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| Figure 1: Capacitance Meter Circuit |
COMATOS has a lot of small details, making it difficult to absorb completely by simply reading the documentation. Perhaps the most useful illustration would be through example.
The system developed in this example is a digital capacitance meter (DCM). It is capable of reading capacitances in the range 1nF~200nF. A measurement is initiated whenever a key is pressed on the keyboard, and the output is sent to the PC through the UART. The circuit used is shown in figure 1. The capacitor to be measured is place in an RC circuit. When the voltage on the capacitor reaches
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| Figure 1: Capacitance Meter Circuit |
This COMATOS implementation of the DCM is divided into three tasks:
The code file begins with the required include directives for the OS, the interrupt vector table, and constant definitions for the keyboard scan table and output message. Note the inclusion of the required include files:
.nolist .include "8515def.inc" ; Contains the macro definitions used in the OS, must always be included .include "comtapi.inc" .list ; Task Number Definitions .equ KbdTask=0 .equ MeasureTask=1 .equ DisplayTask=2 .equ CapIsr=3 ; Measurement State Definitions .equ keyWait =0 .equ dischargeWait =1 .equ measureWait =2 .def key =r17 ;holds raw press value .def butnum =r18 ;final press value .def state =r19 ;debounce state .def lastKey =r20 ;last keypress .def Temp =r21 ;temporary register .def capacit =r19 ;capacitance value .equ waitState=0 .equ gotKeyState=1 .equ TIMSKVal=0x08 ; Only use input capture .equ TCCR1BVal=0b01000011 ; Capture on rising edge, 64x clock div .equ ACSRVal=0b00000111 ; Interrupt enable, rising edge, input capture .cseg .org $0000 rjmp RESET ;reset entry vector reti reti rjmp T1CapISR reti reti reti rjmp OST0ISR ;Timer 0 ISR (MUST ALWAYS BE PRESENT) reti reti rjmp OSTXISR ;UART buffer empty (MUST ALWAYS BE PRESENT) reti reti ; Contains the definitions of the subroutines used within the OS, ; including the scheduler, timer 0 ISR, the OS call functions. ; Always included here .include "comtf.inc" ;Keyboard Scan Table keytbl: .db 0b11101110, 0b11101101, 0b11101011, 0b11100111 .db 0b11011110, 0b11011101, 0b11011011, 0b11010111 .db 0b10111110, 0b10111101, 0b10111011, 0b10110111 .db 0b01111110, 0b01111101, 0b01111011, 0b01110111 capMessage: .db "The capacitance in nF is:"
As usual, execution begins at the reset interrupt vector. After the stack point is initialized, OSInit must be called. In this case, the timeout counter is set to 30ms, which is the interval between executions of the keyboard task. Next, the relevent I/O registers are initialized to set up input capture, a 64x clock divider on timer 1, and the input/output settings on Port B. Following I/O port initialization, we create the three tasks with OSCreateTask as described in table 1. Note also that instead of passsing absolute addresses for the entry point of each task, we pass labels.
