Avoiding HardFault and Optimizing Code Size with TWI Transaction Manager

Question

I recently experimented with the TWI Transaction Manager and found the following: 
 This function would cause HardFault Error 
 void _BAD_read_func(app_twi_t* param_p_app_twi, 
 uint8_t* param_p_reg_addr, 
 uint8_t* param_p_buffer, 
 uint8_t byte_cnt)
{
 app_twi_transfer_t const transfer_1[] = 
 {
 HTS221_READ(param_p_reg_addr, param_p_buffer, byte_cnt)
 };

 app_twi_transaction_t const transaction_1 =
 {
 .callback = custom_app_twi_callback,
 .p_user_data = NULL,
 .p_transfers = transfer_1,
 .number_of_transfers = sizeof(transfer_1)/sizeof(transfer_1[0])
 };

 APP_ERROR_CHECK(app_twi_schedule(param_p_app_twi, &transaction_1));
}
 
 But this would not 
 uint8_t reg_addr = HTS221_WHO_AM_I_REG_ADDR;
uint8_t m_buffer[50];

app_twi_transfer_t transfer_2[] = 
{
 HTS221_READ(&reg_addr, m_buffer, 1)
};

app_twi_transaction_t transaction_2 =
{
 .callback = custom_app_twi_callback,
 .p_user_data = NULL,
 .p_transfers = transfer_2,
 .number_of_transfers = sizeof(transfer_2)/sizeof(transfer_2[0])
};

void _GOOD_read_func(app_twi_t* param_p_app_twi, 
 uint8_t* param_p_reg_addr, 
 uint8_t* param_p_buffer, 
 uint8_t byte_cnt)
{
 APP_ERROR_CHECK(app_twi_schedule(param_p_app_twi, &transaction_2));
}
 
 I am guessing that the TWI peripheral does not touch the app_twi_transaction_t struct nor the app_twi_transfer_t struct before the TWI operation is actually carried out. That is why if I store the variable in the stack as in the first function, when the TWI operation look up the values, the stack variables have already been popped and that is why it caused a HardFault. 
 Is my guess correct? 
 
 Another question is:
I need to optimize my firmware for code size. Therefore I really prefer not having to declare a global or static local app_twi_transfer_t array and a app_twi_transaction_t for every TWI operation I have. 
 With the second approach I posted above, I could modify the global variable depending on what I want to read/write. But then there is the case of two consecutive transactions. When I set up the second transaction and modify the global variables, would that potentially mess up the first transaction?

ovrebekk · Accepted Answer

Your guess is correct. If you want to declare the transfer or transaction structs inside a function other than main() you have to make them static, otherwise the data will be overwritten by the time the TWI driver uses them. The TWI driver runs various asserts on the data to ensure it is valid, and this is likely to fail if the data has been overwritten. 
 Is it code size, RAM size or both that you need to optimize for?
Having all your structures global or static will only affect your RAM consumption, not your code consumption. Even if you only define one structure at a time you still need to have that data in the code somewhere. 
 If you do want to pass one transaction at a time to the TWI library you probably need some kind of state machine in the application, where you wait for TWI events and feed one transaction at a time into the library. That being said I think this will have a larger impact on code usage than simply having all the transactions global/static, so I wouldn't recommend it.

Avoiding HardFault and Optimizing Code Size with TWI Transaction Manager

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