Experiment Ten THE PROJECT

Transcription

1 Experiment Ten THE PROJECT Purpose: This project, which will require the rest of the term to complete, allows you the experience of interfacing the embedded microprocessor (you built the system in EET-L) to the personal computer through the serial port. This provides a realistic experience that one often encounters in industry when doing embedded system development. You are to work in teams of or people and each team is required to have one working embedded system from EET-L. Things that need to be done to accomplish this task include:. modification of your embedded system to include a serial port. development of interrupt service procedures, which handle serial communications for the embedded system. operating system in the embedded microprocessor for the transfer of information between the PC and the embedded system. Windows application (using Visual C) for the PC to transfer a file to the embedded microprocessor through the serial port. development of the protocols for an error free transfer of data Serial Port: The serial port is not TTL compatible, but rather is RS-C compatible, which means that the voltages are not.0v, but at least ±V. Because of this, we must translate the voltage levels between TTL levels and RS-C levels. In the past this may have required additional power supplies and some circuitry. Today we have an integrated circuit to accomplish the interface (MAX-) and also to develop the power supply voltages. The MAX- contains receivers, transmitters, and a DC to-dc converter that develops the ±0V for the interface to the PC. The pin-out is shown below for the MAX- integrated circuit. Vcc Power Supply TTL Inputs 0 RS Outputs TTL Outputs 9 RS Inputs GND All capacitors are.0 µf electrolytic with polarities as shown!

2 You need to purchase a 9-pin DB9 male connector (the cable has the female connector) that will be used to interface the PC serial port to the embedded microprocessor through the MAX- integrated circuit. The pin connections on the DB9 connector are as follows: Pin Number Function Carrier Detect Received Data Transmit Data Data Terminal Ready Signal Ground Data Set Ready Request to Send Clear to Send 9 Not used 9 CONNECTOR DB9 The PC serial port (the 9-pin port COM ) will be connected to the embedded system using a NULL modem cable. This cable can be purchased or built and the connections are as shown below: Embedded System PC The NULL modem cable also has the same connections on the embedded side, but we don t need them because we are not going to use them with our system. The embedded microprocessor will use the channel 0 serial port for the connection to the PC and the simple interface, which you must construct, appears below: 0CEB MAX 9 RXD0 0 TXD0 DB9

3 Now that we have learned how to connect the embedded microprocessor to the serial cable we need to learn how to program it. Programming has three main areas:. initialization for serial port 0. programming the Baud rate generator. developing the interrupt service procedures for both receive and transmit data The mode of operation for the serial port will be data bits, start bit, and stop bit without parity. This is mode operation of the serial port and it is compatible with the COM port of the PC operated in same manner. The serial port is configured through the control register located at PCB address FFH. Ilustrated below is the contents of the control register for serial port 0. M,M,M0 = Mode of operation (our case is mode or 00 ) PEN, EVN = parity control (both bits are 0 to disable parity) REN = enables the receiver when set () CEN = clear to send enable (0 to ignor the CTS pin) TB and SBRK are 0 in our case Our control word is 000H (This initally disables the receiver to prevent errors). The Baud rate generator for serial port 0 is programmed through registers: the Baud rate count register and the Baud rate compare register. We are going to use a Baud rate of 900 with no parity and data bits or a 0 bit frame. We never actually program the Baud rate counter, just the Baud rate compare register for the desired Baud rate. Since our microprocessor s operating frequency is Mhz, we program the Baud rate compare register with an 0H at port FF0H for a Baud rate of 900. The program to intialize our serial port follows: MOV DX,0FF0H ;set Baud rate MOV AX,0H MOV DX,0FFH ;set up serial port, recv off MOV AX, MOV DX,0FFH ;clear pending interrupts IN AX,DX MOV DX,0FFH ;clear interrupt mask bit IN AX,DX AND AX, MOV DX,0FFH ;enable receiver IN AX,DX OR AX,0H

4 This software must be placed in the intialization portion of your system just before the STI instruction, which enables interrupts. Once the serial port is initialized comuncation takes place by a series of interrupts. When data is received an interrupt occurs and when data has been transmitted an interrupt occurs. In the intialization software you will find the following: MOV DX,0FF0H ;address interrupt mask register MOV AX,00FCH ;disable all except for timer OUT DX,AL The contents of the AX register must be changed or no serial port interrupt can occur. The MOV AX,00FCH instruction above must be changed to MOV AX,00FH to enable both the Timer Interrupt and the Serial port interrupt. We also need to install the interupt procedures in the interrupt vector table. These are installed with the following software, again before the STI instruction. MOV AX,CS MOV DS:[H],AX ;receive interrupt vector MOV DS:[0H],OFFSET RECV-00H MOV DS:[H],AX ;transmit interrpt vector MOV DS:[H],OFFSET TRANS-00H MOV BYTE PTR DS:[0H],0 ;Connection Flag We now have installed our receiver interrupt (RECV) and our transmit interrupt (TRANS). All we need to do now is write the interrupt service procedure for the receiver and transmitter. Our system will be comnpletely driven by these two interrupts without any need to change the operating system that we have installed if we choose a good protocol. Protocol: Our main purpose for this interface will allow us to download programs from the personal computer to the embedded controller and then run them. To accomplish this we need to define a protocol for our system before we can write the interupt service procedures. The protocol that we establis will allow us to:. Download a program. Run the program. Stop execution of the program. Ensure an accurate transfer of the program. Establish initial communications between the PC and the embedded system You might call this a remote operating system. We need to chose some commands to accomplish the above tasks: Command Byte 00H 0H 0H 0H 0H 0H Function Start Communication Run Program Download Program from PC Abort program Error in download or invalid command No error in download or command