| Table 1: Task Creation Settings | ||
|---|---|---|
| Task | Timeout Condition | Message Mask |
| Keyboard | 30 - Task runs every 30 ms | 0xff - Not using messaging |
| Measurement | 0 - Not using timeout | 0x01 - Initially waiting on task 0 (keyboard) |
| Display | 0 - Not using timeout | 0x02 - Waiting on the measurement task |
RESET: ldi r20, LOW(RAMEND) ;setup stack pointer out SPL, r20 ldi r20, HIGH(RAMEND) out SPH, r20 OSInit 30, OSDebug ; Specifies 30 ms timeout and debugging features ; Setup timer 1 ldi r16, (TIMSKVal | TimerMask) out TIMSK, r16 ; Setup analog comparator ldi r16, ACSRVal out ACSR, r16 ; Setup PortB for analog comparator ldi r16, 0b11110011 ; Make most pins output except comparator out DDRB, r16 clr r16 out PORTB, r16 ; Start discharging OSCreateTask KbdTaskF, 30, 0xff ; Can't run unless timeout since ; mask is 0xff OSCreateTask MeasureTaskF, 0x00, 0x01 ; Initially can't run unless ; it gets a message from the keyboard OSCreateTask DisplayTaskF, 0x00, 0x02 ; No timeout, wait for message from measurement OSStart ; Run scheduler halt: rjmp halt ; Not really necessary
The keyboard task acts as a state machine. During each run (every 30 ms) it uses OSGetState to determine what state it was in before. The task also stores the last key pressed in its own mailbox by sending a message to itself. The state machine works as follows:
; Pulls a key from the keyboard, debounces it, and sends it to the measurement ; task KbdTaskF: OSGetState state ; get the current state ; Get the key press ldi temp, 0x0f ;set lower four lines to output out DDRC, temp ldi temp, 0xf0 ;and turn on the pullups on the inputs out PORTC, temp nop ;Need some time for the pullups to nop ;charge the port pins nop nop in temp, PINC ;read the high nibble mov key, temp ;and store it (with zeros in the low nibble) ldi temp, 0xf0 ;set upper four lines to outputs out DDRC, temp ldi temp, 0x0f ;and turn on pullups on the inputs out PORTC, temp nop ;As before wait for the pin to charge nop nop nop in temp, PINC ;read the low nibble or key, temp ;combine to make key code ;At the point the raw key code should have exactly one zero each in ;the lower and upper nibbles. Any other number of zeros indicates ;either no-button pressed or multiple-button pressed. ;Now search the table for a match to the raw key code ;and exit with a button number ldi ZL, low(keytbl*2) ;table pointer in FLASH ldi ZH, high(keytbl*2) ;so convert from word to byte addr ldi butnum, 0 tbllp: lpm ;get the table entry cp key, r0 ;match? breq foundit inc butnum ;if not, have we exhaused the cpi butnum, 0x10 ;table breq illegal adiw ZL, 1 ;if not, get the next table entry rjmp tbllp foundit:subi butnum, -1 ;add one for display rjmp TT1_statematch illegal:clr butnum TT1_statematch: OSGetMess lastKey, 0 ;Last key press is stored in our own mailbox tst state ;Check the state breq TT1_state_0 TT1_state_1: OSSetState 0 ; Return state to zero cp lastKey, butnum ; Compare the key presses breq TT1_debounce ; Key is debounced rjmp TT1_false ; False alarm TT1_debounce: tst butnum breq TT1_release OSSendMess butnum, measureTask; Send the key press to the measurement task TT1_release: OSReturn 0 TT1_state_0: cp lastKey, butnum ; Compare the key presses brne TT1_newKey OSReturn 0 TT1_newKey: OSSetState gotKeyState ; New key press OSSendMess butnum, 0 ; Store the last button press OSReturn 0 TT1_false: OSSetState 0 OSReturn 0
This task is the meat of the DCM. It acts as a state machine as well, but it uses a different approach from the keyboard task. Instead of storing a state variable using OSSetState/OSGetState, it uses OSSetEntryPt to redefine where execution begins with each successive run. The states are defined as follows:
