Cell module source code simplified:

State machine

/* Main program entry point. */
int main(void)
{  
  /* CPUINT init */
  /* PORT init */
  /* VREF init */
  /* ADC init */
  /* AC init */
  /* USART init */
  /* TCA init */
  /* EVSYS init */
  
  /* Load settings from the EEPROM and USERROW*/
  /* Init. global variables. */
  
  /* RTC init */
  
  /* FIFO buffers init */
  
  /* Update bootloader flag, set to 0xEA to stay in the application. */
  
  /* WDT init */
  
  /* Main state machine */
  for(;;) 
  {
    /* Read new command byte. */
    uint8_t ch = readSave();
    
    if(ch == CMD_ALARM_CODES)
    {
      /* Handle command */
    }
    else if(ch == CMD_GOTO_BOOTLOADER)
    {
      /* Handle command */
    }
    else
    {
      /* Unknown command, respond with error code. */
      /* Delete received command, we dont want to forwad bad commands. */
    }
    
    /* Forward command, no need if this is the last module. */
  }
}

uint8_t readSave(void)
{
  /* Wait for a new byte received. */
  uint8_t ch = readWait();
  /* Save byte (ch) to the command buffer. */
  /* Return received byte (ch). */
  return ch;
}

uint8_t readWait(void)
{
  /* Do the work while no new data was received. */
  do 
  {
    /* Wait for the real time conter compare event; every  */
    if((RTC_INTFLAGS & RTC_CMP_bm))
    {
      /* Do the work */
      task(&settings);
    }
  }
  while(!FIFOavailable(&RXfifo));
  
  /* New data received, now return the received data. */
  return FIFOread(&RXfifo);
}

Task function performs all the main operations of the cell module BMS.

void task(tSettings *p)
{
  /* Sample two external inputs. */
  /* Sample internal temperature sensor. */
  
  /* Check for overvoltage condition */
  /* Check for charged condition */
  /* Check for empty condition */
  /* Check for undervoltage condition */
  /* Check for overtemperature condition */
  /* Balance cell, if needed apply balancing current. */
  
  /* Send all the alarms, if any occurred. */
  
  /* If undervoltage condition go to sleep.
   * else if empty condition or sleep requested go to standby mode.
   */
  
  /* Call watchdog timer. */
}

All the received commands are saved in a buffer. Once the command has been handled buffer with the command is forwarded to the next module. But if this is the bottom module commands don’t need to be forwarded.

void forwardBufferIfNotBot(void)
{
  /* Is this the bottom module? */
  if(!settings.moduleNum)
  {
    /* Delete received command, we dont want to forwad the command. */
  }
  else
  {
    /* Forward received command. */
  }
}

Communication

void respond(uint8_t data)
{
  /* Is this the bottom module? */
  if(!settings.moduleNum)
  {
    /* Calculate parity bit. */
  }
  
  /* Wait for UART to be ready to send data. */
  
  /* Is this the bottom module? */
  if(!settings.moduleNum)
  { 
    /* Set ninth data bit as parity bit, this converts from 9O1 to 8E2. */
  }
  else
  {
    /* Set ninth data bit to zero, this indicates data is a response. */
  }
  
  /* Send data */
}

void forward(uint8_t data)
{
  /* Wait for UART to be ready to send data. */
  /* Set ninth data bit to one, this indicates data is a command. */
  /* Send data */
}

ISR(AC0_AC_vect)
{
  /* Clear int. flag */
  /* Set invert enable signal depending on AC state to trigger UART receiver. */
}

ISR(USART0_RXC_vect)
{
  /* Read UART status registers. */
  
  /* Has frame or parity error occurred? */
  if(status & (USART_FERR_bm | USART_PERR_bm))
  {
    /* Create frame, parity alarm. */
    /* Send all the alarms. */
    /* Something went wrong, reset itself. */
  }
  /* Has receive complete interrupt occurred? */
  else if(status & USART_RXCIF_bm)
  {
    /* Is this a command? */
    if((status & 0x01) | settings.topModule)
    {
      /* Save the command, so it can be handled outside the ISR. */
    }
    /* This is a respone(not a command). */
    else
    {
      /* Respond with the data. */
    }
  }
}

Voltage measurement

ISR(ADC0_RESRDY_vect)
{
  /* Convert new ADC sample to voltage measurement. */
}

Sleep

void goToSleep(uint8_t mode)
{
  /* Make measure interval short, so we measure as soon as we wake up. */
  /* Wait for any outgoing transmitions to finish. */
  /* Enable PIN1 interrupt (wake up source). */
  /* Enable sleep mode. */
  /* Connect AC output to the PIN3 (UART signal change triggers PIN3 interrupt.)*/
  /* Go to sleep. */
  /* Clear the real time counter compare int. flag (wake up source). */
}

ISR(PORTA_PORT_vect)
{
  /* Disconnect AC output to the PIN3 (needs to be done as soon as possible). */
  /* Disable PIN3 interrupt (wake up source). */
  /* Clear int. flag. */
}

ISR(RTC_PIT_vect)
{
  /* Disconnect AC output to the PIN3 (needs to be done as soon as possible). */
  /* Disable PIN3 interrupt (wake up source). */
  /* Clear int. flag. */
}

ISR(RTC_CNT_vect)
{
  /* Clear the int. flag. */
  /* If sleep was not requested an error has occurred, create alarm. */
  /* Send all the alarms */
  /* Something went wrong, reset itself. */
}