PIC Hardware

Let’s start with the PIC16F1778 since it comes in a 28 pin SOIC package which is a compromise between being easy to solder and having the most pins.

Interconnect

90 degree female header pins are used for power, in-circuit programming and UART communication with the PIC. Male headers will be used to allow daughtercards to plug into the PIC board.

Power

An ADP7104 voltage regulator is used to provide 3.3~V to the circuit. This means the input voltage connected to “Vin” on the header can range from ~4~V up to 44~V. This means the board will run on 3 AA/AAA batteries (minimum) or any of the thousands of 12~V wall warts I seem to have lying around.

The ADP7104 supplies 500~mA of current at 3.3~V. The current draw of the board components is:

Part	Draw (mA)
PIC (32~MHz int osc)	4.2
AT25SF321 FLASH (Programming/Erasing)	16
LEDS (30~mA each)	30
TOTAL	50.2

External FLASH

The brain will have an Adesto AT25SF321 32Mb external FLASH chip for storing measurements, audio clips, etc.

The max SPI clock frequency is $f_{osc}/4$ = 8~MHz, assuming the 32~MHz internal oscillator.
The Read Array command allows data bytes to be read one after the other for a maximum read rate of 8~MHz/8~bits/byte = 1Mbyte/sec.
The slowest read rate would occur assuming a separate Read Array command was issued for each byte. This introduces 4 bytes of command/address overhead which results in a read rate of 8~MHz/40~bits/read = 200~kbyte/sec. This is still easily fast enough for audio applications.

Daughtercard Interface

A group of 4 1x8 headers are used to allow the brain board to plug into a daughter card. The headers are divided into 2 groups of 2. In each group, the headers are separated by 2.54~mm (the header pin spacing) and the centers of the two groups are separated by 32~mm.