UPSD3422_V01 datasheet(83/300 Pages) STMICROELECTRONICS

83 / 300 page

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I/O ports of mcu module

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17.1.4

Alternate functions

There are five SFRs used to control the mapping of alternate functions onto MCU port pins,

and these SFRs are depicted as switches in Figure 15 on page 80.

●

Port 3 uses the SFR, P3SFS (Table 41 on page 84).

●

Port 1 uses SFRs, P1SFS0 (Table 43 on page 84) and P1SFS1 (Table 44 on page 84).

●

Port 4 uses SFRs, P4SFS0 (Table 46 on page 85) and P4SFS1 (Table 47 on page 85).

Since these SFRs are cleared by a reset, then by default all port pins function as GPIO (not

the alternate function) until firmware initializes these SFRs.

Each pin on each of the three ports can be independently assigned a different function on a

pin-by-pin basis.

The peripheral functions Timer 2, UART1, and I2C may be split independently between Port

1 and Port 4 for additional flexibility by giving a wider choice of peripheral usage on a limited

number of device pins.

When the selected alternate function is UART0, UART1, or SPI, then the related pins are in

quasi-bidirectional mode, including the use of the high-side driver for rapid 0-to-1 output

transitions. The high-side driver is enabled for just one MCU_CLK period on 0-to-1

transitions by the delay function at the “digital_alt_func_data_out” signal pictured in

Figure 16 on page 80 through Figure 18 on page 81.

If the alternate function is Timer 0, Timer 1, Timer 2, or PCA input, then the related pins are

in quasi-bidirectional mode, but input only.

If the alternate function is ADC, then for each pin the pull-ups, the high-side driver, and the

low-side driver are disabled. The analog input is routed directly to the ADC unit. Only Port 1

supports analog functions (Figure 16 on page 80). Port 1 is not 5 V tolerant.

If the alternate function is I2C, the related pins will be in open drain mode, which is just like

quasi-bidirectional mode but the high-side driver is not enabled for one cycle when

outputting a 0-to-1 transition. Only the low-side driver and the internal weak pull-ups are

used. Only Port 3 supports open-drain mode (Figure 17 on page 81). I2C requires the use of

an external pull-up resistor on each bus signal, typically 4.7K

Ω to VCC.

If the alternate function is PCA output, then the related pins are in push-pull mode, meaning

the pins are actively driven and held to logic '1' by the high-side driver, or actively driven and

held to logic '0' by the low-side driver. Only Port 4 supports push-pull mode (Figure 18 on

page 81). Port 4 push-pull pins can source IOH current when driving logic '1,' and sink IOL

current when driving logic '0.' This current is significantly more than the capability of pins on

Port 1 or Port 3 (see Table 211 on page 271).

For example, to assign these port functions:

●

Port 1: UART1, ADC[1:0], P1[7:4] are GPIO

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Port 3: UART0, I2C, P3[5:2] are GPIO

●

Port 4: TCM0, SPI, P4[3:1] are GPIO

The following values need to be written to the SFRs:

P1SFS0 = 00001111b, or 0Fh

P1SFS1 = 00000011b , or 03h

P3SFS = 11000011b, or C3h

P4SFS0 = 11110001b, or F1h

P4SFS1 = 11110000b, or F0h

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