MCP23S09 Reset Pin question

Thread Starter

Alex1700

Joined Jan 12, 2020
107
Q1: May I know if the reset pin base on diagram 1 will be in a high or low state?
Q2: May I know will this transistor circuit work when MCU triggers the BJT pull to GND to reset MCP23s09

NOTE that : Datasheet state that need external biased (not understand)

1679551732220.png


1679551745631.png

1679551852669.png
 

StefanZe

Joined Nov 6, 2019
191
The RESET pin in the frist diagram will be high and because the signal is internally inverted the IC is NOT in reset state.
Externally biased means that you have to take care of the pin and it is not allowed to leave the pin floating.
The second diagram will work. Maybe you can remove the transistor and connect the MCU pin directly to the RESET pin.
 

Thread Starter

Alex1700

Joined Jan 12, 2020
107
The RESET pin in the frist diagram will be high and because the signal is internally inverted the IC is NOT in reset state.
Externally biased means that you have to take care of the pin and it is not allowed to leave the pin floating.
The second diagram will work. Maybe you can remove the transistor and connect the MCU pin directly to the RESET pin.
If I directly connect to the MCU means that MCU gives a low signal meaning MCP will be reset
like this diagram?
1679559429501.png
 

Jon Chandler

Joined Jun 12, 2008
1,029
For future reference, the bar over RESET and the bar over CS in the datasheet show these are active LOW signals.

On the chip schematic symbol, active low pins can be indicated by a bar over the label, or like this: /RESET or #RESET.
 

Ian0

Joined Aug 7, 2020
9,679
Do you need to reset the device?
It already has a built-in power-on reset.
You can just connect /Reset to the positive supply.
 

StefanZe

Joined Nov 6, 2019
191
Hi.
It depends what you want to do with the outputs. If you want do drive a load that is connected to VCC (a relay for example) you don't need a pull up.
Whats on the other side from your GPIOs?
There is also MCP23S08 with push-pull GPIOs.
 

Thread Starter

Alex1700

Joined Jan 12, 2020
107
Hi.
It depends what you want to do with the outputs. If you want do drive a load that is connected to VCC (a relay for example) you don't need a pull up.
Whats on the other side from your GPIOs?
There is also MCP23S08 with push-pull GPIOs.
if I connect to the uln2003 driver to control the relay I need to connect a pull-up resistor ?
 

StefanZe

Joined Nov 6, 2019
191
Yes you need a pull-up resistor. Because it is a darlington transistor input the pull up should not be too high. A 1kOhm pull up resistor should work depending how much output current you need out of the uln2003.
 
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