Timers
Timers 1~3 Overview
The Pokemon Mini offers 3 general purpose timer units. Each timer is broken down into several blocks to provide it with variable clock rates, the ability to be broken down into two independent 8-bit timers, and each timer can generate two unique interrupts.
Timers 1~3 Control
Each timer is configured using 6 registers, TIM_SCALE*, TIM_OSCI*, TIM_CTL*_L, TIM_CTL*_H, TIM_PRE*_L and TIM_PRE*_H. These registers provide the ability to change the clock rate of both the low and high 8-bits of the counter, set if there is a borrow chain to the upper 8-bits (16-bit counter mode) as well as enable and reset the timer (load counter from the preset). Beginning with the TIM_OSCI*, each timer has the ability to run from oscillator 1 (System Clock) or oscillator 2 (32768 Hz RTC Clock). The pre-scale is further decided by selecting one of 8 different pre-scale values from a table in the TIM_SCALE* register.
Further more, timers must be enabled individually (by setting the enable flag in TIM_SCALE* TIM_CTL*_L and TIM_CTL*_H) as well as by group (Upper half of TIM_ENA_OSCI1). TIM_ENA_OSCI1 disables oscillator 1 or 2 if either respective bit is clear ($10 and $20).
| Prescale | Clk Div. | Hz |
|---|---|---|
| 0 | CPU / 2 | 2000000 |
| 1 | CPU / 8 | 500000 |
| 2 | CPU / 32 | 125000 |
| 3 | CPU / 64 | 62500 |
| 4 | CPU / 128 | 31250 |
| 5 | CPU / 256 | 15625 |
| 6 | CPU / 1024 | 3906.25 |
| 7 | CPU / 4096 | 976.5625 |
| Prescale | Clk Div. | Hz |
|---|---|---|
| 0 | 32768 / 1 | 32768 |
| 1 | 32768 / 2 | 16384 |
| 2 | 32768 / 4 | 8192 |
| 3 | 32768 / 8 | 4096 |
| 4 | 32768 / 16 | 2048 |
| 5 | 32768 / 32 | 1024 |
| 6 | 32768 / 64 | 512 |
| 7 | 32768 / 128 | 256 |
The timer control registers affect the values of the timers themselves. Enable must be set for timing, this means there are a total of 3 bits that must be enabled for any timer to begin counting. writing a logical 1 to a reset bit in a control register will cause that respective 8-bit section to copy the respective value out of preset. All timers count down. When any timer underflows, it's value is copied from the preset value.
Timers 1~3 in 16-bit mode
When a timer is operating in 16-bit mode, all the upper-8 bit settings are unceremoniously ignored. Enables, reset and and everything no longer control the behavior of the timer. They remain writable, but they no longer actively function. This includes enables, resets and pre-scale values. The lower-8 bit configuration is effective over the full 16-bit value. Additionally, all lower-8 underflow IRQs are effectively disabled. The timer only presets when the full 16-bit value underflows.
Timers 1~3 IRQ Operation
Each timer provides two irqs. These IRQs appear to be fixed function, which provides the only known difference between Timers 1-2 and Timer 3.
Timers 1-3 have a primary IRQ, this fires anytime the upper 8-bit of the counter underflows (16- or 8-bit operations) The secondary IRQ of Timer 1-2 occurs when the lower 8-bit counter underflows (8-bit mode only) The secondary IRQ of Timer 3 occurs when the value of the counter becomes less than or equal-to the value in it's comparator. In 8-bit mode only the upper 8-bit of the value is used.
Sound
Timer 3 is also used for sound within the Pokemon Mini.
256Hz Timer Operation
The 256Hz Timer is enabled by writing the LSB of TIM256_CTL to 1. This activates the timer. It might be a good idea to also reset the timer by writing the 2nd bit to 1 also.
When the timer is active register TIM256_CNT is incremented 256 times a second. Since it's an 8 bit register counting from 0 to 255 it overflows exactly 1 time a second.
IRQs tied to 256 Hz timer
The counter register TIM256_CNT is also used to fire some interrupts at frequencies derived from the 256 Hz clock:
When TIM256_CNT overflows the 1 Hz IRQ $0E is fired.
When TIM256_CNT bit 8 increments (overflow from bit 7) the 2 Hz IRQ $0D is fired.
When TIM256_CNT bit 6 increments (overflow from bit 5) the 8 Hz IRQ $0C is fired.
When TIM256_CNT bit 4 increments (overflow from bit 3) the 32 Hz IRQ $0B is fired.
Second Counter Overview
The second timer, while simple, was misunderstood for a good portion of the initial reverse engineering process. SEC_CNT_* provides a 24 bit second counter that increments with a one second period (based of a 32768hz crystal) and can be enabled, disabled or reset to zero using the SEC_CTL register. Initially, these registers were thought to be a general purpose counter, but is actually one half of the system's real time clock.
When a commercial game sets the time, it resets the timer and stores the current time and date in the EEPROM. Calculating the current time consists of simply adding the number of elapsed seconds to the second counter. Each time the system boots, commercial games compare the current seconds against a 24 bit value stored in EEPROM. This was the last known valid time. If the current second counter is less than the previous know valid, it is assumed that the clock is valid and a game resumes running normally.
Under no circumstances should any homebrew software reset or pause the second counter, as it will invalidate the time on the system. This counter continues to tick even when the system is suspended.