Microprocessor supervisor that would stay in Reset at Power ON - Page 1
Forum section: electronics / beginners

author: jmf11
author_rank: Contributor
timestamp: November 15, 2019, 06:38:10 pm
content: Hello,I'm working on a Class D amp with integrated DSP, based on the TI TAS3251 chip. This chip needs to be controlled by a microcontroller to load the DSP coefficients (I2C) and manage the power on and power off sequence. Especially, the /Reset_Amp of the TAS3251 shall stay to LOW up to the point where the DSP is started and configure. My design is heavily based on the TI TAS3251EVM which uses a TPS3802 as supervisor, controlled.In my design, I would like to pair the TAS3251 with an independant Raspberry PI or STM32 Nucleo board. In that config, I can't ensure that the Microcontroller or RPi will always be powered before the amp. So, I would need that at power up of the amp, instead of having the TPS3802 (or equivalent) going to HIGH when voltage establishes, I would need it to stay low up to the moment the microcontroller will raise /MR to HIGH.So I would need sort of a pull-down to GND on the /MR. But looking at the datasheets, this pin has a pull-up internal resistor (20kR for a MIC8114, did not found the info for the TPS3802).So... How to get a pull-down defaut on chip with pull-up internalsand still controllable to HIGH and LOW by a Microcontroller ?My electronic knowledge is very very rusty, and I'm looking for some help solve this beginners dilemna.Best Regards,JMF

author: TomS_
author_rank: Frequent Contributor
timestamp: November 15, 2019, 07:16:35 pm
content: You would just need to pull the reset signal low using an external resistor with a low enough value such that when combined with the internal pull up you get a voltage low enough to satisfy the reset voltage threshold. Use a resistor divider calculator to play around with values and figure that out.If your microcontroller pins are by default high impedance at boot, you then only need to configure one as an output and set it high to release the peripheral from reset.

author: jmf11
author_rank: Contributor
timestamp: November 16, 2019, 09:44:25 am
content: I've done my home work. I identified 2 possible circuits for my purpose.Option 1: MIC8114 (datasheet:http://ww1.microchip.com/downloads/en/DeviceDoc/mic8114.pdf):Max VCC is 5V. /MR is considered low at 0.25 VCC. There is an internal pull-up resistor that is typically 20K. Let's consider we want an external pull-down resistor that pulls to achieve 0.2VCC, need a pull-down resistor of 0.2*20k/(1-0.2) => 5kRThis seems to be on the low side for a micro-controler external resistor, no ? Is it OK with a STM32 micro-controller GPIO HIGH output (will draw 0.6 mA)?Option 2 (needs confirmation of correct understanding) with TPS3802 (datasheethttp://www.ti.com/lit/ds/symlink/tps3802.pdf)The datasheet states a cryptic (for me) : "Pull-up resistorvalue,RESET : Vpull-up / 50 µA (Ohms)". Does this means that there is in fact no pull-up resistor, and that the chip will just drain 50 µA ?In that case I could simply put a 100k pull-dow resistor on /MR ? This would look like a better option.Thanks for your help. I'm not so confident and would like to avoid big mistakes.Best regards,JMF

author: nali
author_rank: Frequent Contributor
timestamp: November 16, 2019, 09:50:15 am
content: Assuming your MCU defaults to tri-state or inputs at power up -Why not just use a logic-level NMOS to drive the reset line of the amp, and drive the gate of the FET from the MCU. Putting an external pullup resistor to the gate will ensure the FET is holding the reset line until you switch the MCU port to an output and drive it low to turn off.

author: TomS_
author_rank: Frequent Contributor
timestamp: November 16, 2019, 12:17:55 pm
content: Quote from: jmf11 on November 16, 2019, 09:44:25 amAs long as the source current from the uC pin doesnt exceed its maximum rating then it should be fine.QuoteNote that this is specified as the minimum, so at 5V you would want to use a maximum of a 100K resistor to ensure you supply at least 50uA. If youre designing a battery powered device you may go for a higher value resistor like 100K to minimise wasted current, but if power consumption is not a problem you could go for something lower that you are using elsewhere (e.g. 10K), perhaps to minimise the BOM.

author: jmf11
author_rank: Contributor
timestamp: November 17, 2019, 11:52:38 am
content: Thanks TomS_I will go for the TPS3802 option with something like 10k or 20k pull-down resistor on /MR.Best regards,JMF