24-09-2020, 09:09 PM
The Microbee serial mouse interface was released in 1987 with its second version being after the Black Monday stockmarket crash.
At this time the price of a Microsoft or Logitech serial mouse was about $200+. By 1987 there were a few cheaper Taiwanese clones hitting the market but still pretty pricey!
The insides of the mouse consisted a microcontroller with as much processing power as the microbee but with far less ROM and RAM to convert the Gray codes generated by the movement sensors and button pushes into a serial data stream which is sent to the host. The mouse sends differential X and Y values to indicate the change of position since its last transmission. Data is only sent when a mouse event occurs whether it be a mouse movement, button push/release or all.
The mouse is parasitically powered from the host's serial port from -12V on the TXD pin and +12 on its RTS and DTR pin.
Although the mouse can receive host commands, the microbee interface does not transmit any commands to the mouse. It only receives data from it at its default 1200 baud rate. The microbee interface works with the mouse using a baud rate generator crystal of 2.4576MHz, a PEEL (which is like a PAL on protein supplements) and a 4020 binary counter. This is done so that the microbee does not have to use its usual method of software delays and bit bashing to poll the serial port. Instead, the mouse interface is interrupt driven with the interface circuit generating clock pulses on the microbee's serial CLK pin to tell the microbee to read the data presented on its CTS pin and construct the mouse data payload. Doing this puts less strain on the microbee allowing it to run a user application whilst receiving and processing mouse data.
The -12V is generated for the mouse using an ICL7660 negative voltage doubler (-12V).
At this time the price of a Microsoft or Logitech serial mouse was about $200+. By 1987 there were a few cheaper Taiwanese clones hitting the market but still pretty pricey!
The insides of the mouse consisted a microcontroller with as much processing power as the microbee but with far less ROM and RAM to convert the Gray codes generated by the movement sensors and button pushes into a serial data stream which is sent to the host. The mouse sends differential X and Y values to indicate the change of position since its last transmission. Data is only sent when a mouse event occurs whether it be a mouse movement, button push/release or all.
The mouse is parasitically powered from the host's serial port from -12V on the TXD pin and +12 on its RTS and DTR pin.
Although the mouse can receive host commands, the microbee interface does not transmit any commands to the mouse. It only receives data from it at its default 1200 baud rate. The microbee interface works with the mouse using a baud rate generator crystal of 2.4576MHz, a PEEL (which is like a PAL on protein supplements) and a 4020 binary counter. This is done so that the microbee does not have to use its usual method of software delays and bit bashing to poll the serial port. Instead, the mouse interface is interrupt driven with the interface circuit generating clock pulses on the microbee's serial CLK pin to tell the microbee to read the data presented on its CTS pin and construct the mouse data payload. Doing this puts less strain on the microbee allowing it to run a user application whilst receiving and processing mouse data.
The -12V is generated for the mouse using an ICL7660 negative voltage doubler (-12V).