ST9 datasheet(93/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

have its own clock frequency supplied on pins RXCLK and TXCLK for receive and transmit,

respectively. Here, we use the BRG to clock both sections.

At the clock input of the transmit and receive shift registers, two other dividers can be inserted,

to further divide the frequency by 16. It must be used in Asynchronous mode to allow detection

of the start bit using a local clock. If an external clock is supplied, at the same frequency and

phase as the serial incoming data, the 16x divisor must not be used. Here, the predivisor is

used.

The UART is surrounded by logic that allows it to detect errors on reception, a break state on

the line, and also to recognize a match between the character received and a reference char-

acter stored in a register. This last feature is used here. The ACR register is loaded with the

value 10 representing the LF character, and the IMR interrupt mask register is set to enable an

interrupt on a character match. Since DMA is used to transfer the incoming characters to

memory, this technique is used to detect the end of message, if we state that all commands

must end with an LF character.

4.6.1.2 DMA Controller

The DMA is used for the automatic data transfer from memory or a register to the serial trans-

mitter or from the serial receiver to a register or memory, or both.

The working of the DMA in conjunction with the UART merits some detailed explanation, be-

cause it is subtle and you need to take some precautions to make it work properly. Once ini-

tialized, it is easy to use and it consumes very little computing power to initialise transmission

or re-enable reception.

The DMA transfers in each direction are controlled by two registers: the DMA transaction

counter and the DMA pointer. The transaction counter is used to stop the transaction when a

predefined number of characters is transferred. The DMA pointer points to the character to be

sent or to the location that will hold the next character to be received. To initialise a DMA

transfer, the transaction counter must be set to the number of characters to be transferred and

the DMA pointer must be set to the beginning of the storage area that contains the data to be

transmitted or that will hold the data string received.

Transfers can occur between the UART and either the memory or register file.

The transaction counter and the DMA pointer must be registers in the register file. If the reg-

ister file is involved in the transfer, both the transaction counter and the pointer must be single

bytes (the addressing range of the register file is 0 to 255). If the memory is involved, the

counter and the pointer must be each a pair of registers, since the addressing range in

memory is 0 to 65535 for one segment.

Once you have defined in which registers you will put the counter and the pointer, you must let

the SCI know. The SCI has two registers for this purpose called the DMA Address Pointer

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