MeasureTaskF: ; If we did not get a message from the keyboard ; something is wrong OSGetMessageSrc Temp ; Get the list of who sent us messages andi Temp, exp2(kbdTask) brne MT_startDischarge OSReturn 1 ; the keyboard ; If we're here, the keyboard sent a message MT_startDischarge: sbi DDRB, 2 ; Discharge through analog comp cbi PORTB,2 ldi Temp,25 OSSetTimeout Temp ; Wait 25 ms OSSetEntryPt MT_startMeasure ; Next time we start, we want to pick OSReturn 0 ; up at the start measurement point MT_startMeasure: cbi DDRB, 2 ; Make pin 2 an input again clr Temp out TCNT1H, Temp ; Clear timer 1 out TCNT1L, Temp sbi PORTB, 0 ; Raise Vcc to start charging ldi Temp, TCCR1BVal ; Start the timer waiting for charge out TCCR1B, Temp OSSetMessMask exp2(CapISR) ; Now we wait on the completion of charging clr Temp OSSetTimeout Temp OSSetEntryPt MT_finishMeasure ; Next time we start, we will be at OSReturn 0 ; the measurement handling code ;If we're here, the correct value is in the capture register MT_finishMeasure: OSGetMessageSrc Temp ; Get the list of who sent us messages andi Temp, exp2(capISR) ; If the capture ISR did not send it brne MT_fM1 OSReturn 1 ; something's wrong, so return bad error code MT_fM1: clr Temp out TCCR1B, Temp ; Shut off timer 1 cbi PORTB, 0 ; Drop Vcc to discharge capacitor in Temp, ICR1L ; Grab the capacitance value OSSendMess Temp, DisplayTask ; Send the value off to be displayed OSSetMessMask exp2(KbdTask) ; Wait for the next key press OSSetEntryPt MeasureTaskF ; Start from the beginning next time OSReturn 0 ; DoneDisplay Task
The job of this task is to send the capacitance message to the UART. First, it copies the message from the program memory to the SRAM, since that is where OSUARTWait needs it to be. It sets the character count and begins transmission. The final step is to format the value of the capacitance as ASCII characters instead of a hexadecimal number, and then display that to the UART.
DisplayTaskF: ; If we're here, there's a capacitance to print ldi ZL, low(capMessage*2) ; Point to the first character ldi ZH, high(capMessage*2) ; of the output string ldi Temp, 26 sts OSBase+OSUARTCharCnt, Temp ; Tell the UART to 26 characters ldi YL, low(USRBase) ; Point at the beginning of memory ldi YH, high(USRBase) DT_0: ; Copy from program memory to SRAM lpm adiw ZL,1 st Y+,r0 dec Temp brne DT_0 ; Aim Z at the output string ldi ZL, low(USRBase) ldi ZH, high(USRBase) lds Temp, OSBase+OSStatus cbr Temp, exp2(txdone) ; Clear the bit indicating txdone sts OSBase+OSStatus, Temp ld Temp, Z+ ; Put the first character in Temp OSUARTWait Temp ; Send the message ; Now send the capacitance value itself OSGetMess Capacit, MeasureTask ; Format the string ldi ZL, low(USRBase) ldi ZH, high(USRBase) clr Temp DT_1: cpi Capacit, 100 brlo DT_tens subi Capacit, 100 inc Temp rjmp DT_1 DT_tens: subi Temp, -ascii0 st Z+, Temp clr Temp DT_2: cpi Capacit, 10 brlo DT_ones subi Capacit, 10 inc Temp rjmp DT_2 DT_ones: subi Temp, -ascii0 st Z+, Temp subi Capacit, -ascii0 st Z+, Capacit ldi Temp, 0x0d ; Newline character st Z, Temp ; We've done the conversion, now send the value ldi ZH, high(USRBase) ldi ZL, low(USRBase) ldi Temp, 4 sts OSBase+OSUARTCharCnt, Temp ; Tell the UART to 3 characters ld Temp, Z+ ; Put the first character in Temp OSUARTWait Temp OSReturn 0Capture ISR
Like any ISR, the capture ISR is intentionally kept short. It simply sends a message to the measurement task using ISRSendMess. This message does not contain any data and simply makes the measurement task eligible to run again.
; Analog comparator capture complete ; Capacitance in nF is in capture low register T1CapISR: push Temp push Capacit in Temp, SREG push Temp ISRSendMess Temp, measureTask, CapISR ; Send a message to the measurement ; task signalling the available value pop Temp out SREG, Temp pop Capacit pop Temp reti
This example leaves some room for optimization. For example, the ISR could send a message directly to the display task, completely eliminating the measurement task from the last step. Likewise, using OSSetEntryPt to develop state machines tends to involve simpler logic than using OSSetState/OSGetState. Please feel free to experiment with this example, which can be found in the source code section of this document.