5 Special Notes
5.1 XDS100v2 Emulator and SCI (UART) Connectivity
The F280025C controlCARD provides emulation and USB-to-UART adapter functionality on the controlCARD.
This allows for a convenient method to debug and demonstrate the F280025C MCU.
Note that the FTDI chip, its support circuitry, and associated isolation components are placed in Macro A (the
left section of the controlCARD). Each of these components contains an additional “:A” within the component
reference designator (that is, R2:A for resistor 2 in Macro A) (see
The configuration of the switches on S1:A determine if the on-board emulator is active, if an external emulator
can be used, or if the device will boot from FLASH/peripherals (see
).
5.2 Clocking Methodology
This controlCARD is required to support a broad range of TI's baseboards. Several designs rely on GPIO18
and GPIO19 for SPIA , while others require these GPIO to be utilized as a precision clock input source. To
accommodate both of these systems a switch (S3) has been added to the design. This methodology should not
be used in a final system as it increases EMI emissions and creates robustness susceptibilities. It is up to the
system designer to choose the best way to implement the clocking circuity for a given system.
5.3 Evaluation of the Analog-to-Digital Converters (ADCs)
When using the F280025C on-chip ADCs there are some useful guideline to follow to realize the performance
numbers listed in the data sheet. This is especially true for the AC parameters such as: SNR, THD, and SINAD.
Furthermore, it can also be shown that there is a direct correlation between the SNR of the ADC result and the
spread of ADC codes seen for a DC input; as such these tips will improve the range and standard deviation of a
DC input as well. Finally, while topics addressed will be with respect to the controlCARD, they are applicable to
other implementations using the F280025C MCU as well.
On-board resistors and capacitors:
By default all inline resistors to the ADC pins are a simple 0-Ω shunt and
all capacitors to the ground plane are not populated. While this circuit can be used to supply the ADC inputs with
a voltage, likely both the resistor(R) and capacitor(C) will need to be populated based on the voltage source's
characteristics. Referring to the ADC Input Model in the
TMS320F28002x Real-Time Microcontrollers
data sheet,
the ADC input has its own RC network made up of the internal sample and hold capacitor, switch resistance,
and parasitic capacitance. By changing the inline resistance and parallel capacitor we can optimize the input
circuit to assist with settling time and/or filtering the input signal. Finally, it is recommended in general to use
Negative-Positive 0 PPM/°C (NP0/C0G) capacitors as these have better stability over temperature and across
input frequencies than other types of capacitors.
Voltage source and drive circuitry:
While the on-chip ADCs are 12-bit architecture (4096 distinct output codes
when converting an analog signal to the digital domain); the translation will only be as precise as the input
provided to the ADC. The typical rule of thumb when defining the source resolution to realize the full specification
of an ADC is to have a 1-bit better source than the converter. In this case that would mean that ideally the analog
input should be accurate to 13-bits.
Typically voltage supplies or regulators are not designed to be precise, but rather accommodate a wide range of
current loads within a certain tolerance and for this reason are not ideal to show the performance of a higher bit
ADC, like the one on the F280025C. This does also not take into account that many times the supply in question
is providing the main voltage to power the MCU itself; which also introduces noise and other artifacts into the
signal.
In addition to the quality of the input signal there is also the aspect of the load presented to the ADC when it
samples the input. Ideally an input to an ADC would have zero impedance so as not to impact the internal R/C
network when the sampling event takes place. In many applications, however, the voltages that are sampled
by the ADC are derived from a series of resistor networks, often large in value to decrease the active current
consumption of the system. A solution to isolate the source impedance from the ADC sampling network is to
place an operational amplifier in the signal path. Not only does this isolate the impedance of the signal from the
ADC, it also shields the source itself from any effects the sampling network may have on the system.
Recommended source for evaluation:
The
Precision Signal Injector (PSI) EVM
the ADC performance on the F280025C ControlCARD. This EVM supports both single ended as well as
Special Notes
SPRUIR3B – SEPTEMBER 2019 – REVISED JUNE 2022
TMS320F280025C controlCARD Information Guide
7
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