CHAPTER 3 REGISTERS
User’s Manual S19262EJ3V0UM
24
Table 3-8. Auto-Flow Settings (Bits 5 and 1 of MCR and Bit 6 of HCR0)
Bit 5 of MCR
Register
Bit 1 of MCR
Register
Bit 6 of HCR0
Register
Auto-CTS Auto-RTS
0
(Auto-RTS mode 0)
1
1
(Auto-RTS mode 1)
1
0
(RTSZ output is fixed to high level.)
0
Auto-CTS operation:
While the CTSZ pin is pulled low (requesting transmission), data in the transmit buffer (THR/transmit
FIFO) is transmitted. The transmission stops when the pin is pulled high. If the CTSZ pin is pulled high
during transmission, data being transmitted (the data that remains in the transmit shift register) is
transmitted and transmission stops.
Auto-RTS operation (valid only in the FIFO mode):
Auto-RTS mode 0
When the amount of data in the receive FIFO reaches the trigger level, the RTSZ pin is pulled high
(requesting that transmission be stopped). When the data in the receive FIFO is read and the FIFO
empties, the pin is pulled low (requesting transmission).
Auto-RTS mode 1
In the 16-byte FIFO mode, the RTSZ pin is pulled high when the amount of data in the receive FIFO
reaches 14 bytes, and the pin is pulled low when the amount of data becomes 13 bytes or less.
In the 64-byte FIFO mode, the RTSZ pin is pulled high when the amount of data in the receive FIFO
reaches 56 bytes, and the pin is pulled low when the amount of data becomes 55 bytes or less.
Caution
When auto-flow control is not used, flow control is not performed by hardware.
Therefore, CTS monitoring and RTS control must be performed by software to prevent a
FIFO overrun.
When auto-flow is not used (bit 5 of MCR register = 0):
Bit 1 of the MCR register is used to control the RTSZ output (the value set to bit 1 is
reversed and the reversed level is output to RTSZ).
When both a modem status interrupt (bit 3 of IER register = 1) and auto-CTS are set,
auto-CTS is disabled.
In the non-FIFO mode, auto-RTS is disabled.
