it does not match, find which part you misunderstood.
Extend: add one thing the example does not yet have, and keep a photo or video in your portfolio.

Check your understanding

With a 12-bit ADC at Vref 1.8 V, how many millivolts does each step resolve?
Why does the reading jitter slightly even when the pot is not turned?

The answers are in the example’s README and in the code. If you cannot answer one, go back and read the Why / What / How section again.

References

All TESAIoT Dev Kit exercises · commit e5c7722
The code belongs to the Developer Hub and is referenced by link, not copied into this repository

Review questions

Answer on your own first, then open the answer.

The raw ADC value is 2048. What voltage and percentage does Pot Monitor show? (Objective 1)
1. 1.650 V · 50.0 %
2. 0.900 V · 50.0 %
3. 0.900 V · 25.0 %
4. 2.048 V · 50.0 %
Show answer

Answer: B. 0.900 V · 50.0 %

update_channel() ใช้ millivolts = raw × 1800 / 4095 = 900 (หารแบบจำนวนเต็ม) และ percent_tenths = raw × 1000 / 4095 = 500 จึงแสดง 0.900 V และ 50.0 % แรงดันอ้างอิงคือ 1.8 V ไม่ใช่ 3.3 V
A 12-bit ADC with a 1.8 V reference resolves roughly how many millivolts per step? (Objective 2)
1. 0.81 mV
2. 1.76 mV
3. 7.0 mV
4. 0.44 mV
Show answer

Answer: D. 0.44 mV

1800 mV / 4095 ≈ 0.44 mV ต่อขั้น ตัวเลือกอื่นคือความละเอียดเมื่อใช้ 3.3 V (≈ 0.81 mV) ADC 10 บิต (≈ 1.76 mV) และ 8 บิต (≈ 7 mV) ที่ 1.8 V ความละเอียดสูงไม่ได้แปลว่าค่าจะนิ่ง เพราะ noise เพียงไม่กี่ mV ก็ขยับค่าหลายขั้นแล้ว
The pot is untouched but the raw value keeps moving by one to three counts. Which explanation is right? (Objective 2)
1. เป็นเรื่องปกติ: noise ของแหล่งจ่าย แรงดันอ้างอิง และ quantization เพียงไม่กี่ mV ก็มากกว่า 0.44 mV ต่อขั้น ลดได้ด้วยการเฉลี่ยหลายตัวอย่าง
2. ADC เสีย
3. เพราะ lv_timer ทำงานไม่ตรงเวลา
4. เพราะหน้าจอรบกวน ADC ต้องปิดจอ
Show answer

Answer: A. เป็นเรื่องปกติ: noise ของแหล่งจ่าย แรงดันอ้างอิง และ quantization เพียงไม่กี่ mV ก็มากกว่า 0.44 mV ต่อขั้น ลดได้ด้วยการเฉลี่ยหลายตัวอย่าง

README ของ Pot Monitor แนะนำให้เพิ่ม moving average ใน update_channel() เพื่อลด noise ของค่า raw การแกว่ง 1–3 ขั้นคือไม่ถึง 1.5 mV ถือว่าเล็กมากสำหรับงานอ่าน pot
ADC Scope samples every SCOPE_PERIOD_MS = 60 with SCOPE_POINTS = 100. If the period becomes 30 ms, how long a time window does the chart show? (Objective 2)
1. 6 วินาที
2. 12 วินาที
3. 3 วินาที
4. 1.5 วินาที
Show answer

Answer: C. 3 วินาที

LV_CHART_UPDATE_MODE_SHIFT เลื่อนจุดเก่าออกทีละจุด หน้าต่างเวลา = จำนวนจุด × คาบเวลา เดิม 100 × 60 ms = 6 วินาที เป็น 100 × 30 ms = 3 วินาที เห็นรายละเอียดทางเวลามากขึ้นแต่เห็นประวัติสั้นลง
Turning the pot on P15.4 moves the VR2 card in Pot Monitor but the VR1 line in ADC Scope. Why? (Objective 1)
1. สายต่อหลวม
2. ชื่อบนจอเป็นแค่การแมปในซอฟต์แวร์: Pot Monitor แมป VR2 → result index 0 (P15.4) ส่วน ADC Scope แมป VR1 → index 0 สองตัวอย่างจึงตั้งชื่อช่องเดียวกันไม่ตรงกัน ต้องเทียบกับ schematic ของบอร์ด
3. ADC สลับช่องเองตามลำดับการอ่าน
4. สองตัวอย่างใช้ ADC คนละตัว
Show answer

Answer: B. ชื่อบนจอเป็นแค่การแมปในซอฟต์แวร์: Pot Monitor แมป VR2 → result index 0 (P15.4) ส่วน ADC Scope แมป VR1 → index 0 สองตัวอย่างจึงตั้งชื่อช่องเดียวกันไม่ตรงกัน ต้องเทียบกับ schematic ของบอร์ด

pot_channels[] ใน Pot Monitor กำหนด VR1 = P15.5 (index 1) และ VR2 = P15.4 (index 0) ส่วน ADC Scope ใช้ s_ch[] = {0, 1, 2, 3} กับชื่อ VR1–VR4 ตามลำดับ ทั้งคู่อ่าน SAR index 0 ตัวเดียวกัน ชื่อที่ผู้ใช้เห็น ขาจริง และ index ของผลลัพธ์ เป็นสามสิ่งที่ต้องตรวจให้ตรงกันเอง

Cite this lesson

If you teach from this lesson or reuse it in slides or documents, credit it with the text below. If you changed it, add (adapted) after the title.

"Analog voltages with the 12-bit SAR ADC" from TESA Open Knowledge by the Thai Embedded Systems Association (TESA), https://github.com/tesaiot/tesa-qualification-program, licensed under CC BY-NC 4.0

Thai attribution: "อ่านแรงดันอนาล็อกด้วย SAR ADC 12 บิต" จาก TESA Open Knowledge โดยสมาคมสมองกลฝังตัวไทย (Thai Embedded Systems Association: TESA) https://github.com/tesaiot/tesa-qualification-program สัญญาอนุญาต CC BY-NC 4.0

Lesson link: https://tesaiot.github.io/tesa-qualification-program/en/courses/tesaiot-firmware-stack/m04-qwa309-hardware/l02-analog-and-adc/

This lesson adapts the source below; keep its credit too.
https://github.com/tesaiot/developer-hub/blob/e5c772252e7d20f715463e0d27df9ece4e569c38/prac_qwa309_pot_monitor · Code stays in the Developer Hub and is linked at pinned commits, never copied: the episodes, practice codes and main-branch examples are Apache-2.0; the master template and the OPTIGA client carry Infineon/Cypress EULAs.

Full guide: how to cite TESA

Content is licensed CC BY-NC 4.0. Reuse it non-commercially and credit the Thai Embedded Systems Association (TESA) every time. · How to cite TESA