CP/M

home *** CD-ROM | disk | FTP | other *** search

/ CP/M / CPM_CDROM.iso / cpm / modems / modem / inte8251.art < prev next >

Wrap

Text File | 1994-07-13 | 12.9 KB | 396 lines

[ KAY*FOG RBBS | INTE8251.ART | published 05/30/85 | 379 lines 13k ] INTEL8251: Programming the Intel 8251A USART by: Ed Greenberg Kay*Fog CompuServe: 76703,1070 MCIMAIL: EDG Preface ======= This document was inspired by my collection of Intel data books. Of the three databooks, only one contains the following information. Most microcomputer users do not have this information in their computer manuals and do not have access to the Intel manuals. Murphy's law also states that the hobbyist will not have the required manuals at midnight on Saturday when he is busily trying to debug his communications program. [Press ENTER key for more] Introduction ============ In order to communicate via a serial port, a computer is equipped with a device called a Universal Asynchronous Receiver/Transmitter or UART. This device converts a byte (written to an output port) to a series of bits sent one at a time (serially) from a communications port. It also scans the input line on the serial port and, detecting the beginning and end of each character, assembles incoming characters and presents them on an input port. Other names for this part are SIO's (serial input/output) or Communications adapters and USARTS (Universal Synchronous/ Asynchronous Receiver/Transmitters). This document deals with one common part - the Intel I8251a USART. That usart is used in the North Star Horizon, the Morrow Micro Decision, and other common computers. This discussion deals with the 8251A only in asynchronous mode. It is the mode used when communicating with a printer, a modem, a terminal and usually, another microcomputer. Using your USART ================ In order to use a communications port with a usart, one must first initialize it. By doing so, one sets parameters that tells the USART how to do it's job. One defines the parity, the number of stop and data bits, and the divisor (if any) to be applied to the incoming clock signal. After the USART is initialized, one can read and write characters on the data port of the usart. One can also check the status of the usart on the status port. Things to know before you start =============================== Before you can write code for your usart, you must know the port locations on which the usart appears. There are two ports, called the DATA port and the STATUS port. Commands are written to the status port, and the condition of the port is read from it. Actual characters are written to and read from the data port. A good place to find the port numbers is a published I/O MAP in your manual. Another place is in a program listing of the BIOS or a communications program. You will see something like this: STATUS EQU 80H DATA EQU 81H or the like. Sometimes, the word MODEM is worked into the label (e.g. MODDATP for MODem DATa Port.) Be certain that you find the correct set of ports in the case where your computer has more than one usart. A Note About Interrupts ======================= Some computers use the facility called interrupts to signal the processor that a character is ready. In this case, it is important for the amateur programmer to avoid messing up the interrupt structure of the machine. You should not read or write your USART directly when it's interrupt is enabled. Doing so may cause spurious interrupts of the processor. The result is that nothing will work. Determining whether your computer uses interrupts on the communications line is beyond the scope of this document. Initializing the usart ====================== The following code fragment is taken from the MEX overlay for the Morrow Micro Decision. MEX is written and copyrighted by Robert Fowler. The comments are my own. INITMOD: ... ... MVI A,087H ;Take the usart out of OUT MODCTL1 ; the condition for OUT MODCTL1 ; setting the mode (*) MVI A,40H ;Put it back into that OUT MODCTL1 ; condition. This resets ; just about everything for ; new commands. INITMOD1: MVI A,4EH ;This is the MODE BYTE (*) OUT MODCTL1 ;Send it to the control/status port MVI A,37H ;This is the COMMAND BYTE (*) OUT MODCTL1 ;Send it to the control/status port IN PORT ;Clear out the DATA port RET ;Return (*) See the definition below Input and Output of Characters ============================== Below are sample routines for input and output of characters on the 8080 (or Z80) using an 8251A. ;Input character routine. Character is returned in A. INPUT: IN STATUS ;Get the status of the usart ANI 2 ;turn off all bits but RxR (**) JZ INPUT ;go back and check again because ; there is no character ready IN DATA ;There is a character ready, ; so go get it. ;OPTIONAL STEP ANI 7FH ;Remove the high bit RET ;Go back to the caller ;Output character routine. Character is provided in C. OUTPUT: IN STATUS ;Get the status of the usart ANI 1 ;turn off all bits but TxR (**) JZ OUTPUT ;Go back and check again because ; the USART isn't ready for another ; character. MOV A,C ;The USART is ready, so get the ; character in A for output OUT DATA ;Now output it and... RET ;Return to the caller. (**) See the definition of the Status byte below The remainder of this document is concerned with defining the actual bytes sent to (and received from) the usart. ----------------------------------------------------------------- Format of the Mode Byte ======================= 7 6 5 4 3 2 1 0 +----+----+----+----+----+----+----+----+ |s(2)|s(1)| ep |pen |l(2)|l(1)|b(2)|b(1)| +----+----+----+----+----+----+----+----+ Content of the Mode byte ======================== s(2) and s(1) - the stop bit indicators: ---------------------------------------- s(2) s(1) meaning ---- ---- ------- 0 0 Invalid 0 1 1 stop bit 1 0 1.5 stop bits 1 1 2 stop bits ep - the parity bit -------------------- ep meaning -- ------- 0 Odd parity is generated and checked 1 Even parity is generated and checked Note: this bit is only active when pen is set to 1. pen - the parity enable bit --------------------------- pen meaning --- ------- 0 parity disabled 1 parity enabled l(2) and l(1) - the word length bits ------------------------------------ l(2) l(1) meaning ---- ---- ------- 0 0 5 bits 0 1 6 bits 1 0 7 bits 1 1 8 bits Note: For ASCII, we usually use only 7 or 8 bits. Hams and TTY/TDD (for the deaf) use 5 bits. The only thing that 6 bits is used for is a colloquialism for seventy five cents. b(2) and b(1) - the baud rate divisor bits ------------------------------------ b(2) b(1) meaning ---- ---- ------- 0 0 synchronous 0 1 1x 1 0 16x 1 1 64x Note: This should be left alone. Whatever your system comes equipped for... that's it. Format of the Command Byte ========================== 7 6 5 4 3 2 1 0 +----+----+----+----+----+----+----+----+ |EH |IR |RTS |ER |SBRK|RxE |DTR |TxEN| +----+----+----+----+----+----+----+----+ Content of the Command Byte =========================== EH - the Enter Hunt Mode bit ---------------------------- This bit has no effect in async mode! IR - Internal Reset ------------------- 1 Returns 8251A to Mode Instruction format. RTS - Request to Send --------------------- 1 will force RTS high on the RS-232 connector. Note: This assumes that the designer hooked all the signals up on the PC board. This is not necessarily true. On some computers, where the port is wired backwards, this will control CTS rather than RTS. ER - Error Reset ---------------- 1 Will reset error flags in the status word (PE OE and FE will be reset.) See the definition of the status word below. SBRK - Send a break ------------------- 1 Will send a break Note: Usually, one sends a command with this bit set and then, after a delay that equals the length of a break, one sends another command that does not have the break bit on. RxE - Receive Enable -------------------- 1 will enable the receiver. If you are going to disable the receiver for any reason, you should have a data book in front of you and know what you're doing. This bit should almost always be on for any command you send. DTR - Data Terminal Ready ------------------------- 1 will turn on Data Terminal Ready at the RS-232 connector. Note: This assumes that the designer hooked all the signals up on the PC board. This is not necessarily true. On some computers, where the port is wired backwards, this will control DSR rather than DTR. TxE - Transmitter Enable ------------------------ 1 will enable the transmitter. If you are going to disable the transmitter for any reason, you should have a data book in front of you and know what you're doing. This bit should almost always be on for any command you send. Format of the Status Byte ========================= 7 6 5 4 3 2 1 0 +----+----+----+----+----+----+----+----+ |DSR |SDET|RE |OE |PE |TxE |RxR |TxR | +----+----+----+----+----+----+----+----+ Content of the Status Byte ========================== DSR - Data Set Ready -------------------- 1 Indicates that DSR is low! That's right, Low! Note that this bit may be (a) not wired at all or (b) wired to DTR or some other pin on the RS-232 connector. The only way to tell for sure is to look at a schematic. SDET - SYNC Detect/ BREAK Detect -------------------------------- (In the Intel documentation this is called SYNDET, not SDET.) In async mode, this bit "will go high whenever an all zero word of the programmed length (including start bit, data bit, parity bit and *one* stop bit) is received." (***) In other words, when the other end sends *you* a break. This bit stays high until a reset command is issued. FE - Framing Error ------------------ This bit is set when "A valid stop bit is not detected at the end of every character." This bit stays high until a reset command is issued. OE - Overrun Error ------------------ This bit is set when a character has been pending on the USART buffer and another character comes in before the first one has been read. The first character is lost and this bit is set to alert the CPU to the problem. This bit stays high until a reset command is issued. PE - Parity Error ----------------- This bit is set when a parity error is detected. This bit stays high until a reset command is issued. TxEMPTY - Transmit Buffer Empty ------------------------------- "When the 8251A has no characters to transmit, the TxEMPTY ... [bit will be set to 1.] ... " (***) RxRDY - Receiver Ready ---------------------- "This ... [bit set to 1] ... indicates that the 8251A contains a character that is ready to be input to the CPU." (***) This is the usual bit to test to see if a character is available. Usually one sees ANI 2 (on 8080) when this bit is tested. TxRDY - Transmitter Ready ------------------------- "This ... [bit set to a 1] ... signals the CPU that the transmitter is ready to accept a data character." This is the usual bit to test to see if a character may be sent. Usually one sees ANI 1 (on 8080) when this bit is tested. ----------------------- End of INTE8251.ART Text ----------------------- ∩