SAB 82532/SAF 82532
Asynchronous Serial Mode
Semiconductor Group
71
07.96
6.4.2
Flow Control by XON/XOFF (version 2 upward)
Programmable XON and XOFF
Two eight-bit control registers (XON, XOFF) contain the programmable values for XON
and XOFF characters. The number of significant bits in a register is determined by the
programmed character length (right justified).
Two programmable eight-bit registers MXN and MXF serve as mask registers for the
characters in XON and XOFF, respectively:
A ‘1’ in a mask register has the effect that no comparison is performed between the
corresponding bits in the received characters (‘don’t cares’) and the XON and the XOFF
register. At RESET, the mask registers are ‘zero’ed, i.e. all bit positions are compared.
A received character is considered to be recognized as a valid XON or XOFF character
– if it is correctly framed (correct length),
– if its bits match the ones in the XON or XOFF registers over the programmed character
length,
– if it has correct parity (if applicable).
Received XON and XOFF characters are always stored in the receive FIFO, as any other
characters.
In-Band Flow Control of Transmitted Characters
Recognition of an XON or an XOFF character causes always a corresponding maskable
interrupt status to be generated (ISR1:XON / IMR1:XON; ISR1:XOFF / IMR1:XOFF).
Further action depends on the setting of a control bit MODE:FLON (Flow Control On):
0: No further action is automatically taken by the ESCC2.
1: The reception of an XOFF character automatically turns off the transmitter after the
currently transmitted character (if any) has been completely shifted out (XOFF state).
The reception of an XON character automatically makes the transmitter resume
transmitting (XON state).
After hardware RESET, bit MODE:FLON is at ‘0’.
When bit MODE:FLON is made to go from ‘0’ to ‘1’, the transmitter is first in the
‘XON state’, until an XOFF character is received.
When bit MODE:FLON is made to go from ‘1’ to ‘0’, the transmitter always goes in the
‘XON state’, and transmission is only controlled by the user and by the CTS input.
The in-band flow control of the transmitter via received XON and XOFF characters can
be combined with control via CTS pin, i.e. the effect of the CTS pin is independent of
whether in-band control is used or not. The transmitter is enabled only if CTS is ‘low’ and
XON state has been reached.