increase default ADC conversion rate; update example

Increased the default ADC conversion rate from 10 to 20 samples per second to eliminate a racing error where the conversion completion (Cycle Ready) flag wasn't consistently set. The error made it appear that the ADC data wasn't available. Successfully tested conversion rates up to 320 sps. Note: high sample rates are inherently less accurate but may be useful for some applications.

Fixed an error in the simpletest example was causing the example to continuously read the ADC data, ignoring the conversion completion (Cycle Ready) flag.

Both changes are expected to be transparent to all existing applications.

Author: CedarGroveStudios

cedargrove_nau7802.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2022 JG for Cedar Grove Maker Studios
+# SPDX-FileCopyrightText: Copyright (c) 2023 JG for Cedar Grove Maker Studios
 #
 # SPDX-License-Identifier: MIT
 """
@@ -85,8 +85,8 @@ class Gain:
 class ConversionRate:
     """ADC conversion rate settings."""
 
-    RATE_10SPS = 0x0  # 10 samples/sec; _CTRL2[6:4] = 0 (chip default)
-    RATE_20SPS = 0x1  # 20 samples/sec; _CTRL2[6:4] = 1
+    RATE_10SPS = 0x0  # 10 samples/sec; _CTRL2[6:4] = 0 (chip default, too slow)
+    RATE_20SPS = 0x1  # 20 samples/sec; _CTRL2[6:4] = 1 (driver default)
     RATE_40SPS = 0x2  # 40 samples/sec; _CTRL2[6:4] = 2
     RATE_80SPS = 0x3  # 80 samples/sec; _CTRL2[6:4] = 3
     RATE_320SPS = 0x7  # 320 samples/sec; _CTRL2[6:4] = 7
@@ -107,7 +107,7 @@ class NAU7802:
     def __init__(self, i2c_bus, address=0x2A, active_channels=1):
         """Instantiate NAU7802; LDO 3v0 volts, gain 128, 10 samples per second
         conversion rate, disabled ADC chopper clock, low ESR caps, and PGA output
-        stabilizer cap if in single channel mode. Returns True if successful."""
+        stabilizer cap if in single channel mode."""
         self.i2c_device = I2CDevice(i2c_bus, address)
         if not self.reset():
             raise RuntimeError("NAU7802 device could not be reset")
@@ -116,7 +116,7 @@ def __init__(self, i2c_bus, address=0x2A, active_channels=1):
         self.ldo_voltage = "3V0"  # 3.0-volt internal analog power (AVDD)
         self._pu_ldo_source = True  # Internal analog power (AVDD)
         self.gain = 128  # X128
-        self._c2_conv_rate = ConversionRate.RATE_10SPS  # 10 SPS; default
+        self._c2_conv_rate = ConversionRate.RATE_20SPS  # Custom default
         self._adc_chop_clock = 0x3  # 0x3 = Disable ADC chopper clock
         self._pga_ldo_mode = 0x0  # 0x0 = Use low ESR capacitors
         self._act_channels = active_channels
@@ -281,7 +281,7 @@ def reset(self):
         """Resets all device registers and enables digital system power.
         Returns the power ready status bit value: True when system is ready;
         False when system not ready for use."""
-        self._pu_reg_reset = True  # Reset all registers)
+        self._pu_reg_reset = True  # Reset all registers
         time.sleep(0.100)  # Wait 100ms; 10ms minimum
         self._pu_reg_reset = False
         self._pu_digital = True

examples/nau7802_simpletest.py

@@ -1,8 +1,8 @@
-# SPDX-FileCopyrightText: 2021, 2022 Cedar Grove Maker Studios
+# SPDX-FileCopyrightText: 2023 Cedar Grove Maker Studios
 # SPDX-License-Identifier: MIT
 
 """
-nau7802_simpletest.py  2022-04-23 1.0.1  Cedar Grove Maker Studios
+nau7802_simpletest.py  2023-01-12 2.0.0  Cedar Grove Maker Studios
 
 Instantiates both NAU7802 channels with default gain of 128 and sample
 average count of 100.
@@ -17,10 +17,9 @@
 
 
 def zero_channel():
-    """Initiate internal calibration for current channel; return raw zero
-    offset value. Use when scale is started, a new channel is selected, or to
-    adjust for measurement drift. Remove weight and tare from load cell before
-    executing."""
+    """Initiate internal calibration for current channel.Use when scale is started,
+    a new channel is selected, or to adjust for measurement drift. Remove weight
+    and tare from load cell before executing."""
     print(
         "channel %1d calibrate.INTERNAL: %5s"
         % (nau7802.channel, nau7802.calibrate("INTERNAL"))
@@ -29,9 +28,7 @@ def zero_channel():
         "channel %1d calibrate.OFFSET:   %5s"
         % (nau7802.channel, nau7802.calibrate("OFFSET"))
     )
-    zero_offset = read_raw_value(100)  # Read 100 samples to establish zero offset
     print("...channel %1d zeroed" % nau7802.channel)
-    return zero_offset
 
 
 def read_raw_value(samples=100):
@@ -64,7 +61,12 @@ def read_raw_value(samples=100):
 ### Main loop: Read load cells and display raw values
 while True:
     print("=====")
-    for i in range(1, 3):
-        nau7802.channel = i
+    nau7802.channel = 1
+    if nau7802.available():
+        value = read_raw_value()
+        print("channel %1.0f raw value: %7.0f" % (nau7802.channel, value))
+
+    nau7802.channel = 2
+    if nau7802.available():
         value = read_raw_value()
         print("channel %1.0f raw value: %7.0f" % (nau7802.channel, value))