[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH] ibex_uart: fix XOR-as-pow
From: |
Paolo Bonzini |
Subject: |
Re: [PATCH] ibex_uart: fix XOR-as-pow |
Date: |
Wed, 24 Jun 2020 08:33:19 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
On 23/06/20 22:07, Eric Blake wrote:
>>
>> uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>> baud *= 1000;
>> - baud /= 2 ^ 20;
>> + baud >>= 20;
>
> Dividing by 1M instead of 22 seems much more logical, indeed :)
Based on the spec, the "* 1000" is the clock, in other words this is a
fixed point value relative to the clock:
f_baud = NCO * f_clock / 2^20
The example in the spec (https://docs.opentitan.org/hw/ip/uart/doc/) has
f_clock = 50 MHz, while here it's only 1 kHz. And the register is only
16 bit, so the above would only allow a baud rate up to 62 (65535 * 1000
/ 2^20).
Should the clock be a property of the device instead?
Thanks,
Paolo
> It's odd that we are scaling up by 1000, down by 1024*1024, then
>
>> s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10;