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External MCU Design Purpose

BCM2073X is a great SoC for most of IoT/wearable products.But an external MCU may be needed for more complicated use case such as smart watch on which there are LCD, camera etc. This article is a guide on hardware design between BC2073X and external MCU.

System Architecture Introduction

Architecture Diagram

The following Figure shows the usual connection between MCU and BCM2073X chip, together with some other devices.



The Interconnection BUS between MCU and BCM2073X

There are at least five interconnection bus needed between MCU and our BCM2073X chip: UART RX/TX, RESET, INT, and Wake up signals.

UART RX/TX buses are recommended to do data communication work, like notification status reporting data, external sensors inputting data, LCD display data, camera image data etc.

A reset signal from MCU to BCM2073X chip is used to give a reset signal when BCM2073X chip starts
up, or at any time when BCM2073X work abnormally.

An INT signal is an interruption from BCM2073X chip to MCU. For the occasion when MCU is in sleep mode, there are incoming calls,
messages, alarm or other notifications from the smartphones, tablets or other smart devices, which are connected with our BCM2073X chip through Bluetooth4.0/4.1. BCM2073X chip delivers these messages to MCU once it gets them, so that MCU can manage the related external devices to make the related response.The related response includes turning on or off LCD/LED/Vibrator/buzzer, etc., we can see the related devices showed on the above Fig.

A wake up signal is critical to MCU to wake up BCM2073X chip, when BCM2073X is in sleeping mode. For example when there are inputs from sensors, MCU will send a dedicated status update to BCM2073X after calculation, and BCM2073X chip pass them to the smart device for notifying the end user.

SW Download/Debugging Interface

SW downloading and debugging interface is located in MCU side, the software download interface can be   UART or USB. This is determined by the MCU chip we choose. Ask support from MCU vendor for detailed   download method.

The debugging interface can be shared with the download UART interface, or other ports. It’s also determined by MCU chips.

RF Testing Interface

We can do RF testing by separately controlling BCM2073X chip to enter into a testing mode by using Broadcom MBT tool. For HW, we connect BCM2073X chip UART RX/TX pin with a computer’s RS232 port. A level shift is needed for keeping the voltage of the two side at the same level, usually 5V voltage level from the computer side, a Vx(for BCM2073X chip, the voltage range is1.6-3.6V, the dedicated value of Vx is determined by BCM2073X, MCU and all the other device)  voltage level from the board side. We can also have another simple way to add a USB to COM converting chip to connect BCM2073X UART RX/TX pin on the PCB board with the computers by common USB bus. Please refer the picture below for the detail physical connection between the instruments and the tested device.


For the detailed testing procedures we can refer to the application notes of BLE RF Testing Setup Procedures.

Of course, we have some other methods to do the RF testing, since the extra MCU is the master chip of the whole board, we choose MCU UART interface for commands inputting interface, the dedicated testing commands were sent by the putty or hyper terminal tool passing MCU UART interface to control BCM2073X chip, of course the detailed controlling way mostly rely on the MCU system used; or we also can have another method to integrate the dedicated testing commands into the source code of MCU system side,  downloading into the memory on the board, a special testing tool can be added in a APK tool of the smart device in the far end, the special commands of APK tools trig the different testing mode of BLE chip, let the testing work going on by the clear human-computer interaction interface APK tool. MCU pass them to the BCM2073X chips, let it enter into the related testing mode.

Components Selections

For the wearable products, power consumption is the critical side for whole system. Both application requirement and power consumption should be considered during component selection. For MCU, Freescale’s KL25 is one of recommends for wearable products.

LDO or DC to DC convertor is the other key component on the board, the lower working power and the lower idle leakage current is preferred. SII S-1313 is one of recommendation with only 10nA current consumption in sleeping mode.

The other devices like G-sensor, LCD, UV-sensor, etc.Please also consider its working power consumption and standby, sleep power
consumption, make sure they have deep sleep mode and wake up mechanism, when not used, can enter into the deep sleep mode for power consumption reducing and easily return from the deep sleep mode.

Components’ dimension size is needed to be taken into the considerations, which can make the total PCB board smaller and flexible to fulfill the different shape demands of the wearable products. For example, 0201package size is preferred to the RCL parts. CSP and BGA package is recommended for IC chip,and module with more integrated functions is preferred for the ultra small wearable products.

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