6.3.2 UART baudrate negotation

When PIO4, CLK and CMD are all pulled low, BCSP over UART is selected. The speed for this initial

communication should be between 57600bps and 115200bps and must use even parity.

Since the WT21 doesn't know its internal clock frequency nor the baud rate it should communicate at, a form

of baud rate auto-negotiation is implemented on the module. The host needs to send some data packets so

that the WT21 can assess the baud rate the host uses.

Any seemingly non-uniform data can be used but for BCSP sending Link Establishment packets is the natural

choice. Once the WT21 has received enough samples it will start communicating back to the host by replying

to the Link Establishment messages.

Since the H4 doesn't have any link protocol, repeated queries for some known value can be used. Once the

host has started receiving replies for the queries, it can be assumed that that the baud rate has been

established.

Note:

The number of packets the WT21 will need for assessing the baud rate is dependent on the host's UART hardware. With poor

quality UARTs the auto-negotiation may fail, which means the WT21 will use an incorrect baud rate to communicate back. The

host will see this as packets of garbage, i.e. full of crc errors and checksum errors when using BCSP. To restart the negotiation

the host will need to reset the WT21. The host should have some way to power cycle the WT21 to restart the baud rate

negotiation, either by controlling the reset pin or directly the module's VCC.

6.3.3 Loading the PS keys

Once a communications link has been established, BCCMD packets are used to load PS Keys. At least the

following PS Keys need to be set:

Key address

Value

Description

&0001

00dd eeff 00cc aabb

Bluetooth address (aa:bb:cc:dd:ee:ff)

&01be

01d8

UART baud rate (01d8 = 115200)

&01fe

6590

Chrystal frequency. Do not modify.

&025d

0001

VM disable. Do not modify.

&01f9

0001

Host interface, (0001 = BCSP)

For the host interface the following protocols are supported:



BCSP 0001



H4 0003



H5 0006



H4DS 0007

To activate the changes the command for warm reset is sent to the WT21.

After this the module will immediately reboot using the configured settings and thus switch to the specified

baud rate.

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