PS/2 to serial converter using ATtiny4313 and MAX232. There are plenty of similar projects, but almost all of them are external, so here's my internal version which draws power from floppy power connector.
This does essentially the same thing as other PS/2 to serial adapters: it interfaces with PS/2 mouse, and pretends to be a serial mouse to the computer.
It has following features:
- Support for normal 1200 baud serial, or 9600 baud high speed mode
- Support for wheel mouse
- Sample rate is selected from 20/40/60/100/200 with jumpers. For higher than 40, 9600 baud serial is needed.
- Resolution is configured with jumpers
- Option for USB A connector, so no need for a passive USB->PS/2 -adapter
- Option for PS/2 connector is also provided
- Supports hotplugging
- Crystal oscillator for ensuring accurate baud rate
- TVS diodes for ESD protection
- Overcurrent protection using ATtiny4313's onboard analog comparator and a P-type MOSFET as a switch
- No SMD components, so it's easy to assemble
- Programming via SPI header
- Supports combining movement packets in 1200 baud mode as a KVM workaround
Jumpers J1 sets baud rate, closed is high speed mode. J2 and J3 sets sample rate as described in silkscreen, and J4 and J4 set resolution.
Using 9600 baud serial speed requires modified mouse drivers, which can be found for example from this project: https://github.com/LimeProgramming/USB-serial-mouse-adapter/tree/main
There are footprints for both USB-A and mini-DIN6 or PS/2 connector, only one of them should be populated.
Some KVM switches ignore commands to set sample rate. The adapter tries to set baud rate to configured value, but the KVM will ignore it and packets are received faster than they can be sent to PC. As a new feature, this adapter combines data in movement packets so that they don't have to be dropped.
Also, some KVM switches randomly reset in middle of normal operation. There's a workaround for this as well.
ATtiny has both hardware USART and Universal Serial Interface or USI, which is readily adaptable to PS/2, so there's only minimal need for bit banging. USI provides start condition detector with interrupt and counter (increased at PS/2's clock rising and falling edge) overflow interrupt, which removes need for polling. Instead of sampling data on falling clock edge, this adapter uses rising edge as all the devices I have measured change data line state closer to falling than rising edge. In particular, with a KVM switch this was within as little as 3µs from falling edge!
Resistors R8 and R9 form voltage divider for analog comparator positive input pin. R7 is used as a shunt resistor for current detection, voltage after it goes to comparator's negative input pin. Comparator's output is monitored by interrupt from timer 0 every 100µs, and if there's overcurrent for 1ms, power to PS/2 is shut down.
RTS and DTR signals to pins PB2 and PB3 are monitored with pin change interrupt. When either of them goes down and then back up, mouse detection characters are sent if mouse is connected.
Normally PS/2 host doesn't know whether mouse is disconnected or is just still, so the adapter polls mouse by sending command enable data reporting or 0xF4 every two seconds. If mouse is connected, it replys with an ACK or 0xFA, if it's disconnected, there's no clock signal which is detected.
Some KVM switches randomly reset in the middle of normal operation and send 0xAA and 0x00 as during boot, then stopping to send movement data. This adapter attempts to detect this situation and recover from it.
BOM of specific components:
| Ref. | Part. no. | Note |
|---|---|---|
| Q1 | MTD20P06HDL | Any DPAK logic level P-channel enhancement-type MOSFET with similar pinout will work. |
| U1 | ATtiny4313-PU | Microcontroller |
| U2 | MAX232 | RS-232 transceiver |
| R7 | 560mΩ resistor | Anything with max. dissipation > 600mW will work. |
| Y1 | AS-18.432-20 | Other frequencies up to 20M can be used, but will require modifications to software. |
| D2-D4 | P6KE6.8 | TVS diode with 5.8V working voltage |
| J10 | MDC-206 | mini-DIN6 connector |
| J12, J13 | AMT0440051DB0000G | M4 screw terminal |
| C6 | ??? | Exact value of this capacitor isn't important. |
This has been built with Microchip studio, currently there's no Makefile for other toolchains. When programming the MCU, fuses must also be set. Correct setting for high fuse is 0xDF and for low fuse 0xFF.