My Retropie Portable Build

It's been awhile since working on this, I've made a few changes since I originally started.

Change 1 I'm using a different 7inch screen, that is driven by a HDMI and usb for power and Touch https://www.altronics.com.au/p/z6514-7-inches-lcd-touchscreen-800x480-for-raspberry-pi/

Change 2 I've switched to a RPi Zero, its taking up room in my drawer bought it over a year ago , and have yet to use it, so decided on using it in my retro pie portable.

For my button input, I'm using a button matrix designed using the 74HC595 shift register which will allow me to use 4 GPIO to read a number of buttons, thanks to the 595's ability to stack an extra 8 bit shift register and add more input.  I'm going to use 4 shift registers to give me up to 32 button inputs, but the problem I have is trying to assign the key matrix, I was thinking of writing a *.c file like this https://github.com/annem/ADXL362_RaspPi/blob/master/ADXL362_RaspPi.c obviously mine will be spec'd for the shift registers.  But how would I go about building this into a keyboard driver, so I can use a key matrix assignment that Ben heck used on the retro pie build he did in episode 322 and 328.  I'm still relatively new to writing low level code on the RPi but I'd appreciate the help if it comes.

Now the reason why I choose to go with 32 inputs, on my key input device on my hand held I'll only connect approx 12 to 16 inputs, I'm also building a desktop Retro Pie and wanted to make sure I could have the one driver for doing both single and two player control connections.

So the way I have designed the button matrix is as follows, a single bit is loaded on to the shift register, and then is shifted through the entire 32 outputs, each of the outputs is connected to the MOSI line of the SPI bus, if a button is pressed, a logic 1 is sent to the input on the bus at that specific position.  So multiple buttons can be registered, so when the button matrix is read into the pi it is a 32bit number.

Then all I need to do is figure out how to analyse the number most likely via a lookup table or something to confirm which button is pressed, I assume this data analysis is done within the driver.

But I need to try and work out an appropriate polling period, as I'm looking to use a SPI clock of 6.25Mhz which would give me a clock pulse of interval of 160ns, so no where near the lower limit of 25ns for the clock input to register valid data (lowest limit is 20ns want to have a buffer).  But what would be an appropriate poll time, 1 millisecond?  I'd appreciate input on this.  If my math is correct, with a clock pulse of 160ns, and approximately 36 clock cycles to read the input states, that means I need  6 microseconds to read the data in, then I have to decide on a wait period and do another read.

As a friend of mine told me polling is better and easier to debug, versus using an interrupt.  And also cause my method of input is similar to a NES controller, they used a polling method to read the shift register data, so that appears to be the best method to read the button states.

I'll post a partial schematic of the button matrix in due course.

It's been awhile since working on this, I've made a few changes since I originally started.

Change 1 I'm using a different 7inch screen, that is driven by a HDMI and usb for power and Touch https://www.altronics.com.au/p/z6514-7-inches-lcd-touchscreen-800x480-for-raspberry-pi/

Change 2 I've switched to a RPi Zero, its taking up room in my drawer bought it over a year ago , and have yet to use it, so decided on using it in my retro pie portable.

For my button input, I'm using a button matrix designed using the 74HC595 shift register which will allow me to use 4 GPIO to read a number of buttons, thanks to the 595's ability to stack an extra 8 bit shift register and add more input.  I'm going to use 4 shift registers to give me up to 32 button inputs, but the problem I have is trying to assign the key matrix, I was thinking of writing a *.c file like this https://github.com/annem/ADXL362_RaspPi/blob/master/ADXL362_RaspPi.c obviously mine will be spec'd for the shift registers.  But how would I go about building this into a keyboard driver, so I can use a key matrix assignment that Ben heck used on the retro pie build he did in episode 322 and 328.  I'm still relatively new to writing low level code on the RPi but I'd appreciate the help if it comes.

Now the reason why I choose to go with 32 inputs, on my key input device on my hand held I'll only connect approx 12 to 16 inputs, I'm also building a desktop Retro Pie and wanted to make sure I could have the one driver for doing both single and two player control connections.

So the way I have designed the button matrix is as follows, a single bit is loaded on to the shift register, and then is shifted through the entire 32 outputs, each of the outputs is connected to the MOSI line of the SPI bus, if a button is pressed, a logic 1 is sent to the input on the bus at that specific position.  So multiple buttons can be registered, so when the button matrix is read into the pi it is a 32bit number.

Then all I need to do is figure out how to analyse the number most likely via a lookup table or something to confirm which button is pressed, I assume this data analysis is done within the driver.

But I need to try and work out an appropriate polling period, as I'm looking to use a SPI clock of 6.25Mhz which would give me a clock pulse of interval of 160ns, so no where near the lower limit of 25ns for the clock input to register valid data (lowest limit is 20ns want to have a buffer).  But what would be an appropriate poll time, 1 millisecond?  I'd appreciate input on this.  If my math is correct, with a clock pulse of 160ns, and approximately 36 clock cycles to read the input states, that means I need  6 microseconds to read the data in, then I have to decide on a wait period and do another read.

As a friend of mine told me polling is better and easier to debug, versus using an interrupt.  And also cause my method of input is similar to a NES controller, they used a polling method to read the shift register data, so that appears to be the best method to read the button states.

I'll post a partial schematic of the button matrix in due course.