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158 12 Digital Voltmeter with RS232 Interface

it reads all the analog inputs of the ADC, and stores the results in a RAM buffer. If an ASCII character in the range ‘0’ to ‘7’ is received on the asynchronous serial interface, the two bytes needed to represent the 10-bit value of the corresponding analog input are converted to four ASCII digits and transmitted on the serial communication line.

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

Note that the main program loop only contains calls to three subroutines. Get_adc reads all the analog inputs, get_cmd checks the UART receiver data register for an ASCII character between ‘0’ and ‘7’, and returns the CARRY flag set if one of these characters has been received. Finally, exec_cmd extracts the value of the analog line indicated by the command character, converts it to four ASCII digits, and sends them to the serial line.

All other modules contain either variable definitions (MAP.ASM), calls to the initialization routines (INIT.ASM), peripheral specific routines (UART.ASM, SDC.ASM) or miscellaneous data conversion or execution routines (LIB.ASM).

MAP.ASM contains a number of definitions for constants, and RAM variables:

INIT.ASM is a block of code containing the initialization of the stack pointer SP, and calls to the initialization routines associated with the peripheral interfaces used by the application. The resource-specific initialization routines are located in the corresponding module: init_uart in UART.ASM and init_adc in ADC.ASM. The whole block of code contained in INIT.ASM is inserted in the main program at the point specified by the .include directive.

init_sp:

ldi tmp1,high(ramend) out sph,tmp1

ldi tmp1,low(ramend) out spl,tmp1

rcall init_uart rcall init_adc

end_init:

The UART initialization routine, init_uart. has been described in the previous chapters. Below is the listing of the subroutine for the initialization of the ADC subsystem:

This code selects channel number 0 by writing to admux, then selects the ADC clock by programming the prescaler to divide the system clock by 128, and enables the ADC system by setting the bit ADEN (A/D converter enable) in ADCSR. Finally, it starts the conversion on the selected channel, by setting the bit ADSC (Start A/D conversion) in ADCSR.

The subroutine get_adc checks the bit ADIF in ADCSR to determine if the last conversion is completed. If ADIF = 1, then the ADC result value (read as two bytes) is stored in a RAM buffer buf_adc, using the channel number as index in this buffer, then a new conversion is started on the next channel:

The RS232 communication is performed by means of two subroutines called in the main program loop: get_cmd and exec_cmd, both located in the module LIB.ASM.

Get_cmd checks the UART receiver data register for an ASCII code between ‘0’ and ‘7’. If no character is received, or the ASCII code is outside the expected range, get_cmd returns CARRY = 0, otherwise it returns the valid code in the register r16 (tmp1) and CARRY = 1.

162 12 Digital Voltmeter with RS232 Interface

buffer buf_adc, to retrieve the result of the last conversion performed on the selected analog channel.

The accompanying CD contains a terminal emulator, Voltm.exe, which automatically interrogates the voltmeter for the values of all eight analog channels, and displays the values on the screen. The default Comm port is COM2, but this can be changed by the user. The Start/Stop button initiates and interrupts the communication process. The program runs under Windows™ 9x.

12.4 Exercises

X 12.1

Write a subroutine CBCD16 that converts a 16-bit binary number, lower than 9999, received as input in the registers tmp1:tmp2, to four BCD digits placed in the same registers.

X12.2

Modify the software so that the data sent on the serial line contains the decimal representation of the analog value read from the requested channel.

X12.3

Modify the software so that the decimal value of channel 0 is also displayed on the SPI display described in Appendix A14.