bhavanireddy
Newbie level 3
microcontroller output impedance
The DDR SDRAM controller in I.MX25 micro has the following characteristics as per its datasheet/reference manual.
“For DDRTYPE=00 (DDR2 or mDDR) at 25o C in the typical case, the following drive strengths can be referenced: Standard (3.6 mA), High (7.2 mA), Max (10.8 mA) with equivalent impedances of 90ohm, 45ohm, and 20ohmrespectively.”
It also has the logic levels of min 1.44v (0.8 * 1.8v VDD) for Voh (High) and max 0.36v (0.2* 1.8v VDD) for Vol (Low).
The selected drive strength, for example, is 3.6 mA at an impedance of 90ohm. If we apply ohm’s law with logic high voltage of 1.44v and impedance of 90 ohm the source current should be 16 mA but it is 3.6 mA as per the datasheet.
What is wrong with my calculation? The data sheets of most of the microcontrollers do not specify the output impedance at their digital I/O and peripheral pins. So I need to calculate the output impedance (Zo) for impedance matching the PCB traces. How do I know the output impedance (Zo) if it is not explicitly given in the datasheet.
The DDR SDRAM controller in I.MX25 micro has the following characteristics as per its datasheet/reference manual.
“For DDRTYPE=00 (DDR2 or mDDR) at 25o C in the typical case, the following drive strengths can be referenced: Standard (3.6 mA), High (7.2 mA), Max (10.8 mA) with equivalent impedances of 90ohm, 45ohm, and 20ohmrespectively.”
It also has the logic levels of min 1.44v (0.8 * 1.8v VDD) for Voh (High) and max 0.36v (0.2* 1.8v VDD) for Vol (Low).
The selected drive strength, for example, is 3.6 mA at an impedance of 90ohm. If we apply ohm’s law with logic high voltage of 1.44v and impedance of 90 ohm the source current should be 16 mA but it is 3.6 mA as per the datasheet.
What is wrong with my calculation? The data sheets of most of the microcontrollers do not specify the output impedance at their digital I/O and peripheral pins. So I need to calculate the output impedance (Zo) for impedance matching the PCB traces. How do I know the output impedance (Zo) if it is not explicitly given in the datasheet.