Is X register available for writing TIMx_CCRyH/L registers ?

st25 · ‎2010-01-11

Posted on January 11, 2010 at 22:50

st25 · ‎2011-05-17

Posted on May 17, 2011 at 15:07

I am investigating the usage of the PWM output. I found a code to write the TIM3_CCR2H/L registers at the TIM3_OC2Init function of the FWLib source code stm8s_tim3.c The source code behaves two 8-bit write operations for the timer registers as follows.

/* Set the Pulse value */

TIM3->CCR2H = (u8)(TIM3_Pulse >> 8);

TIM3->CCR2L = (u8)(TIM3_Pulse);

I assumed that the program would be more simple when the write operation is described as one 16-bit write operation like following.

volatile u16 TIM3_CCR2 @ 0x00532F;

:

TIM3_CCR2 = (u16)TIM3_Pulse;

But the program using a 16-bit variable to write the timer register has been failed, however the the line is certainly compiled into an LDW instruction sourced from X register.

Why this program has been failed ? Is the WRITE operation order of the LDW instruction different from what the timer register expected ?

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[ This message was edited by: st16 on 10-01-2010 15:34 ]

mozra27 · ‎2011-05-17

Posted on May 17, 2011 at 15:07

Hi,

I think that is a security mechanism that is consist to write first the CCR2H register before the CCR1H.

Regards

Anis

brazov2 · ‎2011-05-17

Posted on May 17, 2011 at 15:07

Hi Anis,

you're right, see reference manual pag. 161 paragraph 17.5.1 (it is for TIM1, but it's valid also for other timers). On 8-bits microcontrollers this kind of read and write mechanism protection is usual 🙂 for r/w access of 16-bit or 10-bit registers as in the case of ADC converted value.

brazov :(

st25 · ‎2011-05-17

Posted on May 17, 2011 at 15:07

Hi Anis, brazov

I assumes that the mechanism is for security reason and many other 8-bit MCU with 16-bit timer have same kind of one. It is reasonable for STM7 because STM7 has 8-bit registers only.

But STM8 has 16-bit registers X and Y. I wonder why the firmware library is still using 8-bit access to the 16-bit timer registers and why STM8 will fail if access to the 16-bit register by the 16-bit X register.

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