Lab 6 - Threshold Scan with SciSDK on the DT1260

In Threshold Discriminator with Event Counter Lab (Lab 3), we developed a firmware on the DT1260 that includes:

• A threshold register (THRESHOLD) for an analog comparator.
• A counter (COUNTS) that increments each time the analog input crosses the threshold.
• A reset register (RESET) to clear the counter.

Now, we want to scan (sweep) the threshold value over a range, measure how many counts occur at each threshold (over a fixed “dwell time”), and then plot the resulting curve.

1. Concept Overview

1. Threshold Scan

   • We define a start threshold (THRS_MIN), an end threshold (THRS_MAX), and a step (THRS_STEP).
   • For each threshold in that range:
     1. Set the THRESHOLD register.
     2. Reset the counter (write 1, then 0 to RESET).
     3. Wait for a dwell time (e.g., 1 second).
     4. Read the COUNTS register to see how many events occurred at that threshold.
   • Append the (threshold, counts) data to arrays/lists for plotting.

2. Plot the Data

   • Use Matplotlib to plot counts (y) versus threshold (x).
   • A typical result might show fewer counts at higher thresholds, assuming the signal amplitude remains the same.

2. Python Script Example

Below is a complete Python script (lab6_threshold_scan.py) illustrating this approach:

# lab6_threshold_scan.py
#
# Scans the threshold register from THRS_MIN to THRS_MAX in steps of THRS_STEP,
# acquiring counts for DWELL_TIME seconds at each threshold. Plots the resulting data.

import time
import matplotlib.pyplot as plt
from scisdk.scisdk import SciSDK
from scisdk.scisdk_defines import *

# -------------------------------
# 1) User Parameters
# -------------------------------
THRS_MIN = 2000   # Start threshold
THRS_MAX = 2500   # End threshold
THRS_STEP = 10    # Step size
DWELL_TIME = 1    # Time in seconds to count at each threshold

# -------------------------------
# 2) Initialize SciSDK and Device
# -------------------------------
sdk = SciSDK()

# Replace "usb:10500" with your board's actual serial number.
# Also ensure you have the matching JSON file from Lab 3 firmware compilation.
result = sdk.AddNewDevice("usb:10500", "dt1260", "./RegisterFile.json", "board0")
if result != 0:
    print("Error adding device:", sdk.s_error(result))
    exit()

print("DT1260 connected.")

# -------------------------------
# 3) Prepare Arrays to Store Data
# -------------------------------
threshold_values = []
count_values = []

# -------------------------------
# 4) Perform the Threshold Scan
# -------------------------------
print("Starting threshold scan...")
for threshold in range(THRS_MIN, THRS_MAX + 1, THRS_STEP):

    # 4a) Set threshold register
    set_result = sdk.SetRegister("board0:/Registers/THRESHOLD", threshold)
    if set_result != 0:
        print("Error setting threshold:", sdk.s_error(set_result))
        continue  # skip this threshold

    # 4b) Reset the counter: write 1, then 0
    reset_result = sdk.SetRegister("board0:/Registers/RESET", 1)
    if reset_result == 0:
        sdk.SetRegister("board0:/Registers/RESET", 0)
    else:
        print("Error resetting counter:", sdk.s_error(reset_result))
        continue

    # 4c) Dwell time: wait for the counter to accumulate events
    time.sleep(DWELL_TIME)

    # 4d) Read the counter
    err, counts = sdk.GetRegister("board0:/Registers/COUNTS")
    if err != 0:
        print("Error reading counts:", sdk.s_error(err))
        counts = 0  # fallback if error

    # 4e) Store the result
    threshold_values.append(threshold)
    count_values.append(counts)

    # Print status
    print(f"Threshold = {threshold}, Counts = {counts}")

print("Threshold scan completed.")

# -------------------------------
# 5) Plot the Results
# -------------------------------
plt.figure("Threshold Scan")
plt.plot(threshold_values, count_values, marker="o", linestyle="-", color="b")
plt.title("Counts vs. Threshold")
plt.xlabel("Threshold (ADC units)")
plt.ylabel("Counts")
plt.grid(True)
plt.show()

# -------------------------------
# 6) Cleanup
# -------------------------------
sdk.DetachDevice("board0")
print("Device detached. Script finished.")

Code Explanation

1. User Parameters:

   • THRS_MIN = 2000, THRS_MAX = 2500, THRS_STEP = 10
   • DWELL_TIME = 1 second

2. AddNewDevice:

   • We connect to the DT1260 using the SciSDK, referencing the JSON file from Lab 3’s firmware output.
   • Adjust the USB serial string ("usb:10500") to match your hardware.

3. Threshold Scan:

   • A for loop increments the threshold in steps of 10 (range(2000, 2501, 10) includes 2500).
   • For each threshold:
     • Set THRESHOLD.
     • Reset the counter by writing 1 then 0 to RESET.
     • Wait 1 second.
     • Read the current COUNTS value.
     • Store (threshold, counts) in arrays.

4. Plot:

   • We use matplotlib to show a line plot of counts vs. threshold.
   • Typically, you’ll see that as the threshold increases, the number of counts might decrease (fewer signals are above a higher threshold).

3. Running the Script

1. Install the needed Python packages:

   pip install scisdk matplotlib
   

2. Run:

   python lab6_threshold_scan.py
   

3. Observe the console output for each threshold step, followed by a Matplotlib window showing the final plot.

4. Interpreting Results

• If your signal amplitude rarely exceeds 2200, you might see higher counts at thresholds below 2200 and lower counts above 2200.
• This measurement can help you find an optimal threshold for a given signal, balancing noise vs. valid events.

5. Further Considerations

• Counting Rate: If your signal is very frequent, you might consider shorter dwell times.
• Averaging: You could perform multiple acquisitions per threshold and average the counts if needed.
• Firmware Variations: Ensure THRESHOLD, RESET, and COUNTS register paths match exactly how you named them in the Lab 3 design.

That’s it! You now have a simple script that automates scanning the threshold in your DT1260 firmware, measuring event counts, and visualizing the results in a straightforward Python workflow.