Copyright © profichip GmbH, 2012
8.3 UART
The transmitter converts the parallel data structure into a serial data flow.
Signal Request-to-Send (RTS) is generated before the first character. The
XCTS input is available for connecting a modem. After RTS active, the
transmitter must hold back the first telegram character until the modem acti-
vates XCTS. XCTS is checked again after each character.
The receiver converts the serial data flow into the parallel data structure
and scans the serial data flow with the four-fold transmission speed. Stop
bit testing can be switched off for test purposes ('Dis_Stop_Control = 1' in
Mode Register 0 or Set_Prm telegram for DP). One requirement of the
PROFIBUS protocol is that no rest states are permitted between the
telegram characters. The VPC3+S transmitter ensures that this
specification is maintained.
The synchronization of the receiver starts with the falling edge of the start
bit. The start bit is checked again in the middle of the bit-time for low level.
The data bits, the parity and the stop bit are also scanned in the middle of
the bit-time. To compensate for the synchronization error, a repeater gen-
erates a 25% distortion of the stop bit at a four-fold scan rate. In this case
the VPC3+ should be parameterized with 'Dis_Start_Control = 1' (in Mode
Register 0 or Set_Prm telegram for DP) in order to increase the permissible
distortion of the stop bit.
8.4 ASIC Test
All output pins and I/O pins can be switched to the high-resistance state via
the XTEST0 test pin. An additional XTEST1 input is provided to test the
chip on automatic test devices (not in the target hardware environment!).
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