Файл: Microcontrollers in Practice (Mitescu M., 2005).pdf

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The CODE segment starts with the interrupt vector area. In this example, only two vectors are used: the reset vector, and the vector for the interrupt for timer0 overflow. The reset vector directs the program to the initialization sequence, and the timer0 overflow vector contains a jump instruction to the associated interrupt service routine, isr_t0_ov.

INIT.ASM contains the initialization sequence, which starts by loading the stack pointer SP with the last address of the internal RAM (Ramend), then calls the initialization routines for timer0, and I/O port B.

After the initialization sequence, the program enables the interrupts, loads the software timer s_timer0 with a constant (time_s_timer0), which defines the time between the moments when port B changes its status, then enters the main loop.

This software timer is the key element of the program. Basically, s_timer0 is an 8-bit RAM variable defined in MAP.ASM, and decremented every 10 milliseconds by the interrupt service routine for timer0 overflow. If the timer is loaded with the value N, it will reach zero after a time T = N × 10 ms.

In this example, N is the constant time_s_timer, defined in MAP.ASM with the value 50, which gives a total time of 500 ms between the moments when port B is toggled. When the software timer expires, port B changes its status, then the timer is reloaded, and the program continues in an endless loop.

The resource-specific routines are placed outside the main loop in the modules TIMER.ASM and IO.ASM. TIMER.ASM contains the initialization sequence for timer0, organized as a subroutine init_timer0, and the interrupt service routine that implements the software timer.

Here is the listing of TIMER.ASM:

init_timer0:

ldi tmp1,prescaler_constant out tccr0,tmp1

ldi tmp1,ft_div mov rint,tmp1 out tcnt0,tmp1 ldi tmp1,$01 out timsk,tmp1 ret

This routine chooses the division factor for the prescaler, and the number of clocks before overflow, then enables the interrupt for timer0 overflow.

isr_t0_ov:

Note how this routine saves and restores the CPU status. The register rint (an alias for r1) is preloaded with the number of clocks before tcnt0 overflows, so that tcnt0 can be reloaded as fast as possible.

soft_timer:

lds tmp1,s_timer0 tst tmp1

breq sft1 dec tmp1

sts s_timer0,tmp1

sft1:

lds tmp1,s_timer1 tst tmp1

breq ex_sft dec tmp1

sts s_timer1,tmp1

ex_sft:

ret

The routine soft_timer implements two independent software timers, s_timer0 and s_timer1. In principle, the number of distinct timers that can be implemented using this technique is limited only by the time required to execute the interrupt service routine. If a larger number of software timers is required, use the technique described in Chap. 9 for HC11 to reduce the execution time of the interrupt service routine.

Step 6. Adding a New Task to an Existing Application

The structure of the software described in the previous paragraph can be used for almost any application. Adding a new task to an existing application involves the following steps:

1.Modify MAP.ASM to define more variables if needed.

2.Write a new software module if a new resource is required. It is recommended to keep all the routines associated with specific resources in separate files (e.g. TIMER.ASM, IO.ASM, UART.ASM, ADC.ASM, etc.)

3.Modify the main program so that it includes calls to subroutines that actually execute the new task.

In the following example, the program reports the status of port B as a string of two ASCII digits, followed by CR+LF, over the asynchronous serial communication line, when the ASCII code for ‘?’ is received on the serial line. Note that port B is toggled every 500 ms, as described in the previous example.

The UART initialization routine and the communication routines are placed in a separate file UART.ASM, invoked in the main module using the “.include” directive. A separate file, LIB.ASM, is used for miscellaneous library routines – in this case hex-to-ASCII conversion routines used to prepare data for serial communication.

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Here is the listing of the main module UARTMAIN.ASM:

.include ‘‘8535def.inc’’

.include ‘‘map.asm’’

.cseg

.org 0

reset:

rjmp init

.org 9 vector_t0_ov:

rjmp isr_t0_ov

init:

.include ‘‘init.asm’’ sei

,include ‘‘timer.asm’’

.include ‘‘uart.asm’’

.include ‘‘io.asm’’

.include ‘‘lib.asm’’

Note the two new subroutine calls included in the main loop: get_uart, and send. Get_uart checks if a character is available in the receiver’s data register, reads the character in tmp1, and sets the carry bit to inform the main program. When no

character is available, get_uart returns carry = 0.

get_uart:

Send prepares and sends over the communication line a string comprising two ASCII digits corresponding to the hexadecimal value of each nibble of port B, plus CR and LF.

send:

Put_uartw waits for the status bit UDRE to be set, then writes the content of tmp1 to UDR. It does not check for valid ASCII characters before sending.

put_uartw:

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10.5 Exercises

SX 10.1

Use two software timers to make the upper and lower nibble of port B toggle at different time intervals.

Solution

The two software timers s_timer0 and s_timer1 described in the previous paragraphs must be loaded with different constants to obtain different time intervals for toggling the lines of port B. Here is how LEDMAIN.ASM should be modified for this:

Each of the subroutines toggle1 and toggle2 must affect only one nibble of port B, leaving the other nibble unchanged:

toggle1:

11

8051 Development Board

11.1 In this Chapter

This chapter contains the description of a simple development board for the study of the 8051 family of microcontrollers.. Unlike the development boards dedicated to the HC11 and AVR families, presented in the previous chapters, this project allows the user to load and execute programs in an external RAM area, addressed to be visible both in the program memory and data memory address space.

11.2 Hardware

The schematic of the board is presented in Fig. 11.1, 11.2, and 11.3. Figure 11.1 shows the microcontroller IC10, the bus demultiplexer IC7, the RS232 interface IC12, the RESET and clock circuits, and the ISP connector SV1.

Note the presence of the two NAND gates IC6C and IC6D, which implement the logic function AND between the signals RD\ (Read) and PSEN\ (Program Store Enable), both active LOW, and generate the signal RDPSEN. RDPSEN is active LOW when either RD\ or PSEN\ is LOW. Connecting this signal to the RD\ input of an external RAM circuit allows the use of this RAM to store program as well as data.

The microcontroller can be any of 8032, 8051, 8052, AT89C51, AT89C52, etc. in a DIP40 package. The input EA\ (External Access) of the MCU is connected to the jumper JP1 to allow the use of the internal ROM, if this is available. Connect the jumper so that EA\ = 1 to use the internal ROM. When the external ROM is used, EA\ must be connected to GND.

The clock circuit comprises the crystal Q1 (14.74 MHz) and the capacitors C6, C7 (22 pF). C5 (10 µF) and R4 (10 K) implement the RESET circuit. The pushbutton S1 is provided to allow manual reset of the circuit.

The latch SN74HC573 (IC7), controlled by the signal ALE (Address Latch Enable), is used to demultiplex the external data bus, by storing the lower half (A0–A&) of the address bus.