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UART Registers The Received Data interrupt is a way to let you know that there is some data waiting for you to pull off of the UART. This is probably the one bit that you will use more than the rest, and has more use. The Transmitter Holding Register Empty Interrupt is to let you know that the output buffer (on more advanced models of the chip like the 16550) has finished sending everything that you pushed into the buffer. This is a way to streamline the data transmission routines so they take up less CPU time. The Receiver Line Status Interrupt indicates that something in the LSR register has probably changed. This is usually an error condition, and if you are going to write an efficient error handler for the UART that will give plain text descriptions to the end user of your application, this is something you should consider. This is certainly something that takes a bit more advanced knowledge of programming. The Modem Status Interrupt is to notify you when something changes with an external modem connected to your computer. This can include things like the telephone "bell" ringing (you can simulate this in your software), that you have successfully connected to another modem (Carrier Detect has been turned on), or that somebody has "hung up" the telephone (Carrier Detect has turned off). It can also help you to know if the external modem or data equipment can continue to receive data (Clear to Send). Essentially, this deals with the other wires in the RS-232 standard other than strictly the transmit and receive wires. The other two modes are strictly for the 16750 chip, and help put the chip into a "low power" state for use on things like a laptop computer or an embedded controller that has a very limited power source like a battery. On earlier chips you should treat these bits as "Reserved", and only put a "0" into them. 3.6.4 Interrupt Identification Register Offset: +2 . This register is to be used to help identify what the unique characteristics of the UART chip that you are using has. This chip has two uses: • Identification of why the UART triggered an interrupt. • Identification of the UART chip itself. Of these, identification of why the interrupt service routine has been invoked is perhaps the most important. The following table explains some of the details of this register, and what each bit on it represents: 41
8250 UART <strong>Programming</strong> Interrupt Identification Register (IIR) Bit Notes Bit 7 Bit 6 0 0 No FIFO on chip 7 and 6 0 1 Reserved condition 1 0 FIFO enabled, but not functioning 1 1 FIFO enabled 5 64 Byte FIFO Enabled (16750 only) 4 Reserved Bit 3 Bit 2 Bit 1 Reset Method 0 0 0 Modem Status Interrupt Reading Modem Status Register(MSR) 3, 2 and 1 0 0 1 Transmitter Holding Register Empty Interrupt Identification Regis- Reading Interrupt ter(IIR) or Writing to Transmit Holding Buffer(THR) 0 1 0 Received Data Available Interrupt Reading Receive Buffer Register(RBR) 0 1 1 Receiver Line Status Interrupt Reading Line Status Register(LSR) 1 0 0 Reserved N/A 1 0 1 Reserved N/A 1 1 0 Time-out Interrupt Pending (16550 & later) Reading Receive Buffer Register(RBR) 1 1 1 Reserved N/A 0 Interrupt Pending Flag 42
Page 1 and 2: Serial Programming Wikibooks.org
Page 3 and 4: Contents 1 Introduction and OSI Mod
Page 6 and 7: 1 Introduction and OSI Model 1.1 In
Page 8 and 9: Software Examples • Serial Networ
Page 10 and 11: 2 RS-232 Connections 2.1 Introducti
Page 12 and 13: Data Terminal/Communications Equipm
Page 14 and 15: Data Terminal/Communications Equipm
Page 16 and 17: Connection Types Figure 1 Female DB
Page 18 and 19: Connection Types Figure 5 mini-ster
Page 20 and 21: Wiring Pins Explained One thing to
Page 22 and 23: Baud Rates Explained 2.4.9 RI (Ring
Page 24 and 25: Baud Rates Explained 2.5.1 Modems E
Page 26 and 27: Signal Bits 2.6.3 Parity Bit To hel
Page 28 and 29: External References Baud Rate Maxim
Page 30 and 31: 3 8250 UART Programming 3.1 Introdu
Page 32 and 33: 8086 I/O ports char test; test = 25
Page 34 and 35: x86 Processor Interrupts 3.3.2 Inte
Page 36 and 37: 8259 PIC (Programmable Interrupt Co
Page 38 and 39: Serial COM Port Memory and I/O Allo
Page 40 and 41: UART Registers UART Registers Base
Page 42 and 43: UART Registers Divisor Latch Byte V
Page 46 and 47: UART Registers When you are writing
Page 48 and 49: UART Registers these to a logical "
Page 50 and 51: UART Registers Each bit the data po
Page 52 and 53: UART Registers Line Status Register
Page 54 and 55: UART Registers Bits 7 and 6 are dir
Page 56 and 57: External References Read the value
Page 58 and 59: 4 Serial DOS 4.1 Introduction It is
Page 60 and 61: Finding the Port I/O Address for th
Page 62 and 63: Making modifications to UART Regist
Page 64 and 65: Basic Serial Input 4.5.1 Polling th
Page 66 and 67: Interrupt Drivers in DOS 4.6 Interr
Page 68 and 69: Interrupt Drivers in DOS able to us
Page 70 and 71: Interrupt Drivers in DOS Interrupt
Page 72 and 73: Terminal Program Revisited Port[Com
Page 74 and 75: Terminal Program Revisited RBR = 0;
Page 76: Terminal Program Revisited build a
Page 79 and 80: Serial Linux Classic Unix systems o
Page 81 and 82: Serial Linux 1. Terminals/teletypes
Page 83 and 84: Serial Linux for more information t
Page 85 and 86: Serial Linux signal and trap that s
Page 87 and 88: Serial Linux 5.2.6 uucp Overview Uu
Page 90 and 91: 6 Serial Java 6.1 Using Java for Se
Page 92 and 93: Using Java for Serial Communication
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JavaComm API 6.2.2 Download & Insta
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JavaComm API wantedPortName); Syste
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JavaComm API 6.2.4 Simple Data Tran
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JavaComm API Before going into deta
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JavaComm API // makes sense to enab
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JavaComm API /** * Clear the buffer
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JavaComm API } } start = i; } retur
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RxTx Like with any other particular
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See also • SerialIO has a free tr
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Forming Data Packets • The footer
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Forming Data Packets bitrate or raw
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8 Error Correction Methods 8.1 Intr
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ACK-NAK A 1-bit sequence number (al
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combination If it sends a message b
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9 Appendex A:Modems and AT Commands
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Introduction forms the base for pro
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Introduction can still also be used
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Modem Programming Basics • By loo
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Modem Programming Basics Whatever i
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Modem Programming Basics 9.2.5 Char
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Changing State 9.4 Changing State 9
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AT Commands with the exception of #
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Result Codes Description: Result
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10 Contributors Edits User 10 Adrig
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List of Figures • GFDL: Gnu Free
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List of Figures 1 GFDL 2 User Mike1
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from that copyright holder, and you
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