Injected ADC Conversion Time off by 1.2usec

[richard23]

Posted on February 20, 2014 at 16:37

I am using TIM1 (PWM Mode) to trigger injected adc conversions in dual mode.  The ADC clock is set to 72 Mhz (PLL Clock).   The ADC sampling is set to 19.5 cycles --> a total conversion time of 19.5 + 12.5 = 32 cycles = 444 nsec.   For N channels the JEOS interrupt should occur at N * 444 nsec after the rising edge of the PWM.   

The problem is that the JEOS interrupt occurs at N * 1.68 usec after the rising edge of the PWM. I have tried this for 1,2 and 3 ADC conversions in the sequence. 

I have confirmed the correct clock by calling RCC_GetClocksFreq().  I also changed the sampling from 19.5 cycles to 61.5 cycles and the time from PWM rising edge  to  JEOS changed by the appropriate amount - 578 nsec. 

Does anyone know what's causing the 1.2usec delay in the ADC conversion

 

[chen]

Posted on February 20, 2014 at 17:07

The reference manual says

''Figure 47. Injected conversion latency

1. The maximum latency value can be found in the electrical characteristics of the STM32F40x and STM32F41x datasheets.''

The datasheet says :

tlat(4) Injection trigger conversion latency

fADC = 30 MHz - - 0.100 μs

                            - - 3(7)μs        1/fADC

(4) Based on characterization, not tested in production.

(7) For external triggers, a delay of 1/fPCLK2 must be added to the latency specified in Table 67.

[richard23]

Posted on February 20, 2014 at 17:38

Thanks for the reply.  But I don't think that latency explains what i see.  

If I add 3.5 cycles for maximum latency the total time from PWM rising edge to the conversion of 1 adc channel, it should be  (35.5 cycles)  = 493 nsec.  I see 1.68 usec.  (ADC clock = 72 MHz)

[Tesla DeLorean]

Posted on February 20, 2014 at 18:06

What part are we talking about, most of the parts have a max ADC clock rate in the 14-18 MHz range.

[richard23]

Posted on February 20, 2014 at 18:43

STM32F302RBT6

[richard23]

Posted on February 20, 2014 at 19:35

This part has the fast ADC with clocks up to 72MHz