How much is the clock in the MSX 2+/ Turbo R VDP?

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By Yukio

Paragon (1541)

Yukio's picture

12-03-2006, 15:53

Weird ...

By Manuel

Ascended (14790)

Manuel's picture

05-09-2007, 13:39

OK, how do we know that the R800 runs on 7.16Mhz, internally?

According to the service manual, the chip has several clock pins:

XTAL1 is a clock (no freq given), and an OUTPUT pin
XTAL0 seems to have 28MHz, and is an INPUT pin
VCLK seems to have 14.3 MHz, and is an OUTPUT pin
SYSCLK has 7.16MHz and is an OUTPUT pin.

Which is used for what and what is the internal clock frequency?

And how is the fact that changing XTAL0 to 40MHz increases R800 speed? I guess the XTAL0 is used to drive the R800 CPU, but it's 28MHz. How do we know that it runs on 7.16 MHz then?

Also note that the turboR's service manual says that the R800 is a 16bit CPU Smile

By Algorythms

Champion (283)

Algorythms's picture

05-09-2007, 14:17

XTAL0 is internally devided by 4. I have one 32Mhz xtal and one 40Mhz xtal connected with a switch because the 28Mhz one broke. Both increased the speed. With 32Mhz it's devided down to 8Mhz, 40MHz is devided down to 10MHz, which also the benchmark numbers show. 40MHz gives some graphics issues on my tbr in Z80 mode.

By Arjan

Paladin (697)

Arjan's picture

05-09-2007, 15:31

manuel: "A CPU can be classified on the basis of the data it can access in a single operation", according to wikipedia. The ALU in R800 is 16bits, making it an 16 bit CPU. If "a single operation" is meant to be "a single assembly instruction" then even the Z80 is a 16 bit CPU Tongue

By Manuel

Ascended (14790)

Manuel's picture

05-09-2007, 16:09

Algorythms: OK, but why didn't they just use a 7.16MHz clock then? And how do you know it's devided by 4?

By Algorythms

Champion (283)

Algorythms's picture

05-09-2007, 18:30

Manuel: Because having a higher frequency xtal devided gives less deviation. As to how I know, it's just from what I have read and heard, and is probably possible to calculate by the amount of cycles an instruction use.

By nikodr

Paladin (721)

nikodr's picture

05-09-2007, 19:00

Can the oscilators be changed so msx can be overclocked?

By AuroraMSX

Paragon (1900)

AuroraMSX's picture

05-09-2007, 19:37

nikodr:Can the oscilators be changed so msx can be overclocked?

Well, 4 or 5 posts up, Algorythms wrote:I have one 32Mhz xtal and one 40Mhz xtal connected with a switch because the 28Mhz one broke. Both increased the speed. With 32Mhz it's devided down to 8Mhz, 40MHz is devided down to 10MHz, which also the benchmark numbers show. 40MHz gives some graphics issues on my tbr in Z80 mode.

I guess that answers your question Wink

By Alex

Master (205)

Alex's picture

05-09-2007, 22:25

Hi,

Regarding MSX turbo R:

The R800 is driven by the 28 MHz crystal which is indeed divided internally to 7.16 MHz. Though, you can take that with a grain of salt because in actual fact, an instruction that takes 1 clock cycle on R800 took 1 M cycle on Z80, which is 4 t-cycles (4 clock cycles) on the Z80. So to make the timing tables simpler (so that they don't have to differentiate between M and T-cycles), they have only mentioned the M-cycles and then said that the CPU runs on7.16 MHz and not on 28 MHz.

The VDP in MSX turbo R has it's own clock crystal at approx. 21 MHz, just like in the MSX2 and MSX2+ models. Though, in MSX2 and MSX2+ models, this 21 MHz is divided by 6 to get the 3.57 MHz clock which feeds the Z80 CPU. So on MSX2 and MSX2+, the CPU and the VDP run perfectly in sync as far as the clock is concerned. On the MSX turbo R however, they run slightly out of sync, because the Z80 and the R800 are both fed from the 28 MHz crystal and the VDP is fed by the 21 MHz crystal.

One consequence is that benchmark programs that measure the CPU speed by counting how many times a certain loop can be done during 1 VDP interrupt cycle, give a slightly different result on MSX turbo R then on other MSX models.

Kind regards,
Alex

By Manuel

Ascended (14790)

Manuel's picture

05-09-2007, 22:53

Alex, so, we can safely say that it does run on 28MHz?

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