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536EX Chipset Developer’s Manual 95
Intel Confidential
Parallel Host Interface 16C450/16C550A UART
9.2 UART Register Definitions
9.2.1 Scratch Register (SCR)
This is an 8-bit read/write register used by the DTE for temporary storage of data.
9.2.2 Modem Status Register (MSR)
This register provides four bits (bits 7:4) that show current modem state and four bits (bits 3:0) that
provide modem change information. Bits 3:0 are set to ‘1’ whenever the control information
changes state. These bits are reset to ‘0’ whenever the DTE reads the MSR register. If the modem
status interrupt is enabled (IER3), the modem generates an interrupt on the
µP HINT pin whenever
MSR bits 3:0 are set to ‘1.’
Figure 14. Scratch Register (SCR)
SCR
Register 7
Figure 15. Modem Status Register (MSR)
DCD RI DSR CTS DDCDD TERI DDSR DCTS
Register 6
Bit 7
Data Carrier Detect (DCD)—When this bit is set to ‘1’, it indicates that the remote modem data carrier has been detected
(refer to the &C command).
Bit 6 Ring Indicate (RI)—This bit indicates when a ring signal has been detected.
Bit 5
Data Set Ready (DSR)—This bit indicates when the modem is ready to establish a communication link.
When entering voice mode, DSR is set to 1. DSR is used for voice playback/record DMA mode to indicate when the DTE
has not responded to a modem DMA data transfer request. DSR is set to 1 when DMA data are being transferred; DSR is
set to 0 when a new DMA transfer has not occurred with 1.7 ms after the previous DMA transfer. DSR works similarly to a
DMA terminal count.
Bit 4 Clear To Send (CTS)—When this bit is set to ‘1’, it indicates to the DTE that the modem is ready to receive data.
Bit 3
Delta Data Carrier Detect (DDCDD)—When this bit is set to ‘1’, it indicates that the DCD bit has changed its value since
the DTE last read the MSR register.
Bit 2 Trailing Edge of Ring Indicator (TERI)—This bit is set to ‘1’ after the RI signal goes from a high to low state.
Bit 1
Delta Data Set Ready (DDSR)—When this bit is set to ‘1’, it indicates that the DSR bit has changed its value since the
DTE last read the MSR register.
Bit 0
Delta Clear to Send (DCTS)—When this bit is set to ‘1’, it indicates that the CTS bit has changed its value since the DTE
last read the MSR register.