Timing measurement

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Timing measurement

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juliancox
Contributor V

I am using a comparator to measure the time for a capacitor to charge up through a resistor, the other end of which is tied to a port pin. The time measured is then used to determine the value of the resistor (actually a NTC thermistor) and hence the temperature. The comparator uses component CMP1 with the on-chip bandgap voltage as the + input. I am using FC161:GetTicks16 to measure the time taken for the comparator to switch. Using the DEMOQE board everything worked fine and I was getting values of 19000 @ 25 degs and 300 @ 150 degs (comparable to results on a different processor type). I then moved over to a target board (S08SH4) which used identical components, but the values were 390 and 250. The comparator input waveforms were the same in both cases and I could see that the comparator tripped at the same point in time and voltage. It would appear that there is a timing issue as not only are the values different but the %ag change is also different.

Any help would be gratefully received.

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Carlos_Mendoza
NXP Employee
NXP Employee

Hi Julian,

Please notice that the Bandgap Voltage Reference for the S08QE MCU has a typical value of 1.17 V (factory trimmed at VDD = 3.0 V) and the S08SH MCU has a typical value of 1.20 V (factory trimmed at VDD = 5.0 V) so you will not have the same reference voltage in the compataros CPM1 + pin. Regarding the timer, are you selecting the same clock
source and prescale factor? Also, are you trimming both devices to the same frequency?

Best Regards,
Carlos Mendoza
Technical Support Engineer

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juliancox
Contributor V

Perhaps I should make my problem a little simpler: -

I have a capacitor being charged through a resistor. The resistor is fed by a port pin. and the capacitor voltage is fed to Comparator CPM1 - pin and the + pin is connected to the internal bandage supply. 

I calculate that the time for the comparator to switch should be approx. 9ms and when I measure the time on an oscilloscope it is approx. 9ms.

When I activate the charging process I also reset and start component FC161:FreeCntr16. In the comparator ISR I read register TPM1CNT.

Using my DEMOQE I read approximately 18000 ticks which, with a 4MHz clock gives a tick frequency of 2MHz, which equates to 18000 ticks for 9ms. So far - so good.

Using my target S08SH4 board the comparator switches after 9ms, but I only get a tick count of 350.

I have checked everything I can, but where do I go next 

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Carlos_Mendoza
NXP Employee
NXP Employee

Hi Julian,

Please notice that the Bandgap Voltage Reference for the S08QE MCU has a typical value of 1.17 V (factory trimmed at VDD = 3.0 V) and the S08SH MCU has a typical value of 1.20 V (factory trimmed at VDD = 5.0 V) so you will not have the same reference voltage in the compataros CPM1 + pin. Regarding the timer, are you selecting the same clock
source and prescale factor? Also, are you trimming both devices to the same frequency?

Best Regards,
Carlos Mendoza
Technical Support Engineer

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