An introduction to electronic weighing
Filtering Data
Before displaying the data, some sort of filtering is usually required that removes fluctuations in the weight display caused by noise and vibration, but filtering comes at the cost of response time. The raw samples from the ADC take 100ms to acquire, so any form of low-pass filtering will slow down the response time into the region by fractions of a second. In the case of a workshop balance, a couple of seconds for the reading to settle is not a problem, but in other cases (e.g., a batch-weighing application), every second adds to production time and therefore costs money. Filtering and ultimate accuracy will always be a trade-off. In this application, 10 samples are simply averaged before the result is passed for display (Listing 3).
Listing 3
Averaging ADC Readings
01 #define SAMPLES 10 02 03 int32_t samples[SAMPLES]; 04 int32_t average = 0; 05 uint8_t sample = 0; 06 07 // main acquisition loop 08 while (true) 09 { 10 samples[sample++] = ADS1232_Read(); 11 if (sample == SAMPLES) 12 { 13 sample = 0; 14 } 15 16 average = 0; 17 for (uint8_t i = 0; i < SAMPLES; i++) 18 { 19 average += samples[i]; 20 } 21 22 average /= SAMPLES; 23 24 // pass the averaged data to the display here 25 }
Practical Precision
In terms of practical precision, you must keep a few things in mind. Of the theoretical 24 bits of precision available from the ADC, the datasheet itself admits that only 19 bits are useful at the highest gain setting. The load cell I have chosen has a maximum output of 4mV (1mV/V, with an excitation of 4V), so it uses only one fifth of the available range (20mV) of the ADC. That's about 100,000 counts. Practically, I have found I can achieve stable readings to 10,000 counts. For a 100g scale, that means one count is equivalent to 10mg. The current software limits that further, giving a resolution of 100mg (i.e., 1,000 counts), which seems more than adequate for the requirements of a general-purpose laboratory scale. There is certainly scope for better filtering and other techniques to improve the precision, but at some point the limitations of the (cheap) load cell will become the limiting factor.
Display Driver
The display has a three-wire serial interface. The chip select (CS) line must be asserted before data can be written, and once that is done, data is presented on the data (DAT) line, being clocked into the chip by asserting the write (WR) line and then negating it (Listing 4). The interface is too slow to keep up with the microcontroller at full speed, so busy-wait loops are necessary to slow the WR line pulses down to just over 3µs.
Listing 4
Writing a Single Bit to the Display
01 /** */ 02 static void LCD_WriteBit(bool bit) 03 { 04 // set the data line 05 HAL_GPIO_WritePin(LCD_DAT_GPIO_Port, LCD_DAT_Pin, bit); 06 07 // assert the WR line 08 HAL_GPIO_WritePin(LCD_NWR_GPIO_Port, LCD_NWR_Pin, 0); 09 10 // busy wait 11 for (uint8_t i = 0; i < 50; i++) 12 ; 13 14 // negate the WR line 15 HAL_GPIO_WritePin(LCD_NWR_GPIO_Port, LCD_NWR_Pin, 1); 16 17 // busy wait 18 for (uint8_t i = 0; i < 50; i++) 19 ; 20 }
The display chip has a small amount of internal memory, and the individual bits in this memory correspond to the different segments in the display characters, as well as decimal points and the battery state indicator. Each transaction with the chip is called a command, and the first three bits of that command determine the type of command. The rest of the transaction comprises an address in the internal memory and the data to be written.
The process of writing a complete digit involves conversion from the required ASCII digit to the memory bit pattern to illuminate the display correctly. This code is not complex, but quite long, so I won't include it here. I refer the interested reader to the source code on my GitHub page [11].
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Direct Download
Read full article as PDF:
Price $2.95
News
-
The 14" Pinebook Pro Linux Laptop is Shipping
After a considerable delay, the 14" version of the Pinebook Pro laptop is, once again, available for purchase.
-
OpenMandriva Lx ROME Technical Preview Released
OpenMandriva’s rolling release distribution technical preview has been released for testing purposes and adds some of the latest/greatest software into the mix.
-
Linux Mint 21 is Now Available
The latest iteration of Linux Mint, codenamed Vanessa, has been released with a new upgrade tool and other fantastic features.
-
Firefox Adds Long-Anticipated Feature
Firefox 103 has arrived and it now includes a feature users have long awaited…sort of.
-
System76 Refreshes Their Popular Oryx Pro Laptop with a New CPU
The System76 Oryx Pro laptop has been relaunched with a 12th Gen CPU and more powerful graphics options.
-
Elive Has Released a New Beta
The Elive team is proud to announce the latest beta version (3.8.30) of its Enlightenment-centric Linux distribution.
-
Rocky Linux 9 Has Arrived
The latest iteration of Rocky Linux is now available and includes a host of new features and support for new architecture.
-
Slimbook Executive Linux Ultrabook Upgrading Their CPUs
The Spanish-based company, Slimbook, has made available their next generation Slimbook Executive Linux ultrabooks with a 12th Gen Intel Alder Lake CPU.
-
Fedora Linux is Coming to the Raspberry Pi 4
Thanks to significant work in the upstream, the upcoming release of Fedora 37 will introduce support for the Raspberry Pi 4.
-
New Linux Ultrabook from TUXEDO Computers
TUXEDO Computers has released a new 15" Ultrabook running Linux.