ST20-GP1 datasheet(68/116 Pages) STMICROELECTRONICS

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ST20-GP1

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upsetting the error status of any complex operation being carried out by the pre-empted low priority

process.

In the event of a processor halting because of HaltOnError, the link will ﬁnish outstanding transfers

before shutting down. If CPUAnalyse is asserted then all inputs continue but outputs will not make

another access to memory for data. Memory refresh will continue to take place.

11.2 Bootstrap

The ST20-GP1 can be bootstrapped from external ROM, internal ROM or from a link. This is

determined by the setting of the BootSource0-1 pins, see Table 9.1 on page 52. If both

BootSource0-1 pins are held low it will boot from a link. If either or both pins are held high, it will

boot from ROM. This is sampled once only by the ST20-GP1, before the ﬁrst instruction is

executed after reset.

11.2.1 Booting from ROM

When booting from ROM, the ST20-GP1 starts to execute code from the top two bytes in external

memory, at address #7FFFFFFE which should contain a backward jump to a program in ROM.

11.2.2 Booting from link

When booting from a link, the ST20-GP1 will wait for the ﬁrst bootstrap message to arrive on the

link. The ﬁrst byte received down the link is the control byte. If the control byte is greater than 1 (i.e.

2 to 255), it is taken as the length in bytes of the boot code to be loaded down the link. The bytes

following the control byte are then placed in internal memory starting at location MemStart.

Following reception of the last byte the ST20-GP1 will start executing code at MemStart. The

memory space immediately above the loaded code is used as work space. A byte arriving on the

bootstrapping link after the last bootstrap byte, is retained and no acknowledge is sent until a

process inputs from the link.

11.2.3 Peek and poke

Any location in internal or external memory can be interrogated and altered when the ST20-GP1 is

waiting for a bootstrap from link.

When booting from link, if the ﬁrst byte (the control byte) received down the link is greater than 1, it

is taken as the length in bytes of the boot code to be loaded down the link.

If the control byte is 0 then eight more bytes are expected on the link. The ﬁrst four byte word is

taken as an internal or external memory address at which to poke (write) the second four byte

word.

If the control byte is 1 the next four bytes are used as the address from which to

peek (read) a word

of data; the word is sent down the output channel of the link.

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