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Does SPI need pullups?
For the SPI operation no pull-ups are needed. Unlike the I2C where pull-ups are needed because the drivers have open collector outputs, no pullups are needed on the SPI signal lines since the drivers have push-pull outputs.
What is a weak pull-down?
A resistor with relatively high resistance is called a “weak” pull-up or pull-down; when the circuit is open, it will pull the output high or low more slowly, but will draw less current.
What is weak pull-up in microcontroller?
Weak pullups provide a method within many microcontrollers such as the Atmel AVR and Microchip PIC microcontrollers to support internal/selectable pull-ups for convenience and reduced parts count. If you review in the microcontroller data sheet, there is no resistance given for the weak pullups.
Is SPI push-pull or open drain?
In general, there is no need for pull up resistors in a SPI implementation, because the pins use a push-pull configuration, rather than Open Drain.
When to use pull down resistors in SPI?
A lot of devices have open-collector outputs, so a pulldown resistor on the line would cause a permanent low state. SPI is a totem-pole system and as such the drivers are push-pull, not open collector.
When to use a pull down on a microchip?
Therefore the MISO input of a processor will be driven by a tri-statable totem pole output from the slave, a pull-down is therefore fine to hold that input into a known state when the slave is not selected. Similarly for the slave’s MOSI input when the master is in reset.
Why do you need a weak pull down resistor?
A weak pull resistor would cause a small drain of current. Theoretically any resistor would work, but for practical purposes, a weak resistor is used because unnecessary high current drains can cause issues and can easily be avoided by sizing the resistor correctly.
Can a SPI master use an open collector?
Agreed, but if the SPI master uses open collector outputs then it’s a bit of a brain dead SPI master, it’ll be slow and subject to noise on rising edges (not good for high speed SPI slaves). SPI is not an open collector driven system, masters should not use open collector drivers!