Chapter 6 Parallel Input/Output Control
MC9S08LG32 MCU Series, Rev. 5
Freescale Semiconductor
99
The pullup device is disabled if the pin is controlled by an analog function regardless of the state of the
corresponding pullup enable register bit.
6.4.2
Port Slew Rate Enable
Slew rate control can be enabled for each port pin by setting the corresponding bit in the slew rate control
register (PTxSEn). When enabled, slew control limits the rate at which an output can transition to reduce
EMC emissions. Slew rate control has no effect on pins that are configured as inputs.
6.4.3
Port Drive Strength Select
An output pin can be configured for high-output drive strength by setting the corresponding bit in the drive
strength select register (PTxDSn). When high drive is selected, a pin is capable of sourcing and sinking
greater current. Even though every I/O pin can be selected as high drive, you must ensure that the total
current source and sink limits for the MCU are not exceeded. Drive strength selection is intended to affect
the DC behavior of I/O pins. However, the AC behavior is also affected. High drive allows a pin to drive
a greater load with the same switching speed as a low drive enabled pin into a smaller load. Because of
this, the EMC emissions may be affected by enabling pins as high drive.
6.5
Open Drain Operation
For most cases, port pins that share functions with the LCD operate as open drain outputs. As an open drain
output, the output high of the pin is dependent upon the pullup resistor. The pullup resistor can be an
internal resistor enabled by the PTxPEx bit or an external resistor.
•
The value of the internal resistor can be in the range of 17.5 to 52.5 k
Ω
•
The value of an external resistor must be carefully selected to ensure it supports the output loads
that are being driven.
6.6
Pin Behavior in Stop Modes
Pin behavior following execution of a STOP instruction depends on the stop mode that is entered. An
explanation of pin behavior for the various stop modes follows:
•
Stop2 mode is a partial power-down mode, whereby I/O latches are maintained in their pre-STOP
instruction state. CPU register status and the state of I/O registers must be saved in RAM before
the STOP instruction is executed to place the MCU in stop2 mode. Upon recovery from stop2
mode, before accessing any I/O, you must examine the state of the PPDF bit in the SPMSC2
register. If the PPDF bit is 0, I/O must be initialized as if a power-on reset had occurred. If the PPDF
bit is 1, I/O register states must be restored from the values saved in RAM before the STOP
instruction was executed. Peripherals may require initialization or restoration to their pre-stop
condition. You must then write a 1 to the PPDACK bit in the SPMSC2 register. Access to I/O is
again permitted in the user application program.
•
If the LCD module is configured to operate in Stop modes, the drive mode of the GPIO shared with
LCD is retained upon stop recovery.
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Страница 26: ...Chapter 1 Device Overview MC9S08LG32 MCU Series Rev 5 26 Freescale Semiconductor...
Страница 40: ...Chapter 2 Pins and Connections MC9S08LG32 MCU Series Rev 5 40 Freescale Semiconductor...
Страница 96: ...Chapter 5 Resets Interrupts and General System Control MC9S08LG32 MCU Series Rev 5 96 Freescale Semiconductor...
Страница 296: ...Chapter 12 Serial Peripheral Interface S08SPIV4 MC9S08LG32 MCU Series Rev 5 296 Freescale Semiconductor...
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