Cypress CY7C638xx Srom Function Descriptions, Srom Function Parameters Variable Name Sram Address

CY7C63310, CY7C638xx

Two important variables that are used for all functions are KEY1 and KEY2. These variables are used to help discriminate between valid SSCs and inadvertent SSCs. KEY1 must always have a value of 3Ah, while KEY2 must have the same value as the stack pointer when the SROM function begins execution. This would be the Stack Pointer value when the SSC opcode is executed, plus three. If either of the keys do not match the expected values, the M8C halts (with the exception of the SWBootReset function). The following code puts the correct value in KEY1 and KEY2. The code starts with a halt, to force the program to jump directly into the setup code and not run into it.

halt

SSCOP: mov [KEY1], 3ah mov X, SP

mov A, X add A, 3

mov [KEY2], A

Table 9-2. SROM Function Parameters

Variable Name	SRAM Address
Key1/Counter/Return Code	0,F8h
Key2/TMP	0,F9h
BlockID	0,FAh
Pointer	0,FBh
Clock	0,FCh
Mode	0,FDh
Delay	0,FEh
PCL	0,FFh

9.4.1 Return Codes

The SROM also features Return Codes and Lockouts.

Return codes aid in the determination of the success or failure of a particular function. The return code is stored in KEY1’s position in the parameter block. The CheckSum and TableRead functions do not have return codes because KEY1’s position in the parameter block is used to return other data.

Table 9-3. SROM Return Codes

Return Code	Description
00h	Success
01h	Function not allowed due to level of protection
	on block.
02h	Software reset without hardware reset.
03h	Fatal error, SROM halted.

9.5 SROM Function Descriptions

9.5.1 SWBootReset Function

The SROM function, SWBootReset, is the function that is responsible for transitioning the device from a reset state to running user code. The SWBootReset function is executed whenever the SROM is entered with an M8C accumulator value of 00h: the SRAM parameter block is not used as an input to the function. This happens by design after a hardware reset, because the M8C's accumulator is reset to 00h or when the user code executes the SSC instruction with an accumulator value of 00h. The SWBootReset function is not executed when the SSC instruction is executed with a bad key value and a non-zero function code. An enCoRe II device executes the HALT instruction if a bad value is given for either KEY1 or KEY2.

The SWBootReset function verifies the integrity of the calibration data by way of a 16-bit checksum, before releasing the M8C to run user code.

9.5.2 ReadBlock Function

The ReadBlock function is used to read 64 contiguous bytes from Flash: a block.

This function first checks the protection bits and determines if the desired BLOCKID is readable. If the read protection is turned on, the ReadBlock function exits setting the accumulator and KEY2 back to 00h. KEY1 has a value of 01h, indicating a read failure. If read protection is not enabled, the function reads 64 bytes from the Flash using a ROMX instruction and stores the results in the SRAM using an MVI instruction. The first of the 64 bytes are stored in the SRAM at the address indicated by the value of the POINTER parameter. When the ReadBlock completes successfully, the accumulator, KEY1, and KEY2 all have a value of 00h.

Table 9-4. ReadBlock Parameters

Name	Address	Description
KEY1	0,F8h	3Ah
KEY2	0,F9h	Stack Pointer value, when SSC is
		executed.
BLOCKID	0,FAh	Flash block number
POINTER	0,FBh	First of 64 addresses in SRAM
		where returned data must be stored.

Read, write, and erase operations may fail if the target block is read or write protected. Block protection levels are set during device programming.

The EraseAll function overwrites data in addition to leaving the entire user Flash in the erase state. The EraseAll function loops through the number of Flash macros in the product, executing the following sequence: erase, bulk program all zeros, erase. After all the user space in all the Flash macros are erased, a second loop erases and then programs each protection block with zeros.

Document 38-08035 Rev. *K Page 15 of 83

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