Overclocking Z80B DART to 7.37 MHz

[Daniel Marks]

Hello,

I have noticed that many designs use a Z80 SIO or Z80 DART at 7.37 MHz.  I have found it difficult to obtain SIO or DART at more than 6 MHz (Z8740B).  However, the Z80B DART I have found (manufactured by Zilog in 8609 and GoldStar in 8635) will not work reliably at all at 7.37 MHz.  Has anyone else has experience with the Z80 DART and can this be expected to work?

Thanks for any feedback,

Dan

[Wayne Hortensius]

Can you use a CMOS part? Digikey has 10MHz SIO/0s (Z84C4010PEG).

Regards,
Wayne

[Spencer Owen]

Hi Daniel,

I haven't got any experience with DART, however, I can confirm that 6MHz SIO/2 chips are totally reliable at 7.37MHz.

Cheers

Spencer 

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[Alan Cox]

On Sun, 15 Aug 2021 at 03:17, Daniel Marks <pro...@gmail.com> wrote:

Hello,

I have noticed that many designs use a Z80 SIO or Z80 DART at 7.37 MHz.  I have found it difficult to obtain SIO or DART at more than 6 MHz (Z8740B).  However, the Z80B DART I have found (manufactured by Zilog in 8609 and GoldStar in 8635) will not work reliably at all at 7.37 MHz.  Has anyone else has experience with the Z80 DART and can this be expected to work?

The DART is basically a cut down SIO without the synchronous modes. So it's a straight swap for any normal RC2014 use case. Again if it's NMOS you'll need to worry about bus loadings and you may need to reduce the size of the resistors on the serial output lines.

In the old naming Z80B versions where 6MHz, Z80H versions 8MHz.

Alan

[Daniel Marks]

I figured out the problem.  It was not that the DART was rated for 6 MHz.  The Grant Searle circuit specifies to use 74HCT00 as the inverter for the CLK oscillator.  Unfortunately, this results in a very asymmetric square wave because of the very different low and high thresholds of HCT logic.  Replacing it with a 74HC00 fixed the problem.