ELE22MIC Lecture 18• Writing 68HC11 software

• 68HC11 Main Timer System– Output Compare– What is different about TOC1?

Writing 68HC11 software

Use Functional Decompositioni.e. Break down complex functions into several simpler functions or subroutines.

For example a long time delay cannot be created using just one 16 bit counter, so break the time delay into two parts. 1. A short time delay subroutine2. A long time delay subroutine that calls the short time delay routine many times.

Software Time Delays

Wait_a_sec: LDX #1000 ; 3 cyclesDelay_lots: JSR Wait_a_bit ; 6 for jsr

; * wait 10 ms* Subroutine Wait_a_bit is on next page DEX ; 3 BNE Delay_lots ; 3 RTS ; 5* ReTurn from Subroutine

Software Time Delays

* 1 us = 2 cycles, so 1ms = 2000 cycles.* 10 cycles per delay_more loop. * 29 cycles in wait_a_sec/jsr/pshx/setup/rts overhead...Wait_a_bit: PSHX ; save x on stack 4 LDX #1971 ; X = 2000-29 3Delay_More: NOP ; waste some time 2 NOP ; 2 DEX ; X - - ; 3 BNE Delay_More ; Is (X = 0)? 3 PULX ; recover x 5 RTS ; ReTurn from Subroutine 5

68HC11 Timer - PreScaler

• An E-Clock PreScaler divides by 1, 4, 8 or 16 • This value is selected by writing to Bits PR0, PR1 of the

TMSK2 register (at $1024). • In normal modes the pre-scale rate may only be changed once

within the first 64 bus cycles after reset.

68HC11 Timer - PreScaler

• Crystal Frequency vs clock Pre-Scaler

Free Running Counter - TCNT (1)

• The pre-scaled clock clocks a free-running counter, TCNT (= $100E..$100F)

• The TCNT Count Value can be read using the instructions:LDD $100E Reads all 16 Bits in one instruction. The Timer value is frozen during this instruction sequence.

DONT read the timer with the following instruction sequence: LDAA $100E LDAB $100F as the counter will be incremented between the instructions

68HC11 Timer System (1)

• The HC11 Timer System is based on a free-running 16 bit counter with a four stage programmable pre-scaler. Assuming PR0=0 and PR1 = 0 => TCNT counts at frequency of divide E by 1

• (8MHz crystal -> 2MHZ E clock -> 0.5us per count).

• Five Output Compare functions are included for generating software timing delays or output signals on pins OC1, OC2, OC3, OC4 and OC5

• OC2..OC5 work in exactly the same manner.

68HC11 Timer System (2)

• Timer Register Summary:– TCNT - 2-byte Free Running Timer Counter Value

– TOCx/TICx - 8 x 2-byte TIC/TOC count registers

• 5 for Timer Ouput Compare & 4 Timer Input Capture (One shared TIC/TOC)

– TMSK1 - 1 byte - Timer Interrupt Enables– TMSK2 - $1024 - PR0/PR1/RTI– OPTION - $1039 - CR0/CR1 - Clock Pre-scaler– CFORC - 1 x byte port for Force output compare

– TCTL2 - 1 x byte register for output port control OMx/OLx

– TFLG1 - 1 x byte flag register indicating state of comparisons

Timer Output Compare - TOC (1)

• The five Output Compare pins can be used independently to create precise timing for

time delays or external logic pulses.

• Each Output Compare Register is compared to the value in the 16 bit counter, and if equal, triggers its Timer Output Compare (TOCn) function.

Timer Output Compare (2)

• The output compare hardware can ensure that intervals and waveforms are not subject to jitter due to interrupt servicing routines

Timer Output Compare (3)

• TCTL2 $1020, OM2 & OL2 - select the relationship of Output Compare to Output Port Pin

OMx OLx where x = 2..5 0 0 No Change 0 1 Toggle Pin state 1 0 Force Pin to 0 1 1 Force Pin to 1

TOC2 code (4)

REGBAS EQU $1000 * Starting address for register blockPORTB EQU $04 * Output port BTCNT EQU $0E * Free running counter (16 bit)TOC2 EQU $18 * OC2 register (16 bit)TCTL1 EQU $20 * OM2,OL2,OM3,OL3;OM4,OL4,OM5,OL5TCTL2 EQU $21 * -,-,EDGlB,EDGlA,EDG2B,EDG2A,EDG3B,

* EDG3ATMSK1 EQU $22 * OC1I,OC2I,OC3I,OC4I;OC51,IC1I,IC2I,IC3ITFLG1 EQU $23 *OC1F,OC2F,OC3F,OC4F;OC5F,IC1F,IC2F,IC3FTMSK2 EQU $24 * TOI,RTII,PAOVI,PAII;-,-,PR1,PR0TFLG2 EQU $25 * TOF,RTIF,PAOVF,PAIF;-,-,-,-*** EVB Routine Addresses & Pseudo Vector EquatesPVOC2 EQU $00DC EVB Pseudo Vector for OC2

