Difference between revisions of "Timer and Divider Registers"
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{{Pandocs|timeranddividerregisters}} | {{Pandocs|timeranddividerregisters}} | ||
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| + | Read this page for a more detailed description of what the registers do: [[Timer Obscure Behaviour]] | ||
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===FF04 - DIV - Divider Register (R/W)=== | ===FF04 - DIV - Divider Register (R/W)=== | ||
This register is incremented at rate of 16384Hz (~16779Hz on SGB). Writing any value to this register resets it to 00h. | This register is incremented at rate of 16384Hz (~16779Hz on SGB). Writing any value to this register resets it to 00h. | ||
Revision as of 18:43, 22 November 2014
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Read this page for a more detailed description of what the registers do: Timer Obscure Behaviour
Contents
FF04 - DIV - Divider Register (R/W)
This register is incremented at rate of 16384Hz (~16779Hz on SGB). Writing any value to this register resets it to 00h.
Note: The divider is affected by CGB double speed mode, and will increment at 32768Hz in double speed.
FF05 - TIMA - Timer counter (R/W)
This timer is incremented by a clock frequency specified by the TAC register ($FF07). When the value overflows (gets bigger than FFh) then it will be reset to the value specified in TMA (FF06), and an interrupt will be requested, as described below.
FF06 - TMA - Timer Modulo (R/W)
When the TIMA overflows, this data will be loaded.
FF07 - TAC - Timer Control (R/W)
Bit 2 - Timer Enable
Bits 1-0 - Input Clock Select
00: CPU Clock / 1024 (DMG, CGB: 4096 Hz, SGB: ~4194 Hz)
01: CPU Clock / 16 (DMG, CGB: 262144 Hz, SGB: ~268400 Hz)
10: CPU Clock / 64 (DMG, CGB: 65536 Hz, SGB: ~67110 Hz)
11: CPU Clock / 256 (DMG, CGB: 16384 Hz, SGB: ~16780 Hz)
Note: The "Timer Enable" bit only affects the timer, the divider is ALWAYS counting.
INT 50 - Timer Interrupt
Each time when the timer overflows (ie. when TIMA gets bigger than FFh), then an interrupt is requested by setting Bit 2 in the IF Register (FF0F). When that interrupt is enabled, then the CPU will execute it by calling the timer interrupt vector at 0050h.
Note
The above described Timer is the built-in timer in the gameboy. It has nothing to do with the MBC3s battery buffered Real Time Clock - that's a completely different thing, described in the chapter about Memory Banking Controllers.
