Since the boards work over a USB converter, what is the polling rate for the keyboards? I assume it’s either 250Hz or 500Hz.
According to xwhatsit’s post at https://deskthority.net/post146397.html#p146397, the controller uses an ATmega32U2 microcontroller clocked with a 16 MHz quartz crystal.
scan.c, line 39:
/* set up primary timer for column-scan interrupt */
TCCR0A |= (1 << WGM01);
OCR0A = 25;
TIMSK0 |= (1 << OCIE0A);
TCCR0B |= (1 << CS01) | (1 << CS00);
Timer control registers are documented on page 105 of the ATmega32U2 datasheet: http://www.atmel.com/images/doc7799.pdf#page=105
First line: Timer restarts when it reaches its target.
Second line: Target = 25.
Third line: Call a function when the timer restarts.
Fourth line: Increment timer every 64 clock cycles.
It looks like the number of columns depends on this counter:
scan.c, line 98:
uint8_t maxLoops = KBD_COLS;
} while (–maxLoops);
currCol = KBD_COLS;
KBD_COLS is defined in a separate file.
kbd_defs.h, line 20:
# if defined(BEAMSPRING)
# define KBD_COLS 23
# define KBD_ROWS 4
# elif defined(MODEL_F)
# define KBD_COLS 16
# define KBD_ROWS 8
Polling Frequency = 16 MHz / 64 / 25 / Number of Columns
Beam Spring USB Controller: 434 Hz.
Model F USB Controller: 625 Hz.