5 The transmit interrupt service procedure: Each time that the serial port transmitter is ready the transmit interrupt occurs. In our system we really don t need this interrupt, but since we enabled interrupts for the port, we need to supply an interrupt service procedure for the transmitter. Below is the interrupt service procedure for the transmitter: TRANS PROC FAR USES AX DX MOV DX,0FF0H MOV AX,000h IRET TRANS ENDP ;issue non-specific EOI All the transmitter interrupt service procedure does is tells the interrupt controller within the microprocessor that the interrupt is complete and to allow another interrupt. The reason we don t need any additional software is that our system never transmits more than a single byte back to the PC. If we needed to transmit more than a byte then the interrupt service procedure for the transmitter would need to send additional bytes from a buffer that we would need to set up in the memory. The receiver interrupt service procedure: The receiver interrupt has the responsibility of interpreting all of our commands and will be longer than the transmitter interrupt service procedure. Each time that a byte is received from the PC through the serial port, the system cause a receiver interrupt. At that time it will be our responsibility to process the received byte to interpret it. CSEND MACRO WHAT MOV DX,0FFAH MOV AX,WHAT ENDM RECV PROC FAR USES DS AX BX BP DX MOV AX,0 MOV DS,AX ;DS addresses SRAM MOV DX,0FFH IN AX,DX ;read status (clear interrupt) MOV DX,0FFH ;read data IN AL,DX.IF BYTE PTR DS:[0H] == 0 ;not connected.if AL == 0 ;connect MOV BYTE PTR DS:[0H], IF BYTE PTR DS:[0H] == ;just connected.if AL == ;load 0000:000 MOV BYTE PTR DS:[0H], MOV WORD PTR DS:[H],00H IF BYTE PTR DS:[0H] == ;get low count MOV BYTE PTR DS:[0H], MOV DS:[H],AL

6 RECV IF BYTE PTR DS:[0H] == ;get high count MOV BYTE PTR DS:[0H], MOV DS:[H],AL MOV WORD PTR DS:[H],0 IF BYTE PTR DS:[0H] == ;get program MOV AH,0 ADD DS:[H],AX ;form checksum MOV BX,DS:[H] ;get address MOV [BX],AL ;save byte INC WORD PTR DS:[H] DEC WORD PTR DS:[H].IF ZERO? ;done reading MOV BYTE PTR DS:[0H], IF BYTE PTR DS:[0H] == MOV DS:[H],AL MOV BYTE PTR DS:[0H], IF BYTE PTR DS:[0H] == MOV AH,AL MOV AL,DS:[H].IF AX == DS:[H] MOV BYTE PTR DS:[0H], MOV BYTE PTR DS:[0H], IF BYTE PTR DS:[0H] ==.IF AL == ;run program MOV BYTE PTR DS:[0H], MOV BP,SP MOV WORD PTR [BP0],00H MOV WORD PTR [BP],0 IF BYTE PTR DS:[0H] ==.IF AL == MOV BYTE PTR DS:[0H], MOV BP,SP MOV WORD PTR [BP0],OFFSET TEMP-00H MOV AX,CS MOV [BP],AX MOV DX,0FF0H MOV AX,000H IRET ENDP

7 We now have a working embedded system that can have a program downloaded to 0000:000 for execution. Since our memory extends to 0000:FFF we should have ample space to download a significant program. Our protocol follows the following flow:. Connection begins when the embedded system is first powered and it receives a 00H from the PC, a good connection is signaled by the return of 0H from the embedded system. Once connected a program can be downloaded by sending a 0H from the PC to the embedded system, followed by the low byte of the count, the high byte of the count, the program bytes, and a two byte checksum. The count is the count of the bytes in the file. The checksum is the simple sum of all the bytes of the program, but not the checksum bytes. If a 0H is returned from the embedded system the program was transferred without error and if a 0H is returned an error occurred and the transfer must be retried.. At this point we can only execute the program. A 0H returned indicates execution is occuring.. At this point we can only abort execution. A good abort is indicated by the return of a 0H.. After an abort the system continues back at step. The PC end with its VC DIALOG BOX application should appear as follows: You wil need the Microsoft MSCOMM ActiveX control to use COM PORT on the PC. The help files give examples of the use of the MSCOMM control. The rest of the program should be fairly easy to write since is is driven by the button click events as in Experiment and 9. Feel free to embellish the Windows dialog box with color or whatever you think will make it look better. To be handed in for this experiment:. Listings of all programs for the microprocessor as well as the VC program. A screen snap shot of your dialog box. A short report describing things that you learned though this project.