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Some more tech info

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2785 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Linus Nielsen Feltzing 2002-10-30 23:39:37 +00:00
parent a02ffd5afa
commit ec72d5a58a
1 changed files with 60 additions and 9 deletions
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docs/TECH
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@ -15,15 +15,21 @@ Booting and (De)Scrambling
scrambling that the original Archos firmware uses so that it can be loaded
by the built-in firmware.
1) The built-in firmware starts
2) It looks in the root directory for a file called "archos.mod" (player)
or "ajbrec.ajz" (recorder)
3) If it finds one, it loads the file, descrambles it and runs it
CPU
The CPU in use is a SH7034 from Hitachi, running at 11.0592MHz or 12MHz.
Most single instructions are excuted in 1 cycle. There is a 4KB internal ram
and a 2MB external ram.
The CPU in use is a SH7034 from Hitachi, running at 11.0592MHz (recorder)
or 12MHz (player).
Most single instructions are executed in 1 cycle. There is a 4KB internal RAM
and a 2MB external RAM.
Memory Usage
All Archos Jukebox models have only 2MB ram. The ram is used for everything,
All Archos Jukebox models have only 2MB RAM. The RAM is used for everything,
including code, graphics and config. To be able to play as long as possible
without having to load more data, the size of the mpeg playing buffer must
remain as big as possible. Also, since we need to be able to do almost
@ -36,7 +42,50 @@ Playing MPEG
The MPEG decoding is performed by an external circuit, MAS3507D (for the
Player/Studio models) or MAS3587F (for the Recorder models).
...
The CPU has a serial connection to the MAS for MP3 playback, using serial
port 0 at approx. 1mbit/s. The MAS has a handshake signal called DEMAND,
that informs the CPU when it wants more MP3 data. Whenever the DEMAND
signal goes high, it wants data sent over the serial line, and it wants it
quickly, within ~1ms. When the MAS has received enough data, it negates the
DEMAND signal and expects the incoming data stream to stop within 1ms.
The DEMAND signal is connected to a port pin on the CPU which can generate
an IRQ, but only on the falling edge. That means that the mpeg driver code
  must poll the DEMAND signal every ms to keep the MAS happy. The mpeg code
does use the IRQ to detect the falling edge when the MAS is "full".
Unfortunately, the serial port on the CPU sends the LSB first, and the MAS
expects the MSB first. Therefore we have to revers the bit order in every
byte in the loaded MP3 data. This is referred to as "bit swapping" in the
Rockbox code.
The internal DMA controller is used to feed the serial port with data. The
driver works roughly like this:
1) Load MP3 data into the RAM buffer
2) Bitswap the data
3) Load the DMA source pointer to the next 64Kbyte block to be transferred
4) Wait until DEMAND is high
5) Enable the DMA
6) Wait until the falling DEMAND pin generates an IRQ
7) Disable the DMA
8) Go to 4
The DMA generates an IRQ when the 64Kbyte block is transferred, and the
IRQ handler updates the DMA source pointer.
_____________________________
| |
DEMAND __________| |_____________
_ _ _ _ _ _ _ _ _
SC0 _____________/ \/ \/ \/ \/ \/ \/ \/ \/ \____________
\_/\_/\_/\_/\_/\_/\_/\_/\_/
^ ^
| |
Poll sees the DEMAND The DEMAND pin generates
signal go high and an IRQ that in turn disables
enables the DMA the DMA again
Spinning The Disk Up/Down
@ -87,10 +136,12 @@ Playing a Directory
Shuffle
Since the playlist is a an array of indexes to where to read the file name,
shuffle modifies the order of these indexes in the array. The randomness is
identical for the same random seed. This is the secret to good resume. Even
when you've shut down your unit and re-starts it, using the same random seed
as the previous time will give exactly the same random order.
shuffle modifies the order of these indexes in the array. The algorithm is
pretty much like shuffling a deck of cards, and it uses a pseudo random
generator called the Mersenne Twister. The randomness is identical for the
same random seed. This is the secret to good resume. Even when you've shut
down your unit and re-starts it, using the same random seed as the previous
time will give exactly the same random order.
Saving Config Data