I use RS485 transceiver ADM2582E, but I met one problem.
If data rate over 7.5M bps, MCU-RXD will receive one LOW pulse after sent data (Format 8N1), as shown in Figure 1.
But when I setup the UART format to 8N2, the problem has disappeared, as shown in Figure 2.
Does anyone know what the problem is?
The following image was captured from Oscilloscope.
Yellow indicates ADM2582E DE/RE(MCU-TXDEN).Blue indicates ADM2582E TXD(MCU-TXD).Purple indicates ADM2582E B-A.Green indicates ADM2582E RXD (MCU-RXD)
The HL transition on the ADM2582E RXD (MCU-RXD) CH4 of Figure 1 appears to line up with the falling edge of the signal on CH1. This signal is disabling the transmitter of the ADM2852E and enabling the receiver. For the duration of transmission, the receiver is disabled, and ADM2582E RXD (MCU-RXD) is high impedance. In your plot the ADM2582E RxD is high for this duration, I expect you have a pull-up resistor on the RxD pin of the ADM2582E?
The ADM2582E receiver has a low to high propagation delay of ~95ns. To avoid a low appearing on your RxD signal, you should wait at least 95ns after the last low-high transition before setting the ADM2582E DE/RE(MCU-TXDEN) signal low to enable the ADM2582E receiver. In the 8N1 case, the receiver is enabled too early, within the 95ns. In the 8N2 case, you have a sufficient delay after the last low - high transition on ADM2582E B-A. before the ADM2582E receiver is enabled.
Thanks a lot for your reply.
I have a pull-up register on the RXD pin of the ADM2582E. So I think it isn’t the primary reason.
And I think the primary reason is that the receiver is enabled too early.
Because the ADM2582E receiver has propagation delay 95 ns, the TXDEN must be have at least 95 ns delay for dropping low. But it is the best to add driver propagation delay 64 ns, right?
That's correct, the TXDEN (CH4) signal should be set low at least 95ns + 64ns after the last LH transition on the TxD signal (CH2)
To guarantee this across supply, temperature and part-to-part variation, I'd recommend a delay of the combined maximum propagation delay : 100ns driver prop + 110ns receiver prop = 210ns max
Well answered. Thanks a lot.