SPRUJ53B April 2024 – September 2024 TMS320F28P550SJ , TMS320F28P559SJ-Q1
The ADCSOCxCTRL.EXTCHSEL field for each SOC can be used to automatically control an external mux with digital output pins ADCxEXTMUX[3:0]. This functionality enables the application to add additional ADC channels using an external mux, with minimal software overhead. The ADCxEXTMUX[3:0] outputs can be mapped to GPIO pins by configuring the GPIO output crossbar, or configuring the GPIO mux accordingly. The EXTCHSEL field supports up to 4-bit muxes, but fewer mux selection output pins can be configured if desired.
To select a specific channel on the external mux, configure ADCSOCxCTRL.CHSEL to select the ADC pin that is connected to the mux output, and configure ADCSOCxCTRL.EXTCHSEL to select the desired mux input channel. There are a variety of potential mux topologies possible. A basic example can be a single external mux connected to a single ADC input channel. This setup is illustrated in Figure 15-12.
Another example can be to use multiple external muxes connected to different ADC inputs. This setup is illustrated in Figure 15-13.
This scheme saves one digital mux pin, at the expense of using two small muxes instead of a single large mux, and a second ADC input pin.
When using an external channel mux, make sure to comprehend the mux selection and switching delay in the sample/hold time requirement for the SOC. This requirement includes the propagation delay for the output X-BAR (if this is used to configure the mux selection pin), any mux switching delays, and the total resistance and capacitance added to the ADC input network by the external mux device. For more information on calculating the acquisition window size, see Section 15.13.2.
Externally multiplexed ADC channels lose the ability to practically use the analog comparators in the comparator subsystem. However, the digital limit compares in the post-processing block can still be used to generate interrupts and/or PWM trips. For more information, see Section 15.8.