Other Parts Discussed in Thread: TXU0104
Good afternoon,
I am working on an embedded board based on a STM32F4 microcontroller that communicates with a Sierra Wireless modem through an UART. Since STM32F4 digital interface is at 3.3V and modem digital interface is at 1.8V a transceiver has been placed between the two devices. In revision 2 of the electronic board a SN74AVC4T774PWR transceiver is used. Communication between microcontroller and modem has been tested and it has always worked correctly so far. Recently we have updated the electronic board from revision 2 to revision 3. In this new revision we have replaced SN74AVC4T774PWR with SN74AXC4T774PWR due to current consumption issues. I have attached an excerpt of the wiring diagram (TEL_DCD, MCU_USART1_RX, MCU_USART1_CTS, TEL_RING, MCU_USART1_TX, MCU_USART1_RTS, TEL_DTR signals are directly connected to the microcontroller, while TEL_DCD_1V8, TEL_RXD_1V8, TEL_CTS_1V8, TEL_RING_1V8, TEL_TXD_1V8, TEL_RTS_1V8, TEL_DTR_1V8 are directly connected to modem). During microcontroller-modem communication tests I experienced some errors. I used an oscilloscope to probe the messages sent and received by microcontroller over the UART and I saw a strange behaviour of the RX signal. After an AT command sent by microcontroller a low level period occurs before modem response (I have attached a picture of the signals detected by oscilloscope). This period lasts for 1.5/2 milliseconds and is interpreted as null character by microcontroller's UART. Unfortunaltey this behaviour gives me some problems reading the buffer where modem response is stored. So I have removed the two SN74AXC4T774PWR transceivers from revision 3 board and I have placed two SN74AVC4T774PWR. After that, the communication between the two devices worked properly again and no low level periods were detected by oscilloscope. All tests have been done with the same firmware loaded on microcontroller and same modem. Any help is appreciated to understand why those low level periods occur when SN74AXC4T774PWRs are mounted on the board.
Best regards.
Massimo G.
