FSK Modem and DTMF Codec
CMX865
5.9
Rx Data Register and USART
A flexible Rx USART is provided for all modem modes. It can be programmed to treat the received data
bit stream as Synchronous data or as Start-Stop characters.
In Synchronous mode the received data bits are all fed into the Rx Data Buffer which is copied into the
C-BUS Rx Data Register after every 8 bits.
In Start-Stop mode the USART Control logic looks for the start of each character, then feeds only the
required number of data bits (not parity) into the Rx Data Buffer. The parity bit (if used) and the presence
of a Stop bit are then checked and the data bits in the Rx Data Buffer copied to the C-BUS Rx Data
Register.
Figure 7a Rx Modem Data Paths
Whenever a new character is copied into the C-BUS Rx Data Register, the Rx Data Ready flag bit of the
Status Register is set to ‘1’ to prompt the µC to read the new data and, in Start-Stop mode, the Even Rx
Parity flag bit of the Status Register is updated.
In Start-Stop mode, if the Stop bit is missing (received as a ‘0’ instead of a ‘1’) the received character will
still be placed into the Rx Data Register and the Rx Data Ready flag bit set, but the Status Register Rx
Framing Error bit will also be set to ‘1’ and the USART will re-synchronise onto the next ‘1’ – ‘0’ (Stop –
Start) transition. The Rx Framing Error bit will remain set until the next character has been received.
Figure 7b Rx USART Function (Start-Stop mode, 8 Data Bits + Parity)
© 2005 CML Microsystems Plc
15
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
