MM9Z1_638: problems caused by channel switch

[Shaojun]

Dear support / community,

during some debugging, the following problems were found:

1.

the switch between internal temperature channel  and external temperature channel(PTB4) created some noise on ISENSE, are there any recommended software configurations to improve the performance.

2.

int. temperature channel  disabled and ext. temperature channel enabled: Voltage drop an VDDA and PTB5 is 2.5V 

int. temperature channel  enabled and ext. temperature channel disabled : Voltage drop an VDDA and PTB5 is 0V

In this case, channel switching causes voltage fluctuations on the NTC. Is there any special consideration for this phenomenon in the design? I am worried that the accuracy of external temperature measurement will be affected.

Thanks!

best regards,

Shaojun

[Q_man]

Shaojun,

yes you can enable both ETMEN + ITMEN, but the resulting behaviour is that the chip itself will perform alternating ETS - ITS -ETS-ITS.... measurements with the PTB5 being GND when ETS is running and PTB5 being OPEN when ITS is running.

Rgds

W

[Q_man · Accepted solution]

Shaojun,

if you enable both ETMEN + ITMEN, then the chip itself will perform alternating measurements of ITS and ETS. The PTB5 will also be automatically be enabled (disabled) when the ETS (ITS) measurement is performed. This will be independent of the GPIO_TSENSE configuration!

To have control over the PTB5 switch you should not enable both bits!

Rgds W.

[Q_man]

Shaojun,

As indicated in the datasheet this is intended behavior to remove the NTC biasing during low power modes and in case no external measurements are performed (e.g. to avoid self heating of the NTC).

The switch is closed with the ACQ_CTL.ETMEN(M) being set and opened when cleared.

No special care has to be taken to handle this as long as there is no significant filter (e.g. RC low pass) causing long setting times.

It's also possible in software to perform multiple measurements, with only the ext. temperature channel enabled, and to discard the first values. You can also check your design and compare the individual measurements to see if the ADC values are settled or not.

Rgds

Wolfgang

[Shaojun]

thank you for your reply Wolfgang @Q_man 

is it possible to enable int. and ext. temperature Measurement(ETMEN + ITMEN) at the same time and the channel is switched by  GPIO TSENSE configuration register (GPIO_TSENSE)?

Rgds

Shaojun