An introduction to electronic weighing

Storing Calibration Parameters

The design provides no non-volatile storage for persistent data such as calibration parameters. In a larger system, it is customary to have an external SPI flash memory or EEPROM chip to provide this function. The current design uses a small portion (one page, 1KB) of the microcontroller's program flash memory as non-volatile storage (Listing 5), and you can tell the linker to exclude a portion of this flash memory from program memory (usually a page at the highest address available). That block can then be labeled in the linker file so that it is accessible from program code, and primitives in the STM32 libraries can erase pages and read/write bytes or words from that area. This technique saves the expense of an external memory chip and the I/O pins that would be required to drive it. Listing 6 from the linker file shows how to reserve a portion of flash memory for parameter storage.

Listing 5

Nonvolatile Storage Code

01 /*
02  * nvdata.c
03  *
04  *  Created on: 19 Mar 2021
05  *      Author: andrew
06  */
07
08 #include "main.h"
09
10 extern uint32_t _nvdata[1024]; // start of non-volatile data (last block of main flash) defined in linker file
11
12 /** */
13 void NVDATA_SaveCalibrationParams(uint32_t span, uint32_t zero)
14 {
15   HAL_FLASH_Unlock();
16
17   FLASH_EraseInitTypeDef eraseInit;
18
19   eraseInit.TypeErase = FLASH_TYPEERASE_PAGES;
20   eraseInit.NbPages = 1;
21   eraseInit.PageAddress = (uint32_t)&_nvdata[0];
22
23   uint32_t sectorError;
24   printf("erasing flash page at %p\n", &_nvdata[0]);
25   HAL_StatusTypeDef res = HAL_FLASHEx_Erase(&eraseInit, &sectorError);
26
27   if (res != HAL_OK)
28   {
29     printf("FLASH erase error %d\n", res);
30   }
31
32   res = HAL_FLASH_Program(FLASH_TYPEPROGRAM_WORD, (uint32_t)&_nvdata[0], span);
33
34   if (res != HAL_OK)
35   {
36     printf("FLASH program error %d\n", res);
37   }
38
39   res = HAL_FLASH_Program(FLASH_TYPEPROGRAM_WORD, (uint32_t)&_nvdata[1], zero);
40
41   if (res != HAL_OK)
42   {
43     printf("FLASH program error %d\n", res);
44   }
45
46   HAL_FLASH_Lock();
47 }
48
49 /** */
50 void NVDATA_LoadCalibrationParams(uint32_t *span, uint32_t *zero)
51 {
52   *span = _nvdata[0];
53   *zero = _nvdata[1];
54 }

Listing 6

Linker File Extract

01 /* Entry Point */
02 ENTRY(Reset_Handler)
03
04 /* Highest address of the user mode stack */
05 _estack = ORIGIN(RAM) + LENGTH(RAM);  /* end of "RAM" Ram type memory */
06 _nvdata = ORIGIN(NVDATA);
07
08 _Min_Heap_Size = 0x200 ;  /* required amount of heap  */
09 _Min_Stack_Size = 0x400 ; /* required amount of stack */
10
11 /* Memories definition */
12 MEMORY
13 {
14   RAM    (xrw)    : ORIGIN = 0x20000000,  LENGTH = 4K
15   FLASH  (rx)     : ORIGIN = 0x8000000,   LENGTH = 15K //reduce flash by 1K
16   NVDATA (rx)     : ORIGIN = 0x8003c00,   LENGTH = 1K //define the nv region
17 }
18
19 /* Sections */
20 SECTIONS
21 {
22  ### ommited ###
23
24   .nvdata : // give the nv region a name so it can be referenced from code
25   {
26     KEEP(*(.nvdata))
27   } >NVDATA
28
29  ### ommited ###
30 }

Calibration

The design has two push buttons for calibration: one for tare and one for span. In the main acquisition loop (Listing 7), the code looks for push-button events. If the tare button is pushed, the current reading is stored and subtracted from subsequent readings to provide a zero output. Similarly, if the span button is pushed, the required span factor is calculated and stored (in this case, the code assumes a 100g calibration weight has been placed on the scale).

Listing 7

Calibration

01 int32_t zero = 0;
02 int32_t span = 1;
03
04 // main acquisition loop
05 while (true)
06 {
07   // get averaged data here
08
09   if (sw1PushedEvent())
10   {
11     zero = average;
12     NVDATA_SaveCalibrationParams(span, zero);
13   }
14
15   if (sw2PushedEvent())
16   {
17     raw = (average - zero) * 100;
18
19     span = raw / 1000;
20
21     NVDATA_SaveCalibrationParams(span, zero);
22   }
23
24   display = ((average * 100 - zero * 100) / span);
25
26   LCD_PrintNumber(display, 0);
27 }

Completed Unit

I used FreeCAD to design an enclosure for the completed instrument and had it printed by a local 3D printing house. FreeCAD is another open source package, and every time I return to FreeCAD, I find the developers have made another step increase in functionality. It really is an exemplary open source project. The STEP files created were sent directly to a local 3D print house and were printed in just a few days.

I purchased a suitable stainless steel pan with a tare weight of just 50g, and the completed assembly can be see in Figure 8. I've left off the front panel to show the mounting arrangement of the PCB and the display. The load cell is mounted behind, with a vertical riser coming up through an aperture in the enclosure. A small circular plastic plate is attached to that, on which the pan sits.

Figure 8: The 3D-printed enclosure with the front panel removed.

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