Initial feedback on USB-based Li-Po SAM4S MCU project schematic

[GarthyD]

I'm looking for some initial input on the schematics for a project I am working on. Basically I am hoping for some feedback on glaring problems, thoughts on things I could tweak, useful things to include, component suggestions, and so forth, whilst there is still time to make changes.

The project is an ARM-based mostly-general-purpose USB-based microcontroller (Atmel SAM4S) board with a number of included peripherals, including a Li-Po charger, some sensors, buttons, and RGB LEDs. The available MCU pins will be broken out to headers. The intention at this point is to produce a hand-solderable PCB.

I've attached the schematics thus far. It is still a work-in-progress. The MCU connections are not complete, most headers are not included, and I haven't settled on specific components (eg. specific MOSFETs). Much of what is here exists as a breadboard prototype. I have some partly-written software to drive it.

I am an experienced software developer but would only consider myself an electronics novice. Chances are that if you are reading this, your electronics knowledge is stronger than mine.

I've designed one PCB before (AVR-based) but this will be my first ARM-based one.

I'll post some more details shortly.

Input most welcome.

[GarthyD]

Some additional details:

External power comes in from USB and goes via an AP2152 where it feeds into the Li-Po charger and battery. The battery and external power are then combined and feed into two regulators: a 3.3V regulator and a 3.6V regulator. These supplies and then split into a number of banks (two for 5V and 3.6V, three for 3.3V). The 3.6V regulator powers the on-board LEDs. The 3.3V regulator supplies most of the board, including the MCU.

The banks are controlled by the MCU, allowing parts of the circuit to be turned on and off as a group. The LEDs sit on 3.6V bank A (3V6A), and most of the on-board peripherals sit on 3.3V bank C (3V3C). Bank B has nothing on it by default.

The MCU itself is an Atmel SAM4S, which seems a good compromise between performance and cost.

There is an SPI bus with two fixed devices: A flash IC and a FRAM IC. SPI address decoding via a 138 provides seven usable slave select lines, of which two are used. Adding an external 138 should allow this to be expanded to fifteen lines.

Three RGB LEDs are included, connected to the PWM/clock outputs of the MCU.

An IR LED is controlled by two inputs, both of which much be enabled to light the LED, which will generally be a PWM out and a UART/USART.

A few sensors are included, connected to the ADC pins on the MCU, either directly or via jumpers. A voltage divider going to an ADC pin allows reading of the battery charge level.

A couple of mostly standalone 4-input buffers are included- one in and one out.

I have not included most of the headers yet, but it is intended to break out all of the MCU I/O pins and a few other odds and ends. I'm leaving the specifics until PCB placement, with the aim to squeeze in as much useful stuff as possible versus making the board too large.

I am designing the board as something mid-ranged cost-wise. I am considering designing two other boards in the future: One a low-cost SAMD21-based board with a whole bunch of on-board RGB LEDs; and another higher-end one based on a SAMS70 and a MAX V CPLD. This means that for this board I am trying to find the sweet-spot between the two, so a reasonable balance of cost/performance is the goal. Anything really fancy can go in the higher-end board, whilst more aggressive cost-cutting can end up in the lower-end one.

[GarthyD]

Just a quick note: Since making my original post I've been experimenting with the PCB layout for this design, and there *may* be large changes to some aspects of the schematic. Please do not give the current schematic more than a cursory glance at this stage.

PS. Pin headers take up a surprising amount of space!