Wirnet™ iStation: Power supply management

The Wirnet iStation gateway includes an internal backup battery allowing to detect power loss.

Level of battery charge can be monitored through VCC_BACKUP ADC sensor. This backup battery is charged on main power called 12V_POE so taking into account hardware implementation, maximum value of VCC_BACKUP is around 11.8V (main 12V - 0.2V).

In the following example, a few time after a cold boot, we can see that Backup battery is charging (5.9V instead 11.8V).

root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +12.03 V  
VCC_BACKUP:   +5.93 V

As examples, power loss could be detected by these 2 methods (to be adapted depending on the application and use-case):

1. basic detection: polling the value of the 12V_POE ADC every second. When it falls under 10V (confirmed with 5 consecutive reads), application must launch power-down sequence.
2. early detection: polling the value of the 12V_POE and VCC_BACKUP. If VCC_BACKUP value is greater than 12V_POE, it means that main power is now supplied by backup battery. Application can launch power-down sequence.

The first thing to do in power-down sequence is to power down lora modem as soon as possible to disable its power consumption.

The voltage values can be obtained with the following methods:

root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +12.02 V  
VCC_BACKUP:  +11.89 V 

Examples with power loss early detected

root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +12.02 V  
VCC_BACKUP:  +11.89 V 
root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +11.35 V  
VCC_BACKUP:  +11.75 V  
root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +11.17 V  
VCC_BACKUP:  +11.57 V  
root@klk-wiis-05005E:~# sensors | egrep 'POE|BACKUP'
12V_POE:     +11.12 V  
VCC_BACKUP:  +11.51 V  

or, in C with lmsensors library:

#include <sensors/sensors.h>
 
static int get_sensor_voltage(const char * pName, double * pVoltage)
{
  const sensors_chip_name *chip;
  const sensors_feature *feature;
  const sensors_subfeature * subfeature;
  int chip_nr, i;
  char *label = NULL;
  double voltage = 0;
  int result = -1;
  int ret;
 
  ret = sensors_init(NULL);
  if (ret != 0) {
    syslog(LOG_ERR, "[%s:%d] err %d in sensors_init\n", __FUNCTION__, __LINE__, ret);
    return result;
  }
 
  chip_nr = 0;
  while ((chip = sensors_get_detected_chips(NULL, &chip_nr))) {
    i = 0;
    while ((feature = sensors_get_features(chip, &i))) {
      label = sensors_get_label(chip, feature);
      if (label != NULL) {
        subfeature = sensors_get_subfeature(chip, feature, SENSORS_SUBFEATURE_IN_INPUT);
        if ((strcmp(pName, label) == 0) && (subfeature != NULL)) {
          if (sensors_get_value(chip, subfeature->number, &voltage) == 0) {
            *pVoltage = voltage;
            result = 0;
          }
        }
        free(label);
        label = NULL;
      }
    }
  }
  sensors_cleanup();
  return result;
}
 
double voltage = 0;
 
get_sensor_voltage("12V_POE", &voltage);

All internal voltage measurement can be obtained using the following tool:

root@klk-wiis-05005E:~# sensors 
klk_wiis_adc-isa-0000
Adapter: ISA adapter
VDD_CORE:     +1.39 V  
NVCC_DRAM:    +1.34 V  
VCC_DIG_1V8:  +1.85 V  
NVCC_3V3:     +3.29 V  
12V_POE:     +12.03 V  
VCC_BACKUP:  +11.53 V  
VCC_5V:       +4.94 V  
VCC_RF:       +1.93 V