ORG $2000 Start variables in RAM (upper half)HDLY RMB 2 Half-cycle delay (in 0.5mS increments)

TOC2 code (5)

TOP5 LDS #$0047 * Top of User’s Stack area on EVB LDAA #$7E * Jump (extended) Opcode STAA PVOC2 * Pseudo Vector see manual text LDX #SV5OC2 * Address of OC2 service routine STX PVOC2+1 * Finish jump instruc to TOF svc LDX #REGBAS * Point to register block LDAA #%01000000 * OM2:OL2 = 0:1 STAA TCTL1,X * Set OC2 for toggle, on compare STAA TFLG1,X * Clear any pending,OC2F STAA TMSK1,X * Enable OC2 interrupts CLI * Enable Interrupts

BRA * * Interrupt driven; sit here* Try varying hdly

TOC2 code (6)

**** SV5OC2 - Output Compare 2 Interrupt Service Routine (ISR)** Called at each OC2 interrupt.***SV5OC2 LDD HDLY * Get delay time for 1/2 cycle ADDD TOC2,X * Add HDLY to last compare value STD TOC2,X * Update OC2 (schedule next edge) BCLR TFLG1,X $BF * Clear OC2F RTI ** Return from OC2 service **

TOC1 Differences

• TOC1 is the same as TOC2..5 in the way we program the Timer register, interrupts and Flags.

• TOC1 is different in that it does not use OMx, OLx to control the output pins.

• i.e. TCNTL2 is used for TOC2..TOC5 only.

TOC1 Output Controls

• TOC1 has much greater flexibility in that it can control all 5 output pins at once, forcing any combination of outputs high or low - TOC1

• OC1M - Output Compare 1 Mask • Setting a bit in this mask enables the Data in OC1D to

be forced onto the output pin.

• OC1D - Output Compare 1 Data• Data to be forced onto pin

TOC1 Output Control Registers

Time Delay using TOC1 (1)If you base your subroutine, delayms, on the TOC1 timer, then the time delay should be independent of other CPU activity as the TOC timers tick at a uniform rate unaffected by other CPU activity

The first time you call delayms, you need to read the value of TCNT, the free running timer counter, then add the delay for the number of milliseconds to delay, write that back to the TOC1 count register, and wait until the TCNT count and TOC1 count matches. At this time TOC1 flag is set

Time Delay using TOC1 (2)* We want a subroutine to provide accurate * time delays:** usage example: wait 10ms

startHereldd #10 ; # of millisecondsjsr delayms ; delay # of msswi ; return to BUFFALO

Time Delay using TOC1 (3)When the TOC1 count matches TCNT, a flag is set OC1F.

If your loop waits for the TOC1 flag to be set then you can stop waiting at this point. The BRCLR instruction can be used here. Eg: brclr TFLG1,#%10000000 *Branch if bits in mask are clear to * (branch back to current instruction)

Time Delay using TOC1 (4)msDelayCount equ 2000* Toc1 is at $1016REGBAS equ $1000TOC1 equ $16

TMSK1 equ $22* Timer Interrupt Mask 1 Register * TMSK1 BITS 7 6 5 4 3 2 1 0* INTS OC1I OC2I OC3I OC4I OC5I IC1I IC2I IC3I

TFLG1 equ $23** TGFLG1 BITS: 7 6 5 4 3 2 1 0* Flags: OC1F OC2F OC3F OC4F OC5F IC1F IC2F IC3F*

Time Delay using TOC1 (5)TCNT equ $0Edelayms* Save number of ms delay in D - on stack

pshapshb

* disable TIC & TOC interrupts, * else this code will not work well at all!

ldx #REGBASldaa #0staa TMSK1,x

Time Delay using TOC1 (6)ldd TCNT,xaddd #(msDelayCount-67)std TOC1,x

* clear the OC1F flag * BIT 7 in TFLG1 register

bclr TFLG1,x #%10000000

morems* Branch if (bit TFLG1 = 0) to *

brclr TFLG1,#%10000000 ** wait here whilst the TOC1 flag is not set* when flag is set, drop through to test* for more ms delays...

Time Delay using TOC1 (7)* Get D again

pulbpula

* Decrement ms delay countsubd #1

* is (D == 0) ?beq doneDelay

* Yes, then done delaying* else save D back on stack again, and delay another ms

pshapshb

Time Delay using TOC1 (8)ldd TOC1,x

* add 20000 clocksaddd msDelayCountstd TOC1,x

* delay another msbra morems

doneDelayrts

Acknowledgments

• Images of the configuration registers, and some source code examples, are derived from the Motorola M68HC11 Reference Manual 11rm.pdf or 11a8td.pdf