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I found the answer to my problem after more experimentation. The key is
to use the FLMC to force the initial delay into the modulus counter and then just let it use the 5mS delay when the first one times out. What seems to have been happening was that the 20mS never got loaded even though I was writing it before MODMC was enabled. So, here is the version that works for anyone who wants to change the modulus counter on the fly. This creates a 20mS delay before the first iteration of the ISR and then a 5mS delay every time after that: 4178 ; //Set up modulus for executive 4178 ; MCCNT = EXECUTIVE_POWERUP_DELAY; 4178 CC7530 ldd #30000 417B 7C0076 std 0x76 417E .dbline 339 417E ; 417E ; //Enable the modulus counter, enable int, mod mode, force load and enable counter at bus/16 417E ; MCCTL = 0b11001111; 417E C6CF ldab #207 4180 7B0066 stab 0x66 4183 .dbline 342 4183 ; 4183 ; /* set the modulus timer for next time and every time thereafter */ 4183 ; MCCNT = EXECUTIVE_TIME_IN_CLOCK_TICKS; 4183 CC1D4C ldd #7500 4186 7C0076 std 0x76 And the interrupt service routine doesn't need to do anything except clear the flag... 4084 ; #pragma interrupt_handler RealTimeOS_ISR 4084 ; void RealTimeOS_ISR() 4084 ; { 4084 .dbline 73 4084 ; /* clear the interrupt flag */ 4084 ; MCFLG |= BIT(7); 4084 1C006780 bset 0x67,#128 4088 .dbline 74 4088 ; Executive(); 4088 164056 jsr _Executive 408B .dbline -2 408B .dbline 76 408B ; 408B ; } 408B L12: 408B .dbline 0 ; func end 408B 0B rti -----Original Message----- From: Daniel White [mailto:] Sent: Thursday, July 01, 2004 10:10 AM To: Subject: [68HC12] Need help with modulus down counter Anyone out there that has experience with the modulus down counter on the S12? Section 3.3.29 of the S12 ECT block user guide (S12ECT16B8V1.pdf) describes the MCCNT register as having a load register. This description says that, if the counter is enabled (MODMC=1), writing to the MCCNT register won't update the counter until it has finished counting down to 0. I am trying to use this feature to my advantage but this doesn't seem to be how it really works. What I want to do is delay 20mS before the first execution of my RTOS and then run it every 5mS after that. I thought this is what I would get if I set MCCNT to 20mS and then enable the counter and then set MCCNT to 5mS. So this is what I tried (bus clock 24MHz): 410B ; //Set up modulus for executive 410B ; MCCNT = EXECUTIVE_POWERUP_DELAY; 410B CCEA60 ldd #0xea60 410E 7C0076 std 0x76 4111 .dbline 296 4111 ; 4111 ; //Enable the modulus counter, enable int, mod mode and enable counter at bus/8 4111 ; MCCTL = 0b11000110; 4111 C6C6 ldab #198 4113 7B0066 stab 0x66 4116 .dbline 299 4116 ; 4116 ; /* set the modulus timer for next time and every time thereafter */ 4116 ; MCCNT = EXECUTIVE_TIME_IN_CLOCK_TICKS; 4116 CC3A98 ldd #15000 4119 7C0076 std 0x76 But on my scope I see that the first interrupt occurs about 3.5mS after powerup (only about 2.8mS after the initialization code) and then every one after that is 5mS apart as desired. The only way I can get the desired behavior is to initialize the counter for 20mS and then re-initialize it to 5mS inside the interrupt which I didn't think I should have to do. I'm concerned that if I reset it every time inside my ISR that it will lose the interrupt latency time and I will have a cumulative error. This means that I would have to put a check in to only initialize it on the first iteration of the ISR. I think I'm just missing the elegant solution. Any help would be appreciated. Daniel White Ph (812) 376-0985 Gale Banks Engineering 2228 Lafayette Ave. Columbus, IN 47201 Visit us on the web: www.bankspower.com ---- -- Yahoo! Groups Links a.. To |