Problem with keeping alive master and slave in a proprietary communication protocol - c

Question

I'm writing a UART communication protocol for my stm32l152c-discovery. Briefly, the master sends the communication start packet to the slave via the user button and the slave, if it receives it correctly, sends an ack start packet and passes to the connected state. In the same way, if the master receives the ack start packet correctly, it goes into connected state. In this state, master and slave keep alive with keep alive packets that must be sent every TPOLL which can be from 100 to 5000 milliseconds. If the TSILENT timeout, which is TPOLL + 100 milliseconds, expires, the board disconnects. The event counter is managed by a timer which increments every millisecond.
I encounter a low solidity problem, in fact after a few packets exchanged one of the two boards disconnects, I tried with various TPOLL values. If I increase TSILENT from TPOLL + 100 to TPOLL +1000 the program works correctly. The problem is that I have no delay in the operations I perform in this state and all the functions to be performed take 12 milliseconds (also counting the 10 for UART reception).timer 3 is initialized before entering the connected state. I've posted parts of the code that might be of interest.
void TIM3_IRQHandler(void)
{
/* USER CODE BEGIN TIM3_IRQn 0 */
/* USER CODE END TIM3_IRQn 0 */
HAL_TIM_IRQHandler(&htim3);
/* USER CODE BEGIN TIM3_IRQn 1 */
GlobalTick++;
if(GlobalTick == 1)
{
TX_BufferReset();
SendPacket(KEEP_ALIVE);
HAL_UART_Transmit_DMA(&huart3, TX_Buffer.buffer, TX_Buffer.count);
}
if(GlobalTick - t_event_poll >= TPOLL)
{
TX_BufferReset();
SendPacket(KEEP_ALIVE);
HAL_UART_Transmit_DMA(&huart3, TX_Buffer.buffer, TX_Buffer.count);
t_event_poll = GlobalTick;
}
if(GlobalTick - t_event_silence > TSILENT)
{
state = STATE_DISCONNECTED;
ResetTime();
}
/* USER CODE END TIM3_IRQn 1 */
}
case STATE_CONNECTED:
RX_BufferReset();
HAL_UART_Receive(&huart3, RX_Buffer.buffer, MAX_PKT_BYTES, 10);
if(WaitFullPacket() == TRUE)
if (ReceivePacket() == CORRECT && RX_Buffer.buffer[3] == KEEP_ALIVE)
t_event_silence = GlobalTick;